hmac_sha256() leaves four stack buffers containing key-derived material
(inner_padding, outer_padding, replacement_key, hash state) on the stack
after returning. The inner_padding and outer_padding arrays contain
key XOR 0x36 and key XOR 0x5c respectively, which are trivially
reversible to recover the original HMAC key.
This function is called with security-sensitive keys including the LUKS
volume key (cryptsetup-util.c), TPM2 PIN (tpm2-util.c), and boot secret
(tpm2-swtpm.c). The key material persists on the stack until overwritten
by later unrelated function calls.
Add CLEANUP_ERASE() to all four local buffers, following the same
pattern applied to tpm2-util.c in commit 6c80ce6 (PR #41394).
Based on the FIPS 198 specification. Not optimized and probably
completely unsafe, to be used only for non-strong-cryptographic
purposes when OpenSSL cannot be used.
Michal Rybecky
Daan De Meyer
Lennart Poettering
Luca Boccassi