Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

> On 29 Mar 2018, at 00:07, Luck, Tony  wrote:
> 
>> The default limit of only 65536 VMAs will also quickly come into play
>> if consecutive anon mmaps don't get merged. Of course this can be
>> raised, but it has significant resource and performance (fork) costs.
> 
> Could the random mmap address chooser look for how many existing
> VMAs have space before/after and the right attributes to merge with the
> new one you want to create? If this is above some threshold (100?) then
> pick one of them randomly and allocate the new address so that it will
> merge from below/above with an existing one.
> 
> That should still give you a very high degree of randomness, but prevent
> out of control numbers of VMAs from being created.

I think this wouldn’t work. For example these 100 allocation may happened on 
process initialization. But when attacker come to the server all his 
allocations would be made on the predictable offsets from each other. So in 
result we did nothing just decrease performance of first 100 allocations. I 
think I can make ioctl to turn off this randomization per process and it could 
be used if needed. For example if application going to allocate big chunk or 
make big memory pressure, etc.

Best regards,
Ilya

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RE: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Luck, Tony]

> The default limit of only 65536 VMAs will also quickly come into play
> if consecutive anon mmaps don't get merged. Of course this can be
> raised, but it has significant resource and performance (fork) costs.

Could the random mmap address chooser look for how many existing
VMAs have space before/after and the right attributes to merge with the
new one you want to create? If this is above some threshold (100?) then
pick one of them randomly and allocate the new address so that it will
merge from below/above with an existing one.

That should still give you a very high degree of randomness, but prevent
out of control numbers of VMAs from being created.

-Tony

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Rich Felker]

On Fri, Mar 30, 2018 at 09:55:08AM +0200, Pavel Machek wrote:
> Hi!
> 
> > Current implementation doesn't randomize address returned by mmap.
> > All the entropy ends with choosing mmap_base_addr at the process
> > creation. After that mmap build very predictable layout of address
> > space. It allows to bypass ASLR in many cases. This patch make
> > randomization of address on any mmap call.
> 
> How will this interact with people debugging their application, and
> getting different behaviours based on memory layout?
> 
> strace, strace again, get different results?

Normally gdb disables ASLR for the process when invoking a program to
debug. I don't see why that would be terribly useful with strace but
you can do the same if you want.

Rich

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Pavel Machek]

On Fri 2018-03-30 12:07:58, Ilya Smith wrote:
> Hi
> 
> > On 30 Mar 2018, at 10:55, Pavel Machek  wrote:
> > 
> > Hi!
> > 
> >> Current implementation doesn't randomize address returned by mmap.
> >> All the entropy ends with choosing mmap_base_addr at the process
> >> creation. After that mmap build very predictable layout of address
> >> space. It allows to bypass ASLR in many cases. This patch make
> >> randomization of address on any mmap call.
> > 
> > How will this interact with people debugging their application, and
> > getting different behaviours based on memory layout?
> > 
> > strace, strace again, get different results?
> > 
> 
> Honestly I’m confused about your question. If the only one way for debugging 
> application is to use predictable mmap behaviour, then something went wrong 
> in 
> this live and we should stop using computers at all.

I'm not saying "only way". I'm saying one way, and you are breaking
that. There's advanced stuff like debuggers going "back in time".

Pavel
-- 
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(cesky, pictures) 
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

Hi

> On 30 Mar 2018, at 10:55, Pavel Machek  wrote:
> 
> Hi!
> 
>> Current implementation doesn't randomize address returned by mmap.
>> All the entropy ends with choosing mmap_base_addr at the process
>> creation. After that mmap build very predictable layout of address
>> space. It allows to bypass ASLR in many cases. This patch make
>> randomization of address on any mmap call.
> 
> How will this interact with people debugging their application, and
> getting different behaviours based on memory layout?
> 
> strace, strace again, get different results?
> 

Honestly I’m confused about your question. If the only one way for debugging 
application is to use predictable mmap behaviour, then something went wrong in 
this live and we should stop using computers at all.

Thanks,
Ilya

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Pavel Machek]

Hi!

> Current implementation doesn't randomize address returned by mmap.
> All the entropy ends with choosing mmap_base_addr at the process
> creation. After that mmap build very predictable layout of address
> space. It allows to bypass ASLR in many cases. This patch make
> randomization of address on any mmap call.

