Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Eric W. Biederman]

Mimi Zohar  writes:

> On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote:
>> Casey Schaufler  writes:
>> 
>> > On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
>> >> Mimi Zohar  writes:
>> >>
>> >>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
>>  Mimi Zohar  writes:
>> 
>> > Allow LSMs and IMA to differentiate between the kexec_load and
>> > kexec_file_load syscalls by adding an "unnecessary" call to
>> > security_kernel_read_file() in kexec_load.  This would be similar to 
>> > the
>> > existing init_module syscall calling security_kernel_read_file().
>>  Given the reasonable desire to load a policy that ensures everything
>>  has a signature I don't have fundamental objections.
>> 
>>  security_kernel_read_file as a hook seems an odd choice.  At the very
>>  least it has a bad name because there is no file reading going on here.
>> 
>>  I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
>>  anywhere.  Which means I could have a kernel compiled without that and I
>>  would be allowed to use kexec_file_load without signature checking.
>>  While kexec_load would be denied.
>> 
>>  Am I missing something here?
>> >>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
>> >>> calls security_kernel_read_file().  So kexec_file_load and kexec_load
>> >>> syscall would be using the same method for enforcing signature
>> >>> verification.
>> >> Having looked at your patches and the kernel a little more I think
>> >> this should be a separate security hook that does not take a file
>> >> parameter.
>> >>
>> >> Right now every other security module assumes !file is init_module.
>> >> So I think this change has the potential to confuse other security
>> >> modules, with the result of unintended policy being applied.
>> >>
>> >> So just for good security module hygeine I think this needs a dedicated
>> >> kexec_load security hook.
>> >
>> > I would rather see the existing modules updated than a new
>> > hook added. Too many hooks spoil the broth. Two hooks with
>> > trivial differences just add to the clutter and make it harder
>> > for non-lsm developers to figure out what to use in their
>> > code.
>> 
>> These are not non-trivial differences.  There is absolutely nothing
>> file related about kexec_load.  Nor for init_module for that matter.
>> 
>> If something is called security_kernel_read_file I think it is wholly
>> appropriate for code that processes such a hook to assume file is
>> non-NULL.
>> 
>> When you have to dance a jig (which is what I see the security modules
>> doing) to figure out who is calling a lsm hook for what purpose I think
>> it is a maintenance problem waiting to happen and that the hook is badly
>> designed.
>> 
>> At this point I don't care what the lsm's do with the hooks but the
>> hooks need to make sense for people outside of the lsm's and something
>> about reading a file in a syscall that doesn't read files is complete
>> and utter nonsense.
>
> Sure, we can define a wrapper around the security_kernel_read_file()
> hook, calling it security_non-fd_syscall() or even
> security_old_syscall().

I really don't see why you want to use the same hook.

I just read through the code of all three users.  None of them.
Especially IMA shares any significant code between the !file case and
the file case.

Eric


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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Mimi Zohar]

On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote:
> Casey Schaufler  writes:
> 
> > On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
> >> Mimi Zohar  writes:
> >>
> >>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
>  Mimi Zohar  writes:
> 
> > Allow LSMs and IMA to differentiate between the kexec_load and
> > kexec_file_load syscalls by adding an "unnecessary" call to
> > security_kernel_read_file() in kexec_load.  This would be similar to the
> > existing init_module syscall calling security_kernel_read_file().
>  Given the reasonable desire to load a policy that ensures everything
>  has a signature I don't have fundamental objections.
> 
>  security_kernel_read_file as a hook seems an odd choice.  At the very
>  least it has a bad name because there is no file reading going on here.
> 
>  I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
>  anywhere.  Which means I could have a kernel compiled without that and I
>  would be allowed to use kexec_file_load without signature checking.
>  While kexec_load would be denied.
> 
>  Am I missing something here?
> >>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
> >>> calls security_kernel_read_file().  So kexec_file_load and kexec_load
> >>> syscall would be using the same method for enforcing signature
> >>> verification.
> >> Having looked at your patches and the kernel a little more I think
> >> this should be a separate security hook that does not take a file
> >> parameter.
> >>
> >> Right now every other security module assumes !file is init_module.
> >> So I think this change has the potential to confuse other security
> >> modules, with the result of unintended policy being applied.
> >>
> >> So just for good security module hygeine I think this needs a dedicated
> >> kexec_load security hook.
> >
> > I would rather see the existing modules updated than a new
> > hook added. Too many hooks spoil the broth. Two hooks with
> > trivial differences just add to the clutter and make it harder
> > for non-lsm developers to figure out what to use in their
> > code.
> 
> These are not non-trivial differences.  There is absolutely nothing
> file related about kexec_load.  Nor for init_module for that matter.
> 
> If something is called security_kernel_read_file I think it is wholly
> appropriate for code that processes such a hook to assume file is
> non-NULL.
> 
> When you have to dance a jig (which is what I see the security modules
> doing) to figure out who is calling a lsm hook for what purpose I think
> it is a maintenance problem waiting to happen and that the hook is badly
> designed.
> 
> At this point I don't care what the lsm's do with the hooks but the
> hooks need to make sense for people outside of the lsm's and something
> about reading a file in a syscall that doesn't read files is complete
> and utter nonsense.

