Subject: Re: [PATCH net] ppp: hold mutex when unbridging channels in unregister path

From: Guillaume Nault <[email-masked]>
Date: Thu, 24 Dec 2020 11:28:18 +0100

On Wed, Dec 23, 2020 at 06:47:30PM +0000, Tom Parkin wrote:
> Channels are bridged using the PPPIOCBRIDGECHAN ioctl, which executes
> with the ppp_mutex held.
> 
> Unbridging may occur in two code paths: firstly an explicit
> PPPIOCUNBRIDGECHAN ioctl, and secondly on channel unregister.  The
> latter may occur when closing the /dev/ppp instance or on teardown of
> the channel itself.
> 
> This opens up a refcount underflow bug if ppp_bridge_channels called via.
> ioctl races with ppp_unbridge_channels called via. file release.
> 
> The race is triggered by ppp_unbridge_channels taking the error path

This is actually ppp_bridge_channels.

> through the 'err_unset' label.  In this scenario, pch->bridge has been
> set, but no reference will be taken on pch->file because the function
> errors out.  Therefore, if ppp_unbridge_channels is called in the window
> between pch->bridge being set and pch->bridge being unset, it will
> erroneously drop the reference on pch->file and cause a refcount
> underflow.
> 
> To avoid this, hold the ppp_mutex while calling ppp_unbridge_channels in
> the shutdown path.  This serialises the unbridge operation with any
> concurrently executing ioctl.
> 
> Signed-off-by: Tom Parkin <[email-masked]>
> ---
>  drivers/net/ppp/ppp_generic.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 09c27f7773f9..e87a05fee2db 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -2938,7 +2938,9 @@ ppp_unregister_channel(struct ppp_channel *chan)
>  	list_del(&pch->list);
>  	spin_unlock_bh(&pn->all_channels_lock);
>  
> +	mutex_lock(&ppp_mutex);
>  	ppp_unbridge_channels(pch);
> +	mutex_unlock(&ppp_mutex);
>  
>  	pch->file.dead = 1;
>  	wake_up_interruptible(&pch->file.rwait);
> -- 
> 2.17.1
> 

The problem is that assigning ->bridge and taking a reference on that
channel isn't atomic. Holding ppp_mutex looks like a workaround for
this problem.

I think the refcount should be taken before unlocking ->upl in
ppp_bridge_channels().

Something like this (completely untested):

---- 8< ----
 static int ppp_bridge_channels(struct channel *pch, struct channel *pchb)
 {
 	write_lock_bh(&pch->upl);
 	if (pch->ppp ||
 	    rcu_dereference_protected(pch->bridge, lockdep_is_held(&pch->upl))) {
 		write_unlock_bh(&pch->upl);
 		return -EALREADY;
 	}
+
+	refcount_inc(&pchb->file.refcnt);
 	rcu_assign_pointer(pch->bridge, pchb);
 	write_unlock_bh(&pch->upl);

	write_lock_bh(&pchb->upl);
	if (pchb->ppp ||
	    rcu_dereference_protected(pchb->bridge, lockdep_is_held(&pchb->upl))) {
		write_unlock_bh(&pchb->upl);
		goto err_unset;
	}
+
+	refcount_inc(&pch->file.refcnt);
	rcu_assign_pointer(pchb->bridge, pch);
	write_unlock_bh(&pchb->upl);

-	refcount_inc(&pch->file.refcnt);
-	refcount_inc(&pchb->file.refcnt);
-
	return 0;

err_unset:
	write_lock_bh(&pch->upl);
	RCU_INIT_POINTER(pch->bridge, NULL);
	write_unlock_bh(&pch->upl);
	synchronize_rcu();
+
+	if (refcount_dec_and_test(&pchb->file.refcnt))
+		ppp_destroy_channel(pchb);
+
	return -EALREADY;
}