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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-08-02 09:28:32 [-0700], Eric Biggers wrote:
> Hi Sebastian,
Hi Eric,

> > 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?

Every other instance in this file had a kernel_fpu_end/ begin() before
skcipher_walk_done() so I though was just missed by chance.

> 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.  

kfree() can't have a might_sleep() because it does not qualify for this
since you can use it in softirq context for instance with an acquired
spinlockt_t on !RT which would trigger it.
On PREEMPT_RT interrupts are threaded, softirq is preemptible,
spintlock_t is a sleeping lock so all these things where a kfree()
would have been invoked in preempt-disable context on !PREEMPT_RT is
actually preemptible on PREEMPT_RT.
This is of course not true in cases where preemption is explicitly
disabled like in this case.

> Is it really necessary that kfree() takes
> a sleepable lock on PREEMPT_RT?
Yes. The locking in SLUB and page allocator is spinlock_t.

> - Eric

Sebastian