How will this interact with people debugging their application, and
getting different behaviours based on memory layout?

strace, strace again, get different results?

Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) 
http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

> On 28 Mar 2018, at 02:49, Matthew Wilcox  wrote:
> 
> On Tue, Mar 27, 2018 at 03:53:53PM -0700, Kees Cook wrote:
>> I agree: pushing this off to libc leaves a lot of things unprotected.
>> I think this should live in the kernel. The question I have is about
>> making it maintainable/readable/etc.
>> 
>> The state-of-the-art for ASLR is moving to finer granularity (over
>> just base-address offset), so I'd really like to see this supported in
>> the kernel. We'll be getting there for other things in the future, and
>> I'd like to have a working production example for researchers to
>> study, etc.
> 
> One thing we need is to limit the fragmentation of this approach.
> Even on 64-bit systems, we can easily get into a situation where there isn't
> space to map a contiguous terabyte.

As I wrote before, shift_random is introduced to be fragmentation limit. Even 
without it, the main question here is ‘if we can’t allocate memory with N size 
bytes, how many bytes we already allocated?’. From these point of view I 
already showed in previous version of patch that if application uses not so big 
memory allocations, it will have enough memory to use. If it uses XX Gigabytes 
or Terabytes memory, this application has all chances to be exploited with 
fully randomization or without. Since it is much easier to find(or guess) any 
usable pointer, etc. For the instance you have only 128 terabytes of memory for 
user space, so probability to exploit this application is 1/128 what is not 
secure at all. This is very rough estimate but I try to make things easier to 
understand.

Best regards,
Ilya



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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

> On 28 Mar 2018, at 01:16, Theodore Y. Ts'o  wrote:
> 
> On Tue, Mar 27, 2018 at 04:51:08PM +0300, Ilya Smith wrote:
>>> /dev/[u]random is not sufficient?
>> 
>> Using /dev/[u]random makes 3 syscalls - open, read, close. This is a 
>> performance
>> issue.
> 
> You may want to take a look at the getrandom(2) system call, which is
> the recommended way getting secure random numbers from the kernel.
> 
>>> Well, I am pretty sure userspace can implement proper free ranges
>>> tracking…
>> 
>> I think we need to know what libc developers will say on implementing ASLR 
>> in 
>> user-mode. I am pretty sure they will say ‘nether’ or ‘some-day’. And 
>> problem 
>> of ASLR will stay forever.
> 
> Why can't you send patches to the libc developers?
> 
> Regards,
> 
>   - Ted

I still believe the issue is on kernel side, not in library.

Best regards,
Ilya


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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

> On 27 Mar 2018, at 17:38, Michal Hocko  wrote:
> 
> On Tue 27-03-18 16:51:08, Ilya Smith wrote:
>> 
>>> On 27 Mar 2018, at 10:24, Michal Hocko  wrote:
>>> 
>>> On Mon 26-03-18 22:45:31, Ilya Smith wrote:
 
> On 26 Mar 2018, at 11:46, Michal Hocko  wrote:
> 
> On Fri 23-03-18 20:55:49, Ilya Smith wrote:
>> 
>>> On 23 Mar 2018, at 15:48, Matthew Wilcox  wrote:
>>> 
>>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
 Current implementation doesn't randomize address returned by mmap.
 All the entropy ends with choosing mmap_base_addr at the process
 creation. After that mmap build very predictable layout of address
 space. It allows to bypass ASLR in many cases. This patch make
 randomization of address on any mmap call.
>>> 
>>> Why should this be done in the kernel rather than libc?  libc is 
>>> perfectly
>>> capable of specifying random numbers in the first argument of mmap.
>> Well, there is following reasons:
>> 1. It should be done in any libc implementation, what is not possible 
>> IMO;
> 
> Is this really so helpful?
 
 Yes, ASLR is one of very important mitigation techniques which are really 
 used 
 to protect applications. If there is no ASLR, it is very easy to exploit 
 vulnerable application and compromise the system. We can’t just fix all 
 the 
 vulnerabilities right now, thats why we have mitigations - techniques 
 which are 
 makes exploitation more hard or impossible in some cases.
 