Sure, we can define a wrapper around the security_kernel_read_file()
hook, calling it security_non-fd_syscall() or even
security_old_syscall().

Mimi


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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Eric W. Biederman]

Casey Schaufler  writes:

> On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
>> Mimi Zohar  writes:
>>
>>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
 Mimi Zohar  writes:

> Allow LSMs and IMA to differentiate between the kexec_load and
> kexec_file_load syscalls by adding an "unnecessary" call to
> security_kernel_read_file() in kexec_load.  This would be similar to the
> existing init_module syscall calling security_kernel_read_file().
 Given the reasonable desire to load a policy that ensures everything
 has a signature I don't have fundamental objections.

 security_kernel_read_file as a hook seems an odd choice.  At the very
 least it has a bad name because there is no file reading going on here.

 I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
 anywhere.  Which means I could have a kernel compiled without that and I
 would be allowed to use kexec_file_load without signature checking.
 While kexec_load would be denied.

 Am I missing something here?
>>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
>>> calls security_kernel_read_file().  So kexec_file_load and kexec_load
>>> syscall would be using the same method for enforcing signature
>>> verification.
>> Having looked at your patches and the kernel a little more I think
>> this should be a separate security hook that does not take a file
>> parameter.
>>
>> Right now every other security module assumes !file is init_module.
>> So I think this change has the potential to confuse other security
>> modules, with the result of unintended policy being applied.
>>
>> So just for good security module hygeine I think this needs a dedicated
>> kexec_load security hook.
>
> I would rather see the existing modules updated than a new
> hook added. Too many hooks spoil the broth. Two hooks with
> trivial differences just add to the clutter and make it harder
> for non-lsm developers to figure out what to use in their
> code.

These are not non-trivial differences.  There is absolutely nothing
file related about kexec_load.  Nor for init_module for that matter.

If something is called security_kernel_read_file I think it is wholly
appropriate for code that processes such a hook to assume file is
non-NULL.

When you have to dance a jig (which is what I see the security modules
doing) to figure out who is calling a lsm hook for what purpose I think
it is a maintenance problem waiting to happen and that the hook is badly
designed.

At this point I don't care what the lsm's do with the hooks but the
hooks need to make sense for people outside of the lsm's and something
about reading a file in a syscall that doesn't read files is complete
and utter nonsense.

Eric



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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Casey Schaufler]

On 5/3/2018 8:51 AM, Eric W. Biederman wrote:
> Mimi Zohar  writes:
>
>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
>>> Mimi Zohar  writes:
>>>
 Allow LSMs and IMA to differentiate between the kexec_load and
 kexec_file_load syscalls by adding an "unnecessary" call to
 security_kernel_read_file() in kexec_load.  This would be similar to the
 existing init_module syscall calling security_kernel_read_file().
>>> Given the reasonable desire to load a policy that ensures everything
>>> has a signature I don't have fundamental objections.
>>>
>>> security_kernel_read_file as a hook seems an odd choice.  At the very
>>> least it has a bad name because there is no file reading going on here.
>>>
>>> I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
>>> anywhere.  Which means I could have a kernel compiled without that and I
>>> would be allowed to use kexec_file_load without signature checking.
>>> While kexec_load would be denied.
>>>
>>> Am I missing something here?
>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
>> calls security_kernel_read_file().  So kexec_file_load and kexec_load
>> syscall would be using the same method for enforcing signature
>> verification.
> Having looked at your patches and the kernel a little more I think
> this should be a separate security hook that does not take a file
> parameter.
>
> Right now every other security module assumes !file is init_module.
> So I think this change has the potential to confuse other security
> modules, with the result of unintended policy being applied.
>
> So just for good security module hygeine I think this needs a dedicated
> kexec_load security hook.

