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fix undefined behavior: clamp shift in B44 Decompress #832

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merged 6 commits into from
Oct 6, 2020
Merged

fix undefined behavior: clamp shift in B44 Decompress #832

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c5d0806
fix undefined behavior: clamp shift in B44 Decompress
peterhillman Sep 8, 2020
80bd450
use unsigned values in shift to prevent undefined behavior
peterhillman Sep 9, 2020
f493356
remove min() in B44 unpack14 for better fix
peterhillman Sep 16, 2020
fb3b565
ignore unused bits in B44 mode detection
peterhillman Sep 16, 2020
0cf62d6
Merge branch 'master' into b44_ubsan_fix
peterhillman Sep 16, 2020
77e7668
Merge branch 'master' into b44_ubsan_fix
peterhillman Oct 6, 2020
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11 changes: 10 additions & 1 deletion OpenEXR/IlmImf/ImfB44Compressor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -380,7 +380,16 @@ unpack14 (const unsigned char b[14], unsigned short s[16])

s[ 0] = (b[0] << 8) | b[1];

unsigned short shift = (b[ 2] >> 2);


//
//TODO unverified fix for undefined behavior
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The TODO comment is confusing, what do you mean by 'unverified'?

// (0x20 << shift) will overflow 32 bit int if 'shift' is greater than 26
// this should only occur in corrupt files. Clamping 'shift' to 26
// prevents undefined behavior in this circumstance (but cast to unsigned short
// will still overflow)
//
unsigned short shift = std::min( 26 , b[ 2] >> 2);
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Why 26, since the type is unsigned short? Why not clamp to 10 instead, since any more would overflow the 16 bits and produce the same 0 results?

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@peterhillman peterhillman Sep 9, 2020 •
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I suppose that's why I felt the need to say the fix was 'unverified' as wasn't fully sure.

IlmImfTest produces shift values of 11, so 10 is definitely not enough. It may actually need to be 31 to be sure this change won't change the decoding of broken images. shift is used to compute bias, as well as in the following lines.

My understanding is this:

All the << operators cause casts to 32 bit signed integers for those lines. That means all those lines involve 32 bit computation, which is then truncated to 16 bits. I think the implicit cast to signed arithmetic may have been unintentional.

It appears that what happens when a<<b overflows is undefined if a is signed, but defined if a is unsigned: the top bits are lost. However if b is equal to or greater than type's bit size (e.g. larger than 31 if a is 32 bit int) then the behavior is always undefined. Perhaps some 32-bit processors only take the bottom 5 bits of b, so a<<33 gives the same result as a<<1

A more conservative approach would be to clamp shift to 31, and switch all the computation to unsigned operations. This would prevent any undefined behavior. Overflows would still happen but now have well defined behavior.

unsigned short bias = (0x20 << shift);

s[ 4] = s[ 0] + ((((b[ 2] << 4) | (b[ 3] >> 4)) & 0x3f) << shift) - bias;
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