 Thats why it is helpful.
>>> 
>>> I am not questioning ASLR in general. I am asking whether we really need
>>> per mmap ASLR in general. I can imagine that some environments want to
>>> pay the additional price and other side effects, but considering this
>>> can be achieved by libc, why to add more code to the kernel?
>> 
>> I believe this is the only one right place for it. Adding these 200+ lines 
>> of 
>> code we give this feature for any user - on desktop, on server, on IoT 
>> device, 
>> on SCADA, etc. But if only glibc will implement ‘user-mode-aslr’ IoT and 
>> SCADA 
>> devices will never get it.
> 
> I guess it would really help if you could be more specific about the
> class of security issues this would help to mitigate. My first
> understanding was that we we need some randomization between program
> executable segments to reduce the attack space when a single address
> leaks and you know the segments layout (ordering). But why do we need
> _all_ mmaps to be randomized. Because that complicates the
> implementation consirably for different reasons you have mentioned
> earlier.
> 

There are following reasons:
1) To protect layout if one region was leaked (as you said). 
2) To protect against exploitation of Out-of-bounds vulnerabilities in some 
cases (CWE-125 , CWE-787)
3) To protect against exploitation of Buffer Overflows in some cases (CWE-120)
4) To protect application in cases when attacker need to guess the address 
(paper ASLR-NG by  Hector Marco-Gisbert and  Ismael Ripoll-Ripoll)
And may be more cases.

> Do you have any specific CVE that would be mitigated by this
> randomization approach?
> I am sorry, I am not a security expert to see all the cosequences but a
> vague - the more randomization the better - sounds rather weak to me.

It is hard to name concrete CVE number, sorry. Mitigations are made to prevent 
exploitation but not to fix vulnerabilities. It means good mitigation will make 
vulnerable application crash but not been compromised in most cases. This means 
the better randomization, the less successful exploitation rate.


Thanks,
Ilya


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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Rob Landley]

On 03/23/2018 02:06 PM, Matthew Wilcox wrote:
> On Fri, Mar 23, 2018 at 02:00:24PM -0400, Rich Felker wrote:
>> On Fri, Mar 23, 2018 at 05:48:06AM -0700, Matthew Wilcox wrote:
>>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
 Current implementation doesn't randomize address returned by mmap.
 All the entropy ends with choosing mmap_base_addr at the process
 creation. After that mmap build very predictable layout of address
 space. It allows to bypass ASLR in many cases. This patch make
 randomization of address on any mmap call.
>>>
>>> Why should this be done in the kernel rather than libc?  libc is perfectly
>>> capable of specifying random numbers in the first argument of mmap.
>>
>> Generally libc does not have a view of the current vm maps, and thus
>> in passing "random numbers", they would have to be uniform across the
>> whole vm space and thus non-uniform once the kernel rounds up to avoid
>> existing mappings.
> 
> I'm aware that you're the musl author, but glibc somehow manages to
> provide etext, edata and end, demonstrating that it does know where at
> least some of the memory map lies.

You can parse /proc/self/maps, but it's really expensive and disgusting.

Rob

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Rich Felker]

On Tue, Mar 27, 2018 at 04:49:04PM -0700, Matthew Wilcox wrote:
> On Tue, Mar 27, 2018 at 03:53:53PM -0700, Kees Cook wrote:
> > I agree: pushing this off to libc leaves a lot of things unprotected.
> > I think this should live in the kernel. The question I have is about
> > making it maintainable/readable/etc.
> > 
> > The state-of-the-art for ASLR is moving to finer granularity (over
> > just base-address offset), so I'd really like to see this supported in
> > the kernel. We'll be getting there for other things in the future, and
> > I'd like to have a working production example for researchers to
> > study, etc.
> 
> One thing we need is to limit the fragmentation of this approach.
> Even on 64-bit systems, we can easily get into a situation where there isn't
> space to map a contiguous terabyte.

The default limit of only 65536 VMAs will also quickly come into play
if consecutive anon mmaps don't get merged. Of course this can be
raised, but it has significant resource and performance (fork) costs.

