6.17-stable review patch. If anyone has any objections, please let me know.
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From: Breno Leitao leitao@debian.org
[ Upstream commit 327c20c21d80e0d87834b392d83ae73c955ad8ff ]
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 called from 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 this 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.
There is a possible TOCTOU issue when checking for the pool length, and queueing the new allocated skb, but, this is not an issue, given that an extra SKB in the pool is harmless and it will be eventually used.
Signed-off-by: Breno Leitao leitao@debian.org Fixes: 248f6571fd4c51 ("netpoll: Optimize skb refilling on critical path") Reviewed-by: Simon Horman horms@kernel.org Link: https://patch.msgid.link/20251103-fix_netpoll_aa-v4-1-4cfecdf6da7c@debian.or... Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/core/netpoll.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/net/core/netpoll.c b/net/core/netpoll.c index 5f65b62346d4e..4ab154c614e33 100644 --- a/net/core/netpoll.c +++ b/net/core/netpoll.c @@ -228,19 +228,16 @@ static void refill_skbs(struct netpoll *np) { struct sk_buff_head *skb_pool; struct sk_buff *skb; - unsigned long flags;
skb_pool = &np->skb_pool;
- spin_lock_irqsave(&skb_pool->lock, flags); - while (skb_pool->qlen < MAX_SKBS) { + while (READ_ONCE(skb_pool->qlen) < MAX_SKBS) { skb = alloc_skb(MAX_SKB_SIZE, GFP_ATOMIC); if (!skb) break;
- __skb_queue_tail(skb_pool, skb); + skb_queue_tail(skb_pool, skb); } - spin_unlock_irqrestore(&skb_pool->lock, flags); }
static void zap_completion_queue(void)