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SubjectRe: [PATCH 1/9] mm: Hardened usercopy
On Thu, Jul 7, 2016 at 3:42 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Wed, 6 Jul 2016, Kees Cook wrote:
>> +
>> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
>> + const void *frame = NULL;
>> + const void *oldframe;
>> +#endif
>
> That's ugly

Yeah, I'd like to have this be controlled by a specific CONFIG, like I
invented for the linear mapping, but I wasn't sure what was the best
approach.

>
>> +
>> + /* Object is not on the stack at all. */
>> + if (obj + len <= stack || stackend <= obj)
>> + return 0;
>> +
>> + /*
>> + * Reject: object partially overlaps the stack (passing the
>> + * the check above means at least one end is within the stack,
>> + * so if this check fails, the other end is outside the stack).
>> + */
>> + if (obj < stack || stackend < obj + len)
>> + return -1;
>> +
>> +#if defined(CONFIG_FRAME_POINTER) && defined(CONFIG_X86)
>> + oldframe = __builtin_frame_address(1);
>> + if (oldframe)
>> + frame = __builtin_frame_address(2);
>> + /*
>> + * low ----------------------------------------------> high
>> + * [saved bp][saved ip][args][local vars][saved bp][saved ip]
>> + * ^----------------^
>> + * allow copies only within here
>> + */
>> + while (stack <= frame && frame < stackend) {
>> + /*
>> + * If obj + len extends past the last frame, this
>> + * check won't pass and the next frame will be 0,
>> + * causing us to bail out and correctly report
>> + * the copy as invalid.
>> + */
>> + if (obj + len <= frame)
>> + return obj >= oldframe + 2 * sizeof(void *) ? 2 : -1;
>> + oldframe = frame;
>> + frame = *(const void * const *)frame;
>> + }
>> + return -1;
>> +#else
>> + return 1;
>> +#endif
>
> I'd rather make that a weak function returning 1 which can be replaced by
> x86 for CONFIG_FRAME_POINTER=y. That also allows other architectures to
> implement their specific frame checks.

Yeah, though I prefer CONFIG-controlled stuff over weak functions, but
I agree, something like arch_check_stack_frame(...) or similar. I'll
build something for this on the next revision.

-Kees

--
Kees Cook
Chrome OS & Brillo Security

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