Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Wed, Dec 9, 2015 at 12:26 AM, Jan Kara  wrote:
> On Mon 07-12-15 16:40:14, Kees Cook wrote:
>> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
>> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  
>> > wrote:
>> >>
>> >>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>> >>>
>> >>> Normally, when a user can modify a file that has setuid or setgid bits,
>> >>> those bits are cleared when they are not the file owner or a member
>> >>> of the group. This is enforced when using write and truncate but not
>> >>> when writing to a shared mmap on the file. This could allow the file
>> >>> writer to gain privileges by changing a binary without losing the
>> >>> setuid/setgid/caps bits.
>> >>>
>> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> >>> during the page fault (due to mmap_sem being held during the fault).
>> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>> >>>
>> >>> Signed-off-by: Kees Cook 
>> >>> Cc: sta...@vger.kernel.org
>> >>> —
>> >>
>> >> is this means mprotect() sys call also need add this check?
>> >> mprotect() can change to PROT_WRITE, then it can write to a
>> >> read only map again , also a secure hole here .
>> >
>> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
>>
>> This continues to look worse and worse.
>>
>> So... to check this at mprotect time, I have to know it's MAP_SHARED,
>> but that's in the vma_flags, which I can only see after holding
>> mmap_sem.
>>
>> The best I can think of now is to strip the bits at munmap time, since
>> you can't execute an mmapped file until it closes.
>>
>> Jan, thoughts on this?
>
> Umm, so we actually refuse to execute a file while someone has it open for
> writing (deny_write_access() in do_open_execat()). So dropping the suid /
> sgid bits when closing file for writing could be plausible. Grabbing
> i_mutex from __fput() context is safe (it gets called from task_work
> context when returning to userspace).
>
> That way we could actually remove the checks done for each write. To avoid
> unexpected removal of suid/sgid bits when someone just opens & closes the
> file, we could mark the file as needing suid/sgid treatment by a flag in
> inode->i_flags when file gets written to or mmaped and then check for this
> in __fput().

Yeah, this is ultimately where I ended up for the v4 (and fixed up in
v5). I added the flag to file, though, not inode. Sending v5 now...

-Kees

>
> I've added Al Viro to CC just in case he is aware of some issues with
> this...
>
> Honza
> --
> Jan Kara 
> SUSE Labs, CR



-- 
Kees Cook
Chrome OS & Brillo Security
--
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Re: [PATCH v2] clear file privilege bits when mmap writing

[Jan Kara]

On Mon 07-12-15 16:40:14, Kees Cook wrote:
> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
> >>
> >>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
> >>>
> >>> Normally, when a user can modify a file that has setuid or setgid bits,
> >>> those bits are cleared when they are not the file owner or a member
> >>> of the group. This is enforced when using write and truncate but not
> >>> when writing to a shared mmap on the file. This could allow the file
> >>> writer to gain privileges by changing a binary without losing the
> >>> setuid/setgid/caps bits.
> >>>
> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
> >>> during the page fault (due to mmap_sem being held during the fault).
> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> >>>
> >>> Signed-off-by: Kees Cook 
> >>> Cc: sta...@vger.kernel.org
> >>> —
> >>
> >> is this means mprotect() sys call also need add this check?
> >> mprotect() can change to PROT_WRITE, then it can write to a
> >> read only map again , also a secure hole here .
> >
> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
> 
> This continues to look worse and worse.
> 
> So... to check this at mprotect time, I have to know it's MAP_SHARED,
> but that's in the vma_flags, which I can only see after holding
> mmap_sem.
> 
> The best I can think of now is to strip the bits at munmap time, since
> you can't execute an mmapped file until it closes.
> 
> Jan, thoughts on this?

Umm, so we actually refuse to execute a file while someone has it open for
writing (deny_write_access() in do_open_execat()). So dropping the suid /
sgid bits when closing file for writing could be plausible. Grabbing
i_mutex from __fput() context is safe (it gets called from task_work
context when returning to userspace).

That way we could actually remove the checks done for each write. To avoid
unexpected removal of suid/sgid bits when someone just opens & closes the
file, we could mark the file as needing suid/sgid treatment by a flag in
inode->i_flags when file gets written to or mmaped and then check for this
in __fput().

I've added Al Viro to CC just in case he is aware of some issues with
this...

