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

[Dmitry Vyukov]

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).