I would rather see the existing modules updated than a new
hook added. Too many hooks spoil the broth. Two hooks with
trivial differences just add to the clutter and make it harder
for non-lsm developers to figure out what to use in their
code. 


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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Eric W. Biederman]

Mimi Zohar  writes:

> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
>> Mimi Zohar  writes:
>> 
>> > Allow LSMs and IMA to differentiate between the kexec_load and
>> > kexec_file_load syscalls by adding an "unnecessary" call to
>> > security_kernel_read_file() in kexec_load.  This would be similar to the
>> > existing init_module syscall calling security_kernel_read_file().
>> 
>> Given the reasonable desire to load a policy that ensures everything
>> has a signature I don't have fundamental objections.
>> 
>> security_kernel_read_file as a hook seems an odd choice.  At the very
>> least it has a bad name because there is no file reading going on here.
>> 
>> I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
>> anywhere.  Which means I could have a kernel compiled without that and I
>> would be allowed to use kexec_file_load without signature checking.
>> While kexec_load would be denied.
>> 
>> Am I missing something here?
>
> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
> calls security_kernel_read_file().  So kexec_file_load and kexec_load
> syscall would be using the same method for enforcing signature
> verification.

Having looked at your patches and the kernel a little more I think
this should be a separate security hook that does not take a file
parameter.

Right now every other security module assumes !file is init_module.
So I think this change has the potential to confuse other security
modules, with the result of unintended policy being applied.

So just for good security module hygeine I think this needs a dedicated
kexec_load security hook.


> This is independent of the architecture specific method for verifying
> signatures.  The coordination between these two methods was included
> in the lockdown patch set, but is being removed, as well the gating of
> kexec_load syscall.  Instead of being based on the lockdown flag, I
> assume the coordination between the two methods will reappear based on
> a secure boot flag of some sort.

I was blind there for a moment.  Yes this is all about the ima xattrs
allowing a file to be loaded.

Eric


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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Mimi Zohar]

On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote:
> Mimi Zohar  writes:
> 
> > Allow LSMs and IMA to differentiate between the kexec_load and
> > kexec_file_load syscalls by adding an "unnecessary" call to
> > security_kernel_read_file() in kexec_load.  This would be similar to the
> > existing init_module syscall calling security_kernel_read_file().
> 
> Given the reasonable desire to load a policy that ensures everything
> has a signature I don't have fundamental objections.
> 
> security_kernel_read_file as a hook seems an odd choice.  At the very
> least it has a bad name because there is no file reading going on here.
> 
> I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
> anywhere.  Which means I could have a kernel compiled without that and I
> would be allowed to use kexec_file_load without signature checking.
> While kexec_load would be denied.
> 
> Am I missing something here?

The kexec_file_load() calls kernel_read_file_from_fd(), which in turn
calls security_kernel_read_file().  So kexec_file_load and kexec_load
syscall would be using the same method for enforcing signature
verification.

This is independent of the architecture specific method for verifying
signatures.  The coordination between these two methods was included
in the lockdown patch set, but is being removed, as well the gating of
kexec_load syscall.  Instead of being based on the lockdown flag, I
assume the coordination between the two methods will reappear based on
a secure boot flag of some sort.

