linux-kernel - Re: [PATCH] tpm: Lazily flush auth session when getting random data

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Message-ID: <Z8sgJl7UZLpmwpp8@kernel.org>
Date: Fri, 7 Mar 2025 18:34:46 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Jonathan McDowell <noodles@...th.li>
Cc: Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm: Lazily flush auth session when getting random data

On Fri, Mar 07, 2025 at 10:58:13AM +0000, Jonathan McDowell wrote:
> From: Jonathan McDowell <noodles@...a.com>
> 
> Lazy flushing of TPM auth sessions was introduced to speed up IMA
> measurments into the TPM. Make use of it in tpm2_get_random as well,
> which has the added benefit of not needlessly cleaning up the session
> that IMA is using when there are no userspace accesses taking place.
> 
> Command trace before for every call:
> 
> hwrng (0x00000161): 14 (52965242 ns)
> hwrng (0x00000176): 48 (161612432 ns)
> hwrng (0x00000165): 10 (2410494 ns)
> hwrng (0x0000017B): 117 (70699883 ns)
> hwrng (0x0000017B): 117 (70959666 ns)
> hwrng (0x00000165): 10 (2756827 ns)
> 
> After, with repeated calls showing no setup:
> 
> hwrng (0x00000161): 14 (53044582 ns)
> hwrng (0x00000176): 48 (160491333 ns)
> hwrng (0x00000165): 10 (2408220 ns)
> hwrng (0x0000017B): 117 (70695037 ns)
> hwrng (0x0000017B): 117 (70994984 ns)
> hwrng (0x0000017B): 117 (70195388 ns)
> hwrng (0x0000017B): 117 (70973835 ns)
> 
> Signed-off-by: Jonathan McDowell <noodles@...a.com>
> ---
>  drivers/char/tpm/tpm2-cmd.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/char/tpm/tpm2-cmd.c b/drivers/char/tpm/tpm2-cmd.c
> index dfdcbd009720..524d802ede26 100644
> --- a/drivers/char/tpm/tpm2-cmd.c
> +++ b/drivers/char/tpm/tpm2-cmd.c
> @@ -359,7 +359,6 @@ int tpm2_get_random(struct tpm_chip *chip, u8 *dest, size_t max)
>  	} while (retries-- && total < max);
>  
>  	tpm_buf_destroy(&buf);
> -	tpm2_end_auth_session(chip);
>  
>  	return total ? total : -EIO;
>  out:
> -- 
> 2.48.1
> 

Thanks for this. It is a good catch! I'll apply this over the weekend
when I apply Arm FF-A driver patches.

Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>

BR, Jarkko