Re: [PATCH] Out-of-bounds access in get_wchan (arch/x86/kernel/process_64.c)

[Dmitry Vyukov]

On Fri, Sep 13, 2013 at 3:03 AM, Wolfram Gloger
<wmglo@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> "H. Peter Anvin" <hpa@xxxxxxxxx> writes:
>
>> Actually, the sanest would be:
>>
>>       if (fp  < (unsigned long)stack ||
>>           fp >= (unsigned long)stack+(THREAD_SIZE-16))
>>
>> ... wouldn't it (since we are accessing an 8-byte datum at offset +8?
>
> Ok, splitting hairs maybe, what about this V2 of the patch?
>
> fp is under userspace control.  stack-8 cannot underflow, otherwise we'd
> have much more serious problems in kernel space.
>
> Hence, for dereferencing fp+8:
>
> The case fp == stack-8 is valid.
> As is the case fp == stack+(THREAD_SIZE-16).
> Everything outside is invalid.
>
> About the worst-case implications: Like I said I have not reproduced it
> and cannot really comment on that.


+kees@, linux-kernel@
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