Honza
-- 
Jan Kara 
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Jan Kara]

On Mon 07-12-15 16:40:14, Kees Cook wrote:
> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
> >>
> >>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
> >>>
> >>> Normally, when a user can modify a file that has setuid or setgid bits,
> >>> those bits are cleared when they are not the file owner or a member
> >>> of the group. This is enforced when using write and truncate but not
> >>> when writing to a shared mmap on the file. This could allow the file
> >>> writer to gain privileges by changing a binary without losing the
> >>> setuid/setgid/caps bits.
> >>>
> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
> >>> during the page fault (due to mmap_sem being held during the fault).
> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> >>>
> >>> Signed-off-by: Kees Cook 
> >>> Cc: sta...@vger.kernel.org
> >>> —
> >>
> >> is this means mprotect() sys call also need add this check?
> >> mprotect() can change to PROT_WRITE, then it can write to a
> >> read only map again , also a secure hole here .
> >
> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
> 
> This continues to look worse and worse.
> 
> So... to check this at mprotect time, I have to know it's MAP_SHARED,
> but that's in the vma_flags, which I can only see after holding
> mmap_sem.
> 
> The best I can think of now is to strip the bits at munmap time, since
> you can't execute an mmapped file until it closes.
> 
> Jan, thoughts on this?

Umm, so we actually refuse to execute a file while someone has it open for
writing (deny_write_access() in do_open_execat()). So dropping the suid /
sgid bits when closing file for writing could be plausible. Grabbing
i_mutex from __fput() context is safe (it gets called from task_work
context when returning to userspace).

That way we could actually remove the checks done for each write. To avoid
unexpected removal of suid/sgid bits when someone just opens & closes the
file, we could mark the file as needing suid/sgid treatment by a flag in
inode->i_flags when file gets written to or mmaped and then check for this
in __fput().

I've added Al Viro to CC just in case he is aware of some issues with
this...

Honza
-- 
Jan Kara 
SUSE Labs, CR
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Wed, Dec 9, 2015 at 12:26 AM, Jan Kara  wrote:
> On Mon 07-12-15 16:40:14, Kees Cook wrote:
>> On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
>> > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  
>> > wrote:
>> >>
>> >>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>> >>>
>> >>> Normally, when a user can modify a file that has setuid or setgid bits,
>> >>> those bits are cleared when they are not the file owner or a member
>> >>> of the group. This is enforced when using write and truncate but not
>> >>> when writing to a shared mmap on the file. This could allow the file
>> >>> writer to gain privileges by changing a binary without losing the
>> >>> setuid/setgid/caps bits.
>> >>>
>> >>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> >>> during the page fault (due to mmap_sem being held during the fault).
>> >>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>> >>>
>> >>> Signed-off-by: Kees Cook 
>> >>> Cc: sta...@vger.kernel.org
>> >>> —
>> >>
>> >> is this means mprotect() sys call also need add this check?
>> >> mprotect() can change to PROT_WRITE, then it can write to a
>> >> read only map again , also a secure hole here .
>> >
>> > Yes, good point. This needs to be added. I will send a new patch. Thanks!
>>
>> This continues to look worse and worse.
>>
>> So... to check this at mprotect time, I have to know it's MAP_SHARED,
>> but that's in the vma_flags, which I can only see after holding
>> mmap_sem.
>>
>> The best I can think of now is to strip the bits at munmap time, since
>> you can't execute an mmapped file until it closes.
>>
>> Jan, thoughts on this?
>
> Umm, so we actually refuse to execute a file while someone has it open for
> writing (deny_write_access() in do_open_execat()). So dropping the suid /
> sgid bits when closing file for writing could be plausible. Grabbing
> i_mutex from __fput() context is safe (it gets called from task_work
> context when returning to userspace).
>
> That way we could actually remove the checks done for each write. To avoid
> unexpected removal of suid/sgid bits when someone just opens & closes the
> file, we could mark the file as needing suid/sgid treatment by a flag in
> inode->i_flags when file gets written to or mmaped and then check for this
> in __fput().

Yeah, this is ultimately where I ended up for the v4 (and fixed up in
v5). I added the flag to file, though, not inode. Sending v5 now...

-Kees

>
> I've added Al Viro to CC just in case he is aware of some issues with
> this...
>
> Honza
> --
> Jan Kara 
> SUSE Labs, CR



-- 
Kees Cook
Chrome OS & Brillo Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
> On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
>>
>>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>>>
>>> Normally, when a user can modify a file that has setuid or setgid bits,
>>> those bits are cleared when they are not the file owner or a member
>>> of the group. This is enforced when using write and truncate but not
>>> when writing to a shared mmap on the file. This could allow the file
>>> writer to gain privileges by changing a binary without losing the
>>> setuid/setgid/caps bits.
>>>
>>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>>> during the page fault (due to mmap_sem being held during the fault).
>>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>>
>>> Signed-off-by: Kees Cook 
>>> Cc: sta...@vger.kernel.org
>>> —
>>
>> is this means mprotect() sys call also need add this check?
>> mprotect() can change to PROT_WRITE, then it can write to a
>> read only map again , also a secure hole here .
>
> Yes, good point. This needs to be added. I will send a new patch. Thanks!

