fix(fsst): grow decode buffer to prevent out-of-bounds write#7267
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Noethix55555 wants to merge 1 commit into
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fix(fsst): grow decode buffer to prevent out-of-bounds write#7267Noethix55555 wants to merge 1 commit into
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decompress only required a 3x output buffer, but an FSST code byte can expand to an 8-byte symbol and the decode loop writes 8 bytes at a time through raw pointers. On data that compresses better than 3:1, honoring the documented minimum overran the buffer (a heap out-of-bounds write, observed as a segfault). Size the output buffer for the worst case before decoding; it is shrunk to the real length afterwards. Adds a regression test.
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Xuanwo
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Jun 19, 2026
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The decompression path still trusts serialized symbol lengths and offsets before entering unsafe decode. Malformed input can write past the allocated buffer or read outside the compressed data.
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Summary
fsst::fsst::decompressonly required a3 * inputoutput buffer, but an FSST code byte can decode to an 8-byte symbol and the decode loop writes 8 bytes at a time through raw pointers. On data that compresses better than 3:1, honoring the documented minimum overran the buffer (a heap out-of-bounds write, observed as a segfault).This sizes the output buffer for the worst case before decoding, then shrinks it to the real length at the end. The change is internal to the decode path and does not change results for callers that already pass a large enough buffer (in-tree callers pass
8 * input).Closes #7266. Related: #2606.
Test
Added
rust/compression/fsst/tests/buffer_bound.rs: it compresses an ~8:1 input and decompresses into the documented 3x buffer. This segfaulted before the change and round-trips correctly after.cargo test -p fsstandcargo clippy -p fsst --testspass.