Rich

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Kees Cook]

On Tue, Mar 27, 2018 at 6:51 AM, Ilya Smith  wrote:
>
>> On 27 Mar 2018, at 10:24, Michal Hocko  wrote:
>>
>> On Mon 26-03-18 22:45:31, Ilya Smith wrote:
>>>
 On 26 Mar 2018, at 11:46, Michal Hocko  wrote:

 On Fri 23-03-18 20:55:49, Ilya Smith wrote:
>
>> On 23 Mar 2018, at 15:48, Matthew Wilcox  wrote:
>>
>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
>>> Current implementation doesn't randomize address returned by mmap.
>>> All the entropy ends with choosing mmap_base_addr at the process
>>> creation. After that mmap build very predictable layout of address
>>> space. It allows to bypass ASLR in many cases. This patch make
>>> randomization of address on any mmap call.
>>
>> Why should this be done in the kernel rather than libc?  libc is 
>> perfectly
>> capable of specifying random numbers in the first argument of mmap.
> Well, there is following reasons:
> 1. It should be done in any libc implementation, what is not possible IMO;

 Is this really so helpful?
>>>
>>> Yes, ASLR is one of very important mitigation techniques which are really 
>>> used
>>> to protect applications. If there is no ASLR, it is very easy to exploit
>>> vulnerable application and compromise the system. We can’t just fix all the
>>> vulnerabilities right now, thats why we have mitigations - techniques which 
>>> are
>>> makes exploitation more hard or impossible in some cases.
>>>
>>> Thats why it is helpful.
>>
>> I am not questioning ASLR in general. I am asking whether we really need
>> per mmap ASLR in general. I can imagine that some environments want to
>> pay the additional price and other side effects, but considering this
>> can be achieved by libc, why to add more code to the kernel?
>
> I believe this is the only one right place for it. Adding these 200+ lines of
> code we give this feature for any user - on desktop, on server, on IoT device,
> on SCADA, etc. But if only glibc will implement ‘user-mode-aslr’ IoT and SCADA
> devices will never get it.

I agree: pushing this off to libc leaves a lot of things unprotected.
I think this should live in the kernel. The question I have is about
making it maintainable/readable/etc.

The state-of-the-art for ASLR is moving to finer granularity (over
just base-address offset), so I'd really like to see this supported in
the kernel. We'll be getting there for other things in the future, and
I'd like to have a working production example for researchers to
study, etc.

-Kees

-- 
Kees Cook
Pixel Security

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Michal Hocko]

On Mon 26-03-18 22:45:31, Ilya Smith wrote:
> 
> > On 26 Mar 2018, at 11:46, Michal Hocko  wrote:
> > 
> > On Fri 23-03-18 20:55:49, Ilya Smith wrote:
> >> 
> >>> On 23 Mar 2018, at 15:48, Matthew Wilcox  wrote:
> >>> 
> >>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
>  Current implementation doesn't randomize address returned by mmap.
>  All the entropy ends with choosing mmap_base_addr at the process
>  creation. After that mmap build very predictable layout of address
>  space. It allows to bypass ASLR in many cases. This patch make
>  randomization of address on any mmap call.
> >>> 
> >>> Why should this be done in the kernel rather than libc?  libc is perfectly
> >>> capable of specifying random numbers in the first argument of mmap.
> >> Well, there is following reasons:
> >> 1. It should be done in any libc implementation, what is not possible IMO;
> > 
> > Is this really so helpful?
> 
> Yes, ASLR is one of very important mitigation techniques which are really 
> used 
> to protect applications. If there is no ASLR, it is very easy to exploit 
> vulnerable application and compromise the system. We can’t just fix all the 
> vulnerabilities right now, thats why we have mitigations - techniques which 
> are 
> makes exploitation more hard or impossible in some cases.
> 
> Thats why it is helpful.

I am not questioning ASLR in general. I am asking whether we really need
per mmap ASLR in general. I can imagine that some environments want to
pay the additional price and other side effects, but considering this
can be achieved by libc, why to add more code to the kernel?

> > 
> >> 2. User mode is not that layer which should be responsible for choosing
> >> random address or handling entropy;
> > 
> > Why?
> 
> Because of the following reasons:
> 1. To get random address you should have entropy. These entropy shouldn’t be 
> exposed to attacker anyhow, the best case is to get it from kernel. So this is
> a syscall.

/dev/[u]random is not sufficient?

> 2. You should have memory map of your process to prevent remapping or big
> fragmentation. Kernel already has this map.

/proc/self/maps?

> You will got another one in libc.
> And any non-libc user of mmap (via syscall, etc) will make hole in your map.
> This one also decrease performance cause you any way call syscall_mmap 
> which will try to find some address for you in worst case, but after you 
> already
> did some computing on it.

