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Micah Elizabeth Scott
a94ce25277
This is a fix for a very rare buffer overflow in hashx, specific to the dynamic compiler on aarch64 platforms. In practice this issue is extremely unlikely to hit randomly, and it's only been seen in unit tests that supply unusual mock PRNG output to the program generator. My best attempt at estimating the probability of hitting the overflow randomly is about 10^-23. Crafting an input with the intent to overflow can be done only as fast as an exhaustive search, so long as Blake2B is unbroken. The root cause is that hashx writes assembly code without any length checks, and it uses an estimated size rather than an absolute maximum size to allocate the buffer for compiled code. Some instructions are much longer than others, especially on aarch64. The length of the overflow is nearly 300 bytes in the worst synthetic test cases I've developed so far. Overflow occurs during hashx_make(), and the subsequent hashx_exec() will always SIGSEGV as the written code crosses outside the region that's been marked executable. In typical use, hashx_exec() is called immediately after hashx_make(). This fix increases the buffer size from 1 page to 2 pages on aarch64, adds an analysis of the compiled code size, and adds runtime checks so we can gracefully fail on overflow. It also adds a unit test (written in Rust) that includes a PRNG sequence exercising the overflow. Without this patch the unit test shows a SIGSEGV on aarch64, with this patch it runs successfully and matches interpreter output. Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>
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