[PATCH net v3] netpoll: Fix deadlock in memory allocation under spinlock

[Breno Leitao <leitao@debian.org>]

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@debian.org>
Fixes: 248f6571fd4c51 ("netpoll: Optimize skb refilling on critical path")
---
Changes in v3:
- Removed the "return" before the exit labels. (Simon)
- Link to v2: https://lore.kernel.org/r/20251014-fix_netpoll_aa-v2-1-dafa6a378649@debian.org

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 | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 60a05d3b7c249..b8729ec1daeb8 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -232,14 +232,27 @@ 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);
 	}
+
+discard:
+	dev_kfree_skb_any(skb);
+unlock:
 	spin_unlock_irqrestore(&skb_pool->lock, flags);
 }
 

---
base-commit: c5705a2a4aa35350e504b72a94b5c71c3754833c
change-id: 20251013-fix_netpoll_aa-c991ac5f2138

Best regards,
--  
Breno Leitao <leitao@debian.org>