[botan-devel] Bug in SHA-160 with resize to zero not allocated empty buffer

William K. Foster wkf at alum.mit.edu
Wed Jun 11 15:32:03 EDT 2014


No comments on this proposed bug fix?


On Tue, Jun 3, 2014 at 3:08 PM, William K. Foster <wkf at alum.mit.edu> wrote:

> Hello,
>
> I have come across a SEGV due to following a null pointer in the SHA-160
> implementation for Botan v1.10.8.  The failure is seen in
> SHA_160::compress_n() being called when W has a null buf and the null
> pointer is dereferenced.  The SHA_160 is constructed by SHA_160_SSE2() in
> file src/hash/sha1_sse2/sha1_sse2.h line 22 where the SHA_160(0) is used,
> overriding the size to be zero. This becomes an issue during construction
> where Botan::MemoryRegion<unsigned int>::resize() in file
> src/alloc/secmem.h line 213 the code is reached with a size of n being
> zero but allocated is also zero, and thus the conditional takes the path
> of keeping the existing allocation for buf (a null pointer) for the
> requested zero length of memory which is then later dereferenced.
>
> I have worked around the bug by changing the condition on line 213 to
> instead be:
>
>    if(n <= allocated && (n || buf))
>
> So that if n is zero and buf is null, the else condition is taken.
>
> Perhaps there is a more elegant solution, but at the least the code I've
> changed here is unsafe in that it allows buf to be null on a resize
> request to zero length that appears to violate assumptions elsewhere that
> the buf is non null.
>
> Please suggest a more permanent fix or let me know if you feel this patch
> is all that is needed.
>
> Thanks.
>
> -William
>
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