This continues to look worse and worse.

So... to check this at mprotect time, I have to know it's MAP_SHARED,
but that's in the vma_flags, which I can only see after holding
mmap_sem.

The best I can think of now is to strip the bits at munmap time, since
you can't execute an mmapped file until it closes.

Jan, thoughts on this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
>
>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> Signed-off-by: Kees Cook 
>> Cc: sta...@vger.kernel.org
>> —
>
> is this means mprotect() sys call also need add this check?
> mprotect() can change to PROT_WRITE, then it can write to a
> read only map again , also a secure hole here .

Yes, good point. This needs to be added. I will send a new patch. Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
>
>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>>
>> Normally, when a user can modify a file that has setuid or setgid bits,
>> those bits are cleared when they are not the file owner or a member
>> of the group. This is enforced when using write and truncate but not
>> when writing to a shared mmap on the file. This could allow the file
>> writer to gain privileges by changing a binary without losing the
>> setuid/setgid/caps bits.
>>
>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>> during the page fault (due to mmap_sem being held during the fault).
>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>
>> Signed-off-by: Kees Cook 
>> Cc: sta...@vger.kernel.org
>> —
>
> is this means mprotect() sys call also need add this check?
> mprotect() can change to PROT_WRITE, then it can write to a
> read only map again , also a secure hole here .

Yes, good point. This needs to be added. I will send a new patch. Thanks!

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[Kees Cook]

On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook  wrote:
> On Thu, Dec 3, 2015 at 5:45 PM, yalin wang  wrote:
>>
>>> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
>>>
>>> Normally, when a user can modify a file that has setuid or setgid bits,
>>> those bits are cleared when they are not the file owner or a member
>>> of the group. This is enforced when using write and truncate but not
>>> when writing to a shared mmap on the file. This could allow the file
>>> writer to gain privileges by changing a binary without losing the
>>> setuid/setgid/caps bits.
>>>
>>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>>> during the page fault (due to mmap_sem being held during the fault).
>>> Instead, clear the bits if PROT_WRITE is being used at mmap time.
>>>
>>> Signed-off-by: Kees Cook 
>>> Cc: sta...@vger.kernel.org
>>> —
>>
>> is this means mprotect() sys call also need add this check?
>> mprotect() can change to PROT_WRITE, then it can write to a
>> read only map again , also a secure hole here .
>
> Yes, good point. This needs to be added. I will send a new patch. Thanks!

This continues to look worse and worse.

So... to check this at mprotect time, I have to know it's MAP_SHARED,
but that's in the vma_flags, which I can only see after holding
mmap_sem.

The best I can think of now is to strip the bits at munmap time, since
you can't execute an mmapped file until it closes.

Jan, thoughts on this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[yalin wang]

> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
> 
> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member
> of the group. This is enforced when using write and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault).
> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> 
> Signed-off-by: Kees Cook 
> Cc: sta...@vger.kernel.org
> —

is this means mprotect() sys call also need add this check?
mprotect() can change to PROT_WRITE, then it can write to a 
read only map again , also a secure hole here .

Thanks

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH v2] clear file privilege bits when mmap writing

[yalin wang]

> On Dec 2, 2015, at 16:03, Kees Cook  wrote:
> 
> Normally, when a user can modify a file that has setuid or setgid bits,
> those bits are cleared when they are not the file owner or a member
> of the group. This is enforced when using write and truncate but not
> when writing to a shared mmap on the file. This could allow the file
> writer to gain privileges by changing a binary without losing the
> setuid/setgid/caps bits.
> 
> Changing the bits requires holding inode->i_mutex, so it cannot be done
> during the page fault (due to mmap_sem being held during the fault).
> Instead, clear the bits if PROT_WRITE is being used at mmap time.
> 
> Signed-off-by: Kees Cook 
> Cc: sta...@vger.kernel.org
> —

is this means mprotect() sys call also need add this check?
mprotect() can change to PROT_WRITE, then it can write to a 
read only map again , also a secure hole here .

Thanks

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/