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

William K. Foster wkf at alum.mit.edu
Tue Jun 3 18:08:35 EDT 2014


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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