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

[Timo Teras <timo.teras@iki.fi>]

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);
>  }
>