Patch for CVE-2021-32629 to stable-v0.26 for security point release. #2919
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Fixes bytecodealliance#2839. See also the issue description and comments in this commits for details of what the fix is about here.
Previously, the x64 backend's ABI code would generate a sign-extending load when loading a less-than-64-bit integer from a spillslot. This is incorrect: e.g., for i32s > 0x80000000, this would result in all high bits set. This interacts poorly with another optimization. Normally, the invariant is that the high bits of a register holding a value of a certain type, beyond that type's bits, are undefined. However, as an optimization, we recognize and use the fact that on x86-64, 32-bit instructions zero the upper 32 bits. This allows us to elide a 32-to-64-bit zero-extend op (turning it into just a move, which can then sometimes disappear entirely due to register coalescing). If a spill and reload happen between the production of a 32-bit value from an instruction known to zero the upper bits and its use, then we will rely on zero upper bits that might actually be set by a sign-extend. This will result in incorrect execution. As a fix, we stick to a simple invariant: we always spill and reload a full 64 bits when handling integer registers on x64. This ensures that no bits are mangled.
alexcrichton
approved these changes
May 21, 2021
iximeow
approved these changes
May 21, 2021
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This PR is a cherrypick of the fix for the CVE (and related #2840) on top of the v0.26.0 release, using a new
stable-v0.26branch. Once this merges, I will version-bump and release v0.26.1 (and Cranelift v0.73.1) off of this branch.