Re: [PATCH] xfrm: policy: Restructure RCU-read locking in xfrm_sk_policy_lookup

[Steffen Klassert <steffen.klassert@secunet.com>]

On Tue, Jun 22, 2021 at 01:51:24PM +0200, Frederic Weisbecker wrote:
> On Tue, Jun 22, 2021 at 01:21:59PM +0200, Steffen Klassert wrote:
> > On Mon, Jun 21, 2021 at 01:05:28PM +0200, Steffen Klassert wrote:
> > > On Mon, Jun 21, 2021 at 11:11:18AM +0200, Varad Gautam wrote:
> > > > 
> > > > Right, I misread the call chain - security_xfrm_policy_lookup does not reach
> > > > xfrm_policy_lookup, making this patch unnecessary. The bug I have is:
> > > > 
> > > > T1, holding hash_resize_mutex and sleeping inside synchronize_rcu:
> > > > 
> > > > __schedule
> > > > schedule
> > > > schedule_timeout
> > > > wait_for_completion
> > > > __wait_rcu_gp
> > > > synchronize_rcu
> > > > xfrm_hash_resize
> > > > 
> > > > And T2 producing RCU-stalls since it blocked on the mutex:
> > > > 
> > > > __schedule
> > > > schedule
> > > > __rt_mutex_slowlock
> > > > rt_mutex_slowlock_locked
> > > > rt_mutex_slowlock
> > > > xfrm_policy_lookup_bytype.constprop.77
> > > 
> > > Ugh, why does xfrm_policy_lookup_bytype use a mutex? This is called
> > > in the receive path inside a sofirq.
> > > 
> > > The bug was introduced by: 
> > > 
> > > commit 77cc278f7b202e4f16f8596837219d02cb090b96
> > > Author: Ahmed S. Darwish <a.darwish@linutronix.de>
> > > Date:   Mon Jul 20 17:55:22 2020 +0200
> > > 
> > >     xfrm: policy: Use sequence counters with associated lock
> > > 
> > >     A sequence counter write side critical section must be protected by some
> > >     form of locking to serialize writers. If the serialization primitive is
> > >     not disabling preemption implicitly, preemption has to be explicitly
> > >     disabled before entering the sequence counter write side critical
> > >     section.
> > > 
> > >     A plain seqcount_t does not contain the information of which lock must
> > >     be held when entering a write side critical section.
> > > 
> > >     Use the new seqcount_spinlock_t and seqcount_mutex_t data types instead,
> > >     which allow to associate a lock with the sequence counter. This enables
> > >     lockdep to verify that the lock used for writer serialization is held
> > >     when the write side critical section is entered.
> > > 
> > >     If lockdep is disabled this lock association is compiled out and has
> > >     neither storage size nor runtime overhead.
> > > 
> > >     Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
> > >     Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > >     Link: https://lkml.kernel.org/r/20200720155530.1173732-17-a.darwish@linutronix.de
> > > 
> > > This uses a seqcount_mutex_t for xfrm_policy_hash_generation, that's
> > > wrong.
> > 
> > Varad, can you try to replace the seqcount_mutex_t for xfrm_policy_hash_generation
> > by a seqcount_spinlock_t? I'm not familiar with that seqcount changes,
> > but we should not end up with using a mutex in this codepath.
> 
> Something like this? (beware, untested, also I don't know if the read side
> should then disable bh, doesn't look necessary for PREEMPT_RT, but I may be
> missing something...)

Looking a bit deeper into this it seems that the problem is that
xfrm_policy_hash_generation and hash_resize_mutex do not protect
the same thing.

hash_resize_mutex protects user configuration against a worker thread
that rebalances the hash buckets. xfrm_policy_hash_generation protects
user configuration against the data path that runs in softirq.

Finally the following line from xfrm_init() relates these two:

seqcount_mutex_init(&xfrm_policy_hash_generation, &hash_resize_mutex);

That looks a bit odd. This line was also introduced with the above
mentioned patch.