Mimi

> 
> > Signed-off-by: Mimi Zohar 
> > ---
> >  kernel/kexec.c | 11 +++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index aed8fb2564b3..d1386cfc6796 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -11,6 +11,7 @@
> >  #include 
> >  #include 
> >  #include 
> > +#include 
> >  #include 
> >  #include 
> >  #include 
> > @@ -195,11 +196,21 @@ static int do_kexec_load(unsigned long entry, 
> > unsigned long nr_segments,
> >  static inline int kexec_load_check(unsigned long nr_segments,
> >unsigned long flags)
> >  {
> > +   int result;
> > +
> > /* We only trust the superuser with rebooting the system. */
> > if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
> > return -EPERM;
> >  
> > /*
> > +* Allow LSMs and IMA to differentiate between kexec_load and
> > +* kexec_file_load syscalls.
> > +*/
> > +   result = security_kernel_read_file(NULL, READING_KEXEC_IMAGE);
> > +   if (result < 0)
> > +   return result;
> > +
> > +   /*
> >  * Verify we have a legal set of flags
> >  * This leaves us room for future extensions.
> >  */
> 


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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Eric W. Biederman]

Mimi Zohar  writes:

> Allow LSMs and IMA to differentiate between the kexec_load and
> kexec_file_load syscalls by adding an "unnecessary" call to
> security_kernel_read_file() in kexec_load.  This would be similar to the
> existing init_module syscall calling security_kernel_read_file().

Given the reasonable desire to load a policy that ensures everything
has a signature I don't have fundamental objections.

security_kernel_read_file as a hook seems an odd choice.  At the very
least it has a bad name because there is no file reading going on here.

I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested
anywhere.  Which means I could have a kernel compiled without that and I
would be allowed to use kexec_file_load without signature checking.
While kexec_load would be denied.

Am I missing something here?

Eric



> Signed-off-by: Mimi Zohar 
> ---
>  kernel/kexec.c | 11 +++
>  1 file changed, 11 insertions(+)
>
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index aed8fb2564b3..d1386cfc6796 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -11,6 +11,7 @@
>  #include 
>  #include 
>  #include 
> +#include 
>  #include 
>  #include 
>  #include 
> @@ -195,11 +196,21 @@ static int do_kexec_load(unsigned long entry, unsigned 
> long nr_segments,
>  static inline int kexec_load_check(unsigned long nr_segments,
>  unsigned long flags)
>  {
> + int result;
> +
>   /* We only trust the superuser with rebooting the system. */
>   if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
>   return -EPERM;
>  
>   /*
> +  * Allow LSMs and IMA to differentiate between kexec_load and
> +  * kexec_file_load syscalls.
> +  */
> + result = security_kernel_read_file(NULL, READING_KEXEC_IMAGE);
> + if (result < 0)
> + return result;
> +
> + /*
>* Verify we have a legal set of flags
>* This leaves us room for future extensions.
>*/

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Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall

[Mimi Zohar]

Hi Eric,

I'd really appreciate your reviewing/ack'ing this patch.

thanks,

Mimi

On Thu, 2018-04-12 at 18:41 -0400, Mimi Zohar wrote:
> Allow LSMs and IMA to differentiate between the kexec_load and
> kexec_file_load syscalls by adding an "unnecessary" call to
> security_kernel_read_file() in kexec_load.  This would be similar to the
> existing init_module syscall calling security_kernel_read_file().
> 
> Signed-off-by: Mimi Zohar 
> ---
>  kernel/kexec.c | 11 +++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index aed8fb2564b3..d1386cfc6796 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -11,6 +11,7 @@
>  #include 
>  #include 
>  #include 
> +#include 
>  #include 
>  #include 
>  #include 
> @@ -195,11 +196,21 @@ static int do_kexec_load(unsigned long entry, unsigned 
> long nr_segments,
>  static inline int kexec_load_check(unsigned long nr_segments,
>  unsigned long flags)
>  {
> + int result;
> +
>   /* We only trust the superuser with rebooting the system. */
>   if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
>   return -EPERM;
> 
>   /*
> +  * Allow LSMs and IMA to differentiate between kexec_load and
> +  * kexec_file_load syscalls.
> +  */
> + result = security_kernel_read_file(NULL, READING_KEXEC_IMAGE);
> + if (result < 0)
> + return result;
> +
> + /*
>* Verify we have a legal set of flags
>* This leaves us room for future extensions.
>*/


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