I do not understand. a) you should be prepared to pay an additional
price for an additional security measures and b) how would anybody punch
a hole into your mapping? 

> 3. The more memory you use in userland for these proposal, the easier for
> attacker to leak it or use in exploitation techniques.

This is true in general, isn't it? I fail to see how kernel chosen and
user chosen ranges would make any difference.

> 4. It is so easy to fix Kernel function and so hard to support memory
> management from userspace.

Well, on the other hand the new layout mode will add a maintenance
burden on the kernel and will have to be maintained for ever because it
is a user visible ABI.

> >> 3. Memory fragmentation is unpredictable in this case
> >> 
> >> Off course user mode could use random ‘hint’ address, but kernel may
> >> discard this address if it is occupied for example and allocate just before
> >> closest vma. So this solution doesn’t give that much security like 
> >> randomization address inside kernel.
> > 
> > The userspace can use the new MAP_FIXED_NOREPLACE to probe for the
> > address range atomically and chose a different range on failure.
> > 
> 
> This algorithm should track current memory. If he doesn’t he may cause
> infinite loop while trying to choose memory. And each iteration increase time
> needed on allocation new memory, what is not preferred by any libc library
> developer.

Well, I am pretty sure userspace can implement proper free ranges
tracking...

-- 
Michal Hocko
SUSE Labs

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Rich Felker]

On Fri, Mar 23, 2018 at 12:29:52PM -0700, Matthew Wilcox wrote:
> On Fri, Mar 23, 2018 at 03:16:21PM -0400, Rich Felker wrote:
> > > Huh, I thought libc was aware of this.  Also, I'd expect a libc-based
> > > implementation to restrict itself to, eg, only loading libraries in
> > > the bottom 1GB to avoid applications who want to map huge things from
> > > running out of unfragmented address space.
> > 
> > That seems like a rather arbitrary expectation and I'm not sure why
> > you'd expect it to result in less fragmentation rather than more. For
> > example if it started from 1GB and worked down, you'd immediately
> > reduce the contiguous free space from ~3GB to ~2GB, and if it started
> > from the bottom and worked up, brk would immediately become
> > unavailable, increasing mmap pressure elsewhere.
> 
> By *not* limiting yourself to the bottom 1GB, you'll almost immediately
> fragment the address space even worse.  Just looking at 'ls' as a
> hopefully-good example of a typical app, it maps:
> 
>   linux-vdso.so.1 (0x7ffef5eef000)
>   libselinux.so.1 => /lib/x86_64-linux-gnu/libselinux.so.1 
> (0x7fb3657f5000)
>   libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x7fb36543b000)
>   libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x7fb3651c9000)
>   libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x7fb364fc5000)
>   /lib64/ld-linux-x86-64.so.2 (0x7fb365c3f000)
>   libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 
> (0x7fb364da7000)
> 
> The VDSO wouldn't move, but look at the distribution of mapping 6 things
> into a 3GB address space in random locations.  What are the odds you have
> a contiguous 1GB chunk of address space?  If you restrict yourself to the
> bottom 1GB before running out of room and falling back to a sequential
> allocation, you'll prevent a lot of fragmentation.

Oh, you're talking about "with random locations" case. Randomizing
each map just hopelessly fragments things no matter what you do on
32-bit. If you reduce the space over which you randomize to the point
where it's not fragmenting/killing your available vm space, there are
so few degrees of freedom left that it's trivial to brute-force. Maybe
"libs randomized in low 1GB, everything else near-sequential in high
addresses" works half decently, but I have a hard time believing you
can get any ASLR that's significantly better than snake oil in a
32-bit address space, and you certainly do pay a high price in total
available vm space.

Rich

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Matthew Wilcox]

On Fri, Mar 23, 2018 at 02:00:24PM -0400, Rich Felker wrote:
> On Fri, Mar 23, 2018 at 05:48:06AM -0700, Matthew Wilcox wrote:
> > On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
> > > Current implementation doesn't randomize address returned by mmap.
> > > All the entropy ends with choosing mmap_base_addr at the process
> > > creation. After that mmap build very predictable layout of address
> > > space. It allows to bypass ASLR in many cases. This patch make
> > > randomization of address on any mmap call.
> > 
> > Why should this be done in the kernel rather than libc?  libc is perfectly
> > capable of specifying random numbers in the first argument of mmap.
> 
> Generally libc does not have a view of the current vm maps, and thus
> in passing "random numbers", they would have to be uniform across the
> whole vm space and thus non-uniform once the kernel rounds up to avoid
> existing mappings.

