Re: [PATCH] crypto: x86/aes-gcm: Disable FPU around skcipher_walk_done().

[Eric Biggers <ebiggers@kernel.org>]

Hi Sebastian,

On Fri, Aug 02, 2024 at 12:23:33PM +0200, Sebastian Andrzej Siewior wrote:
> kernel_fpu_begin() disables preemption. gcm_crypt() has a
> skcipher_walk_done() invocation within a preempt disabled section.
> skcipher_walk_done() can invoke kfree() which requires sleeping locks on
> PREEMPT_RT and must not be invoked with disabled preemption.
> 
> Keep FPU access enabled while skcipher_walk_done() is invoked.
> 
> Fixes: b06affb1cb580 ("crypto: x86/aes-gcm - add VAES and AVX512 / AVX10 optimized AES-GCM")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  arch/x86/crypto/aesni-intel_glue.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/crypto/aesni-intel_glue.c b/arch/x86/crypto/aesni-intel_glue.c
> index cd37de5ec4046..be92e4c3f9c7f 100644
> --- a/arch/x86/crypto/aesni-intel_glue.c
> +++ b/arch/x86/crypto/aesni-intel_glue.c
> @@ -1403,7 +1403,9 @@ gcm_crypt(struct aead_request *req, int flags)
>  			aes_gcm_update(key, le_ctr, ghash_acc,
>  				       walk.src.virt.addr, walk.dst.virt.addr,
>  				       nbytes, flags);
> +			kernel_fpu_end();
>  			err = skcipher_walk_done(&walk, 0);
> +			kernel_fpu_begin();
>  			/*
>  			 * The low word of the counter isn't used by the
>  			 * finalize, so there's no need to increment it here.

Can you make this conditional on CONFIG_PREEMPT_RT so that it doesn't hurt
performance for everyone else?

Note that kfree() lacks a might_sleep(), and its kerneldoc does not say that it
can sleep.  Have you checked for other instances of this same problem?  It seems
it would be quite common kernel-wide.  Is it really necessary that kfree() takes
a sleepable lock on PREEMPT_RT?

- Eric