Re: [RFC PATCH 0/2] arm64: optional paranoid __{get,put}_user checks

[Kees Cook]

On Wed, Nov 1, 2017 at 2:13 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
> On 11/01/2017 05:05 AM, Mark Rutland wrote:
>> On Tue, Oct 31, 2017 at 04:56:39PM -0700, Laura Abbott wrote:
>>> On 10/26/2017 02:09 AM, Mark Rutland wrote:
>>>> In Prague, Kees mentioned that it would be nice to have a mechanism to
>>>> catch bad __{get,put}_user uses, such as the recent CVE-2017-5123 [1,2]
>>>> issue with unsafe_put_user() in waitid().
>>>>
>>>> These patches allow an optional access_ok() check to be dropped in
>>>> arm64's __{get,put}_user() primitives. These will then BUG() if a bad
>>>> user pointer is passed (which should only happen in the absence of an
>>>> earlier access_ok() check).
>>
>>> Turning on the option fails as soon as we hit userspace. On my buildroot
>>> based environment I get the help text for ld.so (????) and then a message
>>> about attempting to kill init.
>>
>> Ouch. Thanks for the report, and sorry about this.
>>
>> The problem is that I evaluate the ptr argument twice in
>> __{get,put}_user(), and this may have side effects.
>>
>> e.g. when the ELF loader does things like:
>>
>>   __put_user((elf_addr_t)p, sp++)
>>
>> ... we increment sp twice, and write to the wrong user address, leaving
>> sp corrupt.
>>
>> I have an additional patch [1] to fix this, which is in my
>> arm64/access-ok branch [2].
>>
>> Thanks,
>> Mark.
>>
>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/access-ok&id=ebb7ff83eb53b8810395d5cf48712a4ae6d678543
>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/access-ok
>>
>
> Thanks, the updated patch works. I wrote an LKDTM test to verify
> the expected behavior (__{get,put}_user panic whereas {get,put}_user
> do not). You're welcome to add Tested-by or I can wait for v2.

Nice. :) Out of curiosity, can you check if this correctly BUG()s on a
waitid() call when the fixes are reverted?

96ca579a1ecc ("waitid(): Avoid unbalanced user_access_end() on
access_ok() error")
1c9fec470b81 ("waitid(): Add missing access_ok() checks")

-Kees

-- 
Kees Cook
Pixel Security