linux-kernel - Re: [PATCH] xfrm: policy: Restructure RCU-read locking in xfrm_sk_policy

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Date:   Mon, 21 Jun 2021 10:29:49 +0200
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Varad Gautam <varad.gautam@...e.com>
CC:     <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        <netdev@...r.kernel.org>, Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        Florian Westphal <fw@...len.de>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        <stable@...r.kernel.org>
Subject: Re: [PATCH] xfrm: policy: Restructure RCU-read locking in
 xfrm_sk_policy_lookup

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