ChaCha20: Fix stream-length overflow check #216
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Tests 1, 2, 3, 6, and 7 fail. I suspect they behave differently with the `avx2` backend since it depends on where the boundary of the buffer is. I don't think it's easy to exploit this unless you're trying really hard. That said, I believe it's possible for sufficiently determined user code to bypass the MAX_BLOCKS check and generate looped/overflowed keystream past 256GB.
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One sec, forgot I was running nightly... |
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@tarcieri not sure if you're the right person to tag but this could use review |
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Great, thank you! This part of the code has been problematic in part because it's duplicated between The added tests should hopefully ensure it does not regress. |
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It seems that the stream-length-overflow check has been through a few iterations, and at some point along the way it developed troublesome edge-case behavior. This adds 7 unit tests to exercise the behavior, a confusing subset of which (1,2,3,6,7) fail on the current upstream branch. it also adds a simplified stream-length-overflow check and enforces stream-position checking in
try_seek(). Those changes make all the new tests pass.Based on a quick glance through the history, I'm not sure when this problem appeared. It doesn't seem easy to exploit unless you're very deliberately extracting more keystream than you should be allowed to, but it definitely needs a fix.