I'm aware that you're the musl author, but glibc somehow manages to
provide etext, edata and end, demonstrating that it does know where at
least some of the memory map lies.  Virtually everything after that is
brought into the address space via mmap, which at least glibc intercepts,
so it's entirely possible for a security-conscious libc to know where
other things are in the memory map.  Not to mention that what we're
primarily talking about here are libraries which are dynamically linked
and are loaded by ld.so before calling main(); not dlopen() or even
regular user mmaps.

> Also this would impose requirements that libc be
> aware of the kernel's use of the virtual address space and what's
> available to userspace -- for example, on 32-bit archs whether 2GB,
> 3GB, or full 4GB (for 32-bit-user-on-64-bit-kernel) is available, and
> on 64-bit archs where fewer than the full 64 bits are actually valid
> in addresses, what the actual usable pointer size is. There is
> currently no clean way of conveying this information to userspace.

Huh, I thought libc was aware of this.  Also, I'd expect a libc-based
implementation to restrict itself to, eg, only loading libraries in
the bottom 1GB to avoid applications who want to map huge things from
running out of unfragmented address space.

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Rich Felker]

On Fri, Mar 23, 2018 at 05:48:06AM -0700, Matthew Wilcox wrote:
> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
> > Current implementation doesn't randomize address returned by mmap.
> > All the entropy ends with choosing mmap_base_addr at the process
> > creation. After that mmap build very predictable layout of address
> > space. It allows to bypass ASLR in many cases. This patch make
> > randomization of address on any mmap call.
> 
> Why should this be done in the kernel rather than libc?  libc is perfectly
> capable of specifying random numbers in the first argument of mmap.

Generally libc does not have a view of the current vm maps, and thus
in passing "random numbers", they would have to be uniform across the
whole vm space and thus non-uniform once the kernel rounds up to avoid
existing mappings. Also this would impose requirements that libc be
aware of the kernel's use of the virtual address space and what's
available to userspace -- for example, on 32-bit archs whether 2GB,
3GB, or full 4GB (for 32-bit-user-on-64-bit-kernel) is available, and
on 64-bit archs where fewer than the full 64 bits are actually valid
in addresses, what the actual usable pointer size is. There is
currently no clean way of conveying this information to userspace.

Rich

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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

> On 23 Mar 2018, at 15:48, Matthew Wilcox  wrote:
> 
> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
>> Current implementation doesn't randomize address returned by mmap.
>> All the entropy ends with choosing mmap_base_addr at the process
>> creation. After that mmap build very predictable layout of address
>> space. It allows to bypass ASLR in many cases. This patch make
>> randomization of address on any mmap call.
> 
> Why should this be done in the kernel rather than libc?  libc is perfectly
> capable of specifying random numbers in the first argument of mmap.
Well, there is following reasons:
1. It should be done in any libc implementation, what is not possible IMO;
2. User mode is not that layer which should be responsible for choosing
random address or handling entropy;
3. Memory fragmentation is unpredictable in this case

Off course user mode could use random ‘hint’ address, but kernel may
discard this address if it is occupied for example and allocate just before
closest vma. So this solution doesn’t give that much security like 
randomization address inside kernel.
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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

Hello, Andrew

Thanks for reading this patch.

> On 22 Mar 2018, at 23:57, Andrew Morton  wrote:
> 
> On Thu, 22 Mar 2018 19:36:36 +0300 Ilya Smith  wrote:
> 
>> Current implementation doesn't randomize address returned by mmap.
>> All the entropy ends with choosing mmap_base_addr at the process
>> creation. After that mmap build very predictable layout of address
>> space. It allows to bypass ASLR in many cases.
> 
> Perhaps some more effort on the problem description would help.  *Are*
> people predicting layouts at present?  What problems does this cause? 
> How are they doing this and are there other approaches to solving the
> problem?
> 
Sorry, I’ve lost it in first version. In short - memory layout could be easily 
repaired by single leakage. Also any Out of Bounds error may easily be 
exploited according to current implementation. All because mmap choose address 
just before previously allocated segment. You can read more about it here: 
http://www.openwall.com/lists/oss-security/2018/02/27/5
Some test are available here https://github.com/blackzert/aslur. 
To solve the problem Kernel should randomize address on any mmap so
attacker could never easily gain needed addresses.

