Re: [PATCH] lib/string: avoid reading beyond src buffer in strscpy

[Kees Cook]

On Fri, Dec 8, 2017 at 7:29 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Fri, Dec 8, 2017 at 4:29 PM, Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>> On 12/07/2017 09:26 PM, Kees Cook wrote:
>>> On Thu, Dec 7, 2017 at 3:33 AM, Eryu Guan <eguan@xxxxxxxxxx> wrote:
>>>> strscpy() tries to copy sizeof(unsigned long) bytes a time from src
>>>> to dest when possible, and stops the loop when 'max' is less than
>>>> sizeof(unsigned long). But it doesn't check if (src+res) goes beyond
>>>> src buffer and does out-of-bound access to the underlying memory.
>>>>
>>>> KASAN reported global-out-of-bound bug when reading seccomp
>>>> actions_logged file in procfs:
>>>>
>>>>   cat /proc/sys/kernel/seccomp/actions_logged
>>>>
>>>> Because seccomp_names_from_actions_logged() is copying short strings
>>>> (less than sizeof(unsigned long)) to buffer 'names'. e.g.
>>>>
>>>>   ret = strscpy(names, " ", size);
>>>
>>> This is a false positive:
>>> https://marc.info/?l=linux-kernel&m=150768944030805&w=2
>>>
>>> Given that we keep getting these reports (this is the third), I wonder
>>> if can adjust the seccomp code to work around the bug in KASAN...
>>>
>>
>> We should fix this in strscpy() somehow. We just need to decide how to do this.
>> Last time we haven't came to agreement.
>>
>> So, possible solutions are:
>>
>> 1) Simply disable word-at-a-time optimization in strscpy(). I seriously doubt
>> that this optimization have noticeable performance impact in real workloads.
>>
>> 2) Apply patch https://lkml.kernel.org/r/20170718171523.32208-1-aryabinin@xxxxxxxxxxxxx
>> It's a simple, minimally intrusive way to fix the bug.
>> However, the patch changes semantics/implementation of the strscpy() which is bad idea.
>> It basically means that we use one implementation of the strscpy() but test with KASAN another implementation.
>> For that reason patch wasn't liked by Linus.
>>
>> 3) Introduce read_once_partial_check() function and use it in strscpy().
>> read_once_partial_check() supposed to read one word, but KASAN will check only the first byte of the access.
>> Dmitry didn't like this. I'm also not fan of this solution, because we may not notice some real bugs, e.g.:
>>
>>         char dst[8];
>>         char *src;
>>
>>         src = kmalloc(5, GFP_KERNEL);
>>         memset(src, 0xff, 5);
>>         strscpy(dst, src, 8);
>>
>> stscpy() will copy from 6 up to 8 bytes (exact size depends on what is stored in src[5-7])
>> from non-null terminated src. With read_once_partial_check() used in strscpy() KASAN will
>> not report such bug.
>>
>>
>> So, what it's gonna be? Let's finally decide something and deal with the problem.
>> My vote belongs to 1) or 2).
>
>
> My vote is for 1) if we agree that the optimization is not worth it,
> otherwise for 2).

Who would be the best person to measure the difference for 1)?

-Kees

-- 
Kees Cook
Pixel Security