linux-kernel - Re: [PATCH net v2] netpoll: Fix deadlock in memory allocation under spinlock

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Message-ID: <53jkbptcchxc2sxho56rrj7bcwizpd536bsox45hw75uk2fydg@k33edbzrjdvr>
Date: Tue, 14 Oct 2025 09:13:18 -0700
From: Breno Leitao <leitao@...ian.org>
To: Simon Horman <horms@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	usamaarif642@...il.com, riel@...riel.com, kernel-team@...a.com
Subject: Re: [PATCH net v2] netpoll: Fix deadlock in memory allocation under
 spinlock

On Tue, Oct 14, 2025 at 03:11:51PM +0100, Simon Horman wrote:
> On Tue, Oct 14, 2025 at 03:10:51AM -0700, Breno Leitao wrote:
> > Fix a AA deadlock in refill_skbs() where memory allocation while holding
> > skb_pool->lock can trigger a recursive lock acquisition attempt.
> > 
> > The deadlock scenario occurs when the system is under severe memory
> > pressure:
> > 
> > 1. refill_skbs() acquires skb_pool->lock (spinlock)
> > 2. alloc_skb() is called while holding the lock
> > 3. Memory allocator fails and calls slab_out_of_memory()
> > 4. This triggers printk() for the OOM warning
> > 5. The console output path calls netpoll_send_udp()
> > 6. netpoll_send_udp() attempts to acquire the same skb_pool->lock
> > 7. Deadlock: the lock is already held by the same CPU
> > 
> > Call stack:
> >   refill_skbs()
> >     spin_lock_irqsave(&skb_pool->lock)    <- lock acquired
> >     __alloc_skb()
> >       kmem_cache_alloc_node_noprof()
> >         slab_out_of_memory()
> >           printk()
> >             console_flush_all()
> >               netpoll_send_udp()
> >                 skb_dequeue()
> >                   spin_lock_irqsave(&skb_pool->lock)     <- deadlock attempt
> > 
> > This bug was exposed by commit 248f6571fd4c51 ("netpoll: Optimize skb
> > refilling on critical path") which removed refill_skbs() from the
> > critical path (where nested printk was being deferred), letting nested
> > printk being calld form inside refill_skbs()
> > 
> > Refactor refill_skbs() to never allocate memory while holding
> > the spinlock.
> > 
> > Another possible solution to fix this problem is protecting the
> > refill_skbs() from nested printks, basically calling
> > printk_deferred_{enter,exit}() in refill_skbs(), then, any nested
> > pr_warn() would be deferred.
> > 
> > I prefer tthis approach, given I _think_ it might be a good idea to move
> > the alloc_skb() from GFP_ATOMIC to GFP_KERNEL in the future, so, having
> > the alloc_skb() outside of the lock will be necessary step.
> > 
> > Signed-off-by: Breno Leitao <leitao@...ian.org>
> > Fixes: 248f6571fd4c51 ("netpoll: Optimize skb refilling on critical path")
> > ---
> > Changes in v2:
> > - Added a return after the successful path (Rik van Riel)
> > - Changed the Fixes tag to point to the commit that exposed the problem.
> > - Link to v1: https://lore.kernel.org/r/20251013-fix_netpoll_aa-v1-1-94a1091f92f0@debian.org
> > ---
> >  net/core/netpoll.c | 20 +++++++++++++++++---
> >  1 file changed, 17 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> > index 60a05d3b7c249..c19dada9283ce 100644
> > --- a/net/core/netpoll.c
> > +++ b/net/core/netpoll.c
> > @@ -232,14 +232,28 @@ static void refill_skbs(struct netpoll *np)
> >  
> >  	skb_pool = &np->skb_pool;
> >  
> > -	spin_lock_irqsave(&skb_pool->lock, flags);
> > -	while (skb_pool->qlen < MAX_SKBS) {
> > +	while (1) {
> > +		spin_lock_irqsave(&skb_pool->lock, flags);
> > +		if (skb_pool->qlen >= MAX_SKBS)
> > +			goto unlock;
> > +		spin_unlock_irqrestore(&skb_pool->lock, flags);
> > +
> >  		skb = alloc_skb(MAX_SKB_SIZE, GFP_ATOMIC);
> >  		if (!skb)
> > -			break;
> > +			return;
> >  
> > +		spin_lock_irqsave(&skb_pool->lock, flags);
> > +		if (skb_pool->qlen >= MAX_SKBS)
> > +			/* Discard if len got increased (TOCTOU) */
> > +			goto discard;
> >  		__skb_queue_tail(skb_pool, skb);
> > +		spin_unlock_irqrestore(&skb_pool->lock, flags);
> >  	}
> > +
> > +	return;
> 
> Maybe it is worth leaving alone for clarity.
> And certainly it does no harm.
> But the line above is never reached.

Thanks for the catch. I will remove it since it is useless and respin.