> Mainly: what value does this patchset have to our users?  This reader
> is unable to determine that from the information which you have
> provided.  Full details, please.

The value of this patch is to decrease successful rate of exploitation
vulnerable applications.These could be either remote or local vectors.


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Re: [RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Matthew Wilcox]

On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote:
> Current implementation doesn't randomize address returned by mmap.
> All the entropy ends with choosing mmap_base_addr at the process
> creation. After that mmap build very predictable layout of address
> space. It allows to bypass ASLR in many cases. This patch make
> randomization of address on any mmap call.

Why should this be done in the kernel rather than libc?  libc is perfectly
capable of specifying random numbers in the first argument of mmap.

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[RFC PATCH v2 0/2] Randomization of address chosen by mmap.

[Ilya Smith]

Current implementation doesn't randomize address returned by mmap.
All the entropy ends with choosing mmap_base_addr at the process
creation. After that mmap build very predictable layout of address
space. It allows to bypass ASLR in many cases. This patch make
randomization of address on any mmap call.

---
v2: Changed the way how gap was chosen. Now we don't get all possible
gaps. Random address generated and used as a tree walking direction.
Tree walked with backtracking till suitable gap will be found.
When the gap was found, address randomly shifted from next vma start.

The vm_unmapped_area_info structure was extended with new field random_shift
what might be used to set arch-depended limit on shift to next vma start.
In case of x86-64 architecture this shift is 256 pages for 32 bit applications
and 0x100 pages for 64 bit.

To get the entropy pseudo-random is used. This is because on Intel x86-64
processors instruction RDRAND works very slow if buffer is consumed -
after about 1 iterations.

This feature could be enabled by setting randomize_va_space with 4.

---
Performance:
After applying this patch single mmap took about 7% longer according to
following test:

before = rdtsc();
addr = mmap(0, SIZE, PROT_READ | PROT_WRITE, 
 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
after = rdtsc();
diff = after - before;
munmap(addr, SIZE)
...
unsigned long long total = 0;
for(int i = 0; i < count; ++i) {
total += one_iteration();
}
printf("%lld\n", total);

Time is consumed by div instruction in computation of the address.

make kernel:
echo 2 > /proc/sys/kernel/randomize_va_space 
make mrproper && make defconfig && time make 
real11m9.925s
user10m17.829s
sys 1m4.969s

echo 4 > /proc/sys/kernel/randomize_va_space 
make mrproper && make defconfig && time make 
real11m12.806s
user10m18.305s
sys 1m4.281s


Ilya Smith (2):
  Randomization of address chosen by mmap.
  Architecture defined limit on memory region random shift.

 arch/alpha/kernel/osf_sys.c |   1 +
 arch/arc/mm/mmap.c  |   1 +
 arch/arm/mm/mmap.c  |   2 +
 arch/frv/mm/elf-fdpic.c |   1 +
 arch/ia64/kernel/sys_ia64.c |   1 +
 arch/ia64/mm/hugetlbpage.c  |   1 +
 arch/metag/mm/hugetlbpage.c |   1 +
 arch/mips/mm/mmap.c |   1 +
 arch/parisc/kernel/sys_parisc.c |   2 +
 arch/powerpc/mm/hugetlbpage-radix.c |   1 +
 arch/powerpc/mm/mmap.c  |   2 +
 arch/powerpc/mm/slice.c |   2 +
 arch/s390/mm/mmap.c |   2 +
 arch/sh/mm/mmap.c   |   2 +
 arch/sparc/kernel/sys_sparc_32.c|   1 +
 arch/sparc/kernel/sys_sparc_64.c|   2 +
 arch/sparc/mm/hugetlbpage.c |   2 +
 arch/tile/mm/hugetlbpage.c  |   2 +
 arch/x86/kernel/sys_x86_64.c|   4 +
 arch/x86/mm/hugetlbpage.c   |   4 +
 fs/hugetlbfs/inode.c|   1 +
 include/linux/mm.h  |  17 ++--
 mm/mmap.c   | 165 
 23 files changed, 213 insertions(+), 5 deletions(-)

-- 
2.7.4


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