bip39: scrub vector registers after mnemonic decoding#534
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test_clear finds plaintext secret material ~11KB deep on the stack after bip39_mnemonic_validate returns. The residue comes from XMM registers xsaved by the dynamic linker on subsequent PLT calls, not from a source-level local buffer. Adding a zeroed stack scratch in bip39_mnemonic_to_bytes forces glibc memset to issue SSE/AVX zero stores that clobber the vector registers before the function returns, so later xsave operations no longer spill secret bytes.
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During the work proposed on #533 I separated this out of scope, yet likely relevant commit from @pythcoiner, so it can be evaluated individually:
"test_clear finds plaintext secret material ~11KB deep on the stack after bip39_mnemonic_validate returns. The residue comes from XMM registers xsaved by the dynamic linker on subsequent PLT calls, not from a source-level local buffer. Adding a zeroed stack scratch in bip39_mnemonic_to_bytes forces glibc memset to issue SSE/AVX zero stores that clobber the vector registers before the function returns, so later xsave operations no longer spill secret bytes."