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    Subject[PATCH 5.13 222/266] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype
    Date
    From: Varad Gautam <varad.gautam@suse.com>

    commit d7b0408934c749f546b01f2b33d07421a49b6f3e upstream.

    xfrm_policy_lookup_bytype loops on seqcount mutex xfrm_policy_hash_generation
    within an RCU read side critical section. Although ill advised, this is fine if
    the loop is bounded.

    xfrm_policy_hash_generation wraps mutex hash_resize_mutex, which is used to
    serialize writers (xfrm_hash_resize, xfrm_hash_rebuild). This is fine too.

    On PREEMPT_RT=y, the read_seqcount_begin call within xfrm_policy_lookup_bytype
    emits a mutex lock/unlock for hash_resize_mutex. Mutex locking is fine, since
    RCU read side critical sections are allowed to sleep with PREEMPT_RT.

    xfrm_hash_resize can, however, block on synchronize_rcu while holding
    hash_resize_mutex.

    This leads to the following situation on PREEMPT_RT, where the writer is
    blocked on RCU grace period expiry, while the reader is blocked on a lock held
    by the writer:

    Thead 1 (xfrm_hash_resize) Thread 2 (xfrm_policy_lookup_bytype)

    rcu_read_lock();
    mutex_lock(&hash_resize_mutex);
    read_seqcount_begin(&xfrm_policy_hash_generation);
    mutex_lock(&hash_resize_mutex); // block
    xfrm_bydst_resize();
    synchronize_rcu(); // block
    <RCU stalls in xfrm_policy_lookup_bytype>

    Move the read_seqcount_begin call outside of the RCU read side critical section,
    and do an rcu_read_unlock/retry if we got stale data within the critical section.

    On non-PREEMPT_RT, this shortens the time spent within RCU read side critical
    section in case the seqcount needs a retry, and avoids unbounded looping.

    Fixes: 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock")
    Signed-off-by: Varad Gautam <varad.gautam@suse.com>
    Cc: linux-rt-users <linux-rt-users@vger.kernel.org>
    Cc: netdev@vger.kernel.org
    Cc: stable@vger.kernel.org # v4.9
    Cc: Steffen Klassert <steffen.klassert@secunet.com>
    Cc: Herbert Xu <herbert@gondor.apana.org.au>
    Cc: "David S. Miller" <davem@davemloft.net>
    Cc: Jakub Kicinski <kuba@kernel.org>
    Cc: Florian Westphal <fw@strlen.de>
    Cc: "Ahmed S. Darwish" <a.darwish@linutronix.de>
    Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
    Acked-by: Ahmed S. Darwish <a.darwish@linutronix.de>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    net/xfrm/xfrm_policy.c | 21 ++++++++++++++-------
    1 file changed, 14 insertions(+), 7 deletions(-)

    --- a/net/xfrm/xfrm_policy.c
    +++ b/net/xfrm/xfrm_policy.c
    @@ -2092,12 +2092,15 @@ static struct xfrm_policy *xfrm_policy_l
    if (unlikely(!daddr || !saddr))
    return NULL;

    - rcu_read_lock();
    retry:
    - do {
    - sequence = read_seqcount_begin(&xfrm_policy_hash_generation);
    - chain = policy_hash_direct(net, daddr, saddr, family, dir);
    - } while (read_seqcount_retry(&xfrm_policy_hash_generation, sequence));
    + sequence = read_seqcount_begin(&xfrm_policy_hash_generation);
    + rcu_read_lock();
    +
    + chain = policy_hash_direct(net, daddr, saddr, family, dir);
    + if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) {
    + rcu_read_unlock();
    + goto retry;
    + }

    ret = NULL;
    hlist_for_each_entry_rcu(pol, chain, bydst) {
    @@ -2128,11 +2131,15 @@ static struct xfrm_policy *xfrm_policy_l
    }

    skip_inexact:
    - if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence))
    + if (read_seqcount_retry(&xfrm_policy_hash_generation, sequence)) {
    + rcu_read_unlock();
    goto retry;
    + }

    - if (ret && !xfrm_pol_hold_rcu(ret))
    + if (ret && !xfrm_pol_hold_rcu(ret)) {
    + rcu_read_unlock();
    goto retry;
    + }
    fail:
    rcu_read_unlock();


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