linux-kernel - Re: [PATCH v8 04/12] tpm: Change tpm_get

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Message-ID: <aUUd760l89lrNOs-@earth.li>
Date: Fri, 19 Dec 2025 09:42:07 +0000
From: Jonathan McDowell <noodles@...th.li>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: linux-integrity@...r.kernel.org,
	"David S . Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Eric Biggers <ebiggers@...nel.org>, Peter Huewe <peterhuewe@....de>,
	Jason Gunthorpe <jgg@...pe.ca>, David Howells <dhowells@...hat.com>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:KEYS/KEYRINGS" <keyrings@...r.kernel.org>,
	"open list:SECURITY SUBSYSTEM" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH v8 04/12] tpm: Change tpm_get_random() opportunistic

On Tue, Dec 16, 2025 at 11:21:38AM +0200, Jarkko Sakkinen wrote:
>hwrng framework does not have a requirement that the all bytes requested
>need to be provided. By enforcing such a requirement internally, TPM driver
>can cause unpredictability in latency, as a single tpm_get_random() call
>can result multiple TPM commands.
>
>Especially, when TCG_TPM2_HMAC is enabled, extra roundtrips could have
>significant effect to the system latency.
>
>Thus, send TPM command only once and return bytes received instead of
>committing to the number of requested bytes.

Function comment for tpm_get_random needs updated as well, as it 
currently says "until all of the @max bytes have been received", which 
is no longer true with this patch. With that:

Reviewed-by: Jonathan McDowell <noodles@...a.com>

>Cc: David S. Miller <davem@...emloft.net>
>Cc: Herbert Xu <herbert@...dor.apana.org.au>
>Cc: Eric Biggers <ebiggers@...nel.org>
>Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
>---
>v7:
>- Given that hwrng is now only caller for tpm_get_random(), remove the
>  wait parameter.
>v4:
>- Fixed grammar mistakes.
>---
> drivers/char/tpm/tpm-interface.c | 28 +++++-----------------------
> 1 file changed, 5 insertions(+), 23 deletions(-)
>
>diff --git a/drivers/char/tpm/tpm-interface.c b/drivers/char/tpm/tpm-interface.c
>index d157be738612..677dcef05dfb 100644
>--- a/drivers/char/tpm/tpm-interface.c
>+++ b/drivers/char/tpm/tpm-interface.c
>@@ -626,10 +626,6 @@ static int tpm2_get_random(struct tpm_chip *chip, u8 *out, size_t max)
>  */
> int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
> {
>-	u32 num_bytes = max;
>-	u8 *out_ptr = out;
>-	int retries = 5;
>-	int total = 0;
> 	int rc;
>
> 	if (!out || !max || max > TPM_MAX_RNG_DATA)
>@@ -646,28 +642,14 @@ int tpm_get_random(struct tpm_chip *chip, u8 *out, size_t max)
> 		rc = tpm2_start_auth_session(chip);
> 		if (rc)
> 			return rc;
>-	}
>-
>-	do {
>-		if (chip->flags & TPM_CHIP_FLAG_TPM2)
>-			rc = tpm2_get_random(chip, out_ptr, num_bytes);
>-		else
>-			rc = tpm1_get_random(chip, out_ptr, num_bytes);
>-
>-		if (rc < 0)
>-			goto err;
>-
>-		out_ptr += rc;
>-		total += rc;
>-		num_bytes -= rc;
>-	} while (retries-- && total < max);
>
>-	tpm_put_ops(chip);
>-	return total ? total : -EIO;
>+		rc = tpm2_get_random(chip, out, max);
>+	} else {
>+		rc = tpm1_get_random(chip, out, max);
>+	}
>
>-err:
> 	tpm_put_ops(chip);
>-	return rc;
>+	return rc != 0 ? rc : -EIO;
> }
> EXPORT_SYMBOL_GPL(tpm_get_random);
>
>-- 
>2.39.5
>
>

J.

-- 
... We are talking one charming motherf**king pig.