Re: [PATCH] xfrm: Fix RCU vs hash_resize_mutex lock inversion

[Varad Gautam]

On 6/30/21 8:57 AM, Steffen Klassert wrote:
> On Mon, Jun 28, 2021 at 03:34:28PM +0200, Frederic Weisbecker wrote:
>> xfrm_bydst_resize() calls synchronize_rcu() while holding
>> hash_resize_mutex. But then on PREEMPT_RT configurations,
>> xfrm_policy_lookup_bytype() may acquire that mutex while running in an
>> RCU read side critical section. This results in a deadlock.
>>
>> In fact the scope of hash_resize_mutex is way beyond the purpose of
>> xfrm_policy_lookup_bytype() to just fetch a coherent and stable policy
>> for a given destination/direction, along with other details.
>>
>> The lower level net->xfrm.xfrm_policy_lock, which among other things
>> protects per destination/direction references to policy entries, is
>> enough to serialize and benefit from priority inheritance against the
>> write side. As a bonus, it makes it officially a per network namespace
>> synchronization business where a policy table resize on namespace A
>> shouldn't block a policy lookup on namespace B.
>>
>> Fixes: 77cc278f7b20 (xfrm: policy: Use sequence counters with associated lock)
>> Cc: stable@xxxxxxxxxxxxxxx
>> Cc: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
>> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>> Cc: Varad Gautam <varad.gautam@xxxxxxxx>
>> Cc: Steffen Klassert <steffen.klassert@xxxxxxxxxxx>
>> Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
>> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
>> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> 
> Your patch has a conflicht with ("commit d7b0408934c7 xfrm: policy: Read
> seqcount outside of rcu-read side in xfrm_policy_lookup_bytype")
> from Varad. Can you please rebase onto the ipsec tree?
> 
> Btw. Varad, your above mentioned patch tried to fix the same issue.
> Do we still need it, or is it obsolete with the fix from Frederic?
> 

The patch "xfrm: policy: Read seqcount outside of rcu-read side in
xfrm_policy_lookup_bytype" shouldn't be needed after Frederic's fix since
the offending mutex is now gone. It can be dropped.

Regards,
Varad

> Thanks!
> 

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