Re: [PATCH v2] xfrm: policy: Fix doulbe free in xfrm_policy_timer

Timo Teras
2020-03-23T06:53:20Z

Hi

On Mon, 23 Mar 2020 09:41:55 +0800
YueHaibing <yuehaibing@huawei.com> wrote:

> After xfrm_add_policy add a policy, its ref is 2, then
>
> xfrm_policy_timer
> read_lock
> xp->walk.dead is 0
> ....
> mod_timer()
> xfrm_policy_kill
> policy->walk.dead = 1
> ....
> del_timer(&policy->timer)
> xfrm_pol_put //ref is 1
> xfrm_pol_put //ref is 0
> xfrm_policy_destroy
> call_rcu
> xfrm_pol_hold //ref is 1
> read_unlock
> xfrm_pol_put //ref is 0
> xfrm_policy_destroy
> call_rcu
>
> xfrm_policy_destroy is called twice, which may leads to
> double free.

I believe the timer changes were added later in commit e7d8f6cb2f which
added holding a reference when timer is running. I think it fails to
properly account for concurrently running timer in xfrm_policy_kill().

The time when commit ea2dea9dacc2 was done this was not the case.

I think it would be preferable if the concurrency issue could be solved
without additional locking.

> Call Trace:
> RIP: 0010:refcount_warn_saturate+0x161/0x210
> ...
> xfrm_policy_timer+0x522/0x600
> call_timer_fn+0x1b3/0x5e0
> ? __xfrm_decode_session+0x2990/0x2990
> ? msleep+0xb0/0xb0
> ? _raw_spin_unlock_irq+0x24/0x40
> ? __xfrm_decode_session+0x2990/0x2990
> ? __xfrm_decode_session+0x2990/0x2990
> run_timer_softirq+0x5c5/0x10e0
>
> Fix this by use write_lock_bh in xfrm_policy_kill.
>
> Fixes: ea2dea9dacc2 ("xfrm: remove policy lock when accessing
> policy->walk.dead") Signed-off-by: YueHaibing <yuehaibing@huawei.com>

Should be instead:
Fixes: e7d8f6cb2f ("xfrm: Add refcount handling to queued policies")

> ---
> v2: Fix typo 'write_lock_bh'--> 'write_unlock_bh' while unlocking
> ---
> net/xfrm/xfrm_policy.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> index dbda08ec566e..ae0689174bbf 100644
> --- a/net/xfrm/xfrm_policy.c
> +++ b/net/xfrm/xfrm_policy.c
> @@ -434,6 +434,7 @@ EXPORT_SYMBOL(xfrm_policy_destroy);
>
> static void xfrm_policy_kill(struct xfrm_policy *policy)
> {
> + write_lock_bh(&policy->lock);
> policy->walk.dead = 1;
>
> atomic_inc(&policy->genid);
> @@ -445,6 +446,7 @@ static void xfrm_policy_kill(struct xfrm_policy
> *policy) if (del_timer(&policy->timer))
> xfrm_pol_put(policy);
>
> + write_unlock_bh(&policy->lock);
> xfrm_pol_put(policy);
> }
>