tpm_buf_check_hmac_response(9)

💅 tpm_buf_check_hmac_response(9)

okay so bestie, we're in the Kernel API Manual today and this one is lowkey iconic — it's tpm_buf_check_hmac_response, and it does exactly what the name screams: it checks the TPM return HMAC for correctness. no cap. 🦋

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✨ NAME ✨

tpm_buf_check_hmac_response — check the TPM return HMAC for correctness

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🌸 SYNOPSIS 🌸

int tpm_buf_check_hmac_response (struct tpm_chip *chip , struct tpm_buf
*buf , int rc );

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🎀 ARGUMENTS 🎀

• chip — the TPM chip structure. that's your whole TPM vibe right there 💖
• buf — the original command buffer, which now contains the response. it's giving "same buffer, new era" ✨
• rc — the return code from tpm_transmit_cmd. yes, the lowercase tpm_transmit_cmd, not the function above it, don't get it twisted 🙈

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🧚 DESCRIPTION 🧚

okay so first case, bestie: if rc is non zero, buf may not contain an actual return. so rc is passed through as the return, and the session gets cleaned up and de-allocated if required. and when is it required? literally when TPM2_SA_CONTINUE_SESSION was not specified as a session flag. noted. 💅

second case, and this is the main character moment: if rc is zero, the response HMAC is computed against the returned buf and matched to the TPM one in the session area. if there's a mismatch, an error is logged and -EINVAL is returned. boom, roasted. 🔥

and the reason for all this? the command issue and HMAC check sequence should look like this:

rc = tpm_transmit_cmd(...); rc = tpm_buf_check_hmac_response(buf, auth,
rc); if (rc) ...

which is easily layered into the current contrl flow. 🫶

ngl that code block is the whole point — it's the "check your HMAC before you do anything else" pattern, and it slots right into the flow you already have.

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👑 RETURN 👑

0 on success or an error. that's it. that's the tea. ⭐

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🌈 SEE ALSO 🌈

• Kernel file ./drivers/char/tpm/tpm2-sessions.c
• tpm_buf_append_name(9)
• tpm_buf_append_hmac_session(9)
• tpm_buf_fill_hmac_session(9)
• tpm2_end_auth_session(9)
• tpm2_start_auth_session(9)
• tpm2_parse_create_primary(9)
• tpm2_create_primary(9)
• tpm2_sessions_init(9)

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and that's the whole page, bestie 🩷 this little icon lives in ./drivers/char/tpm, straight from the October 2026 drops — go read the source, it's right there, iykyk 💜🧚