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

On Mon, Jun 21, 2021 at 11:11:18AM +0200, Varad Gautam wrote:

On 6/21/21 10:29 AM, Steffen Klassert wrote:

quoted

On Fri, Jun 18, 2021 at 04:11:01PM +0200, Varad Gautam wrote:

quoted

Commit "xfrm: policy: Read seqcount outside of rcu-read side in
xfrm_policy_lookup_bytype" [Linked] resolved a locking bug in
xfrm_policy_lookup_bytype that causes an RCU reader-writer deadlock on
the mutex wrapped by xfrm_policy_hash_generation on PREEMPT_RT since
77cc278f7b20 ("xfrm: policy: Use sequence counters with associated
lock").

However, xfrm_sk_policy_lookup can still reach xfrm_policy_lookup_bytype
while holding rcu_read_lock(), as:
xfrm_sk_policy_lookup()
  rcu_read_lock()
  security_xfrm_policy_lookup()
    xfrm_policy_lookup()

Hm, I don't see that call chain. security_xfrm_policy_lookup() calls
a hook with the name xfrm_policy_lookup. The only LSM that has
registered a function to that hook is selinux. It registers
selinux_xfrm_policy_lookup() and I don't see how we can call
xfrm_policy_lookup() from there.

Did you actually trigger that bug?

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 [off-list ref]
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 [off-list ref]
    Signed-off-by: Peter Zijlstra (Intel) [off-list ref]
    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.