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charset="utf-8" Message-Id: <20261023-netlink-dump-livelock-v1-1-478214a635d2@gmail.com> References: <20261023-netlink-dump-livelock-v1-0-478214a635d2@gmail.com> In-Reply-To: <20261023-netlink-dump-livelock-v1-0-478214a635d2@gmail.com> To: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Shuah Khan Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Kuniyuki Iwashima , Jhonata Poma-Hansen X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=openpgp-sha256; l=2113; i=jhonata.poma@gmail.com; h=from:subject:message-id; bh=wpiSXo5U0PAbR1oO+UlU/ssinpjnck+7Z7pQkTuLTv8=; b=owGbwMvMwCX21S6Y7aK9oS7jabUkhqzbLv4TN7/T0dzqESTLuX5WpoopX9pVz5DKved6xaR7j x/ReqfdUcrCIMbFICumyOJz9nEPt+LHHf8f7rgMM4eVCWQIAxenAEyEj53hF9Pm395xN2rPsbas n3ZZWjZEf0/StDndDSGNKTeWfthwbSIjw8q23LKf0QdzH/J/F1dOWVp2kEHm+M/NNYr9m+aofwh 5ywIA X-Developer-Key: i=jhonata.poma@gmail.com; a=openpgp; fpr=4CCDE38C0B21F1B8FFE1B8D3F53E5306D13F312D X-Endpoint-Received: by B4 Relay for jhonata.poma@gmail.com/default with auth_id=1288 X-Original-From: Jhonata Poma-Hansen Reply-To: jhonata.poma@gmail.com Content-Transfer-Encoding: 7bit From: Jhonata Poma-Hansen netlink_dump() sizes the skb after the largest buffer the reader passed to recvmsg(), up to 32KiB. Since commit ae8f160e7eb2 ("netlink: Fix wraparounds of sk->sk_rmem_alloc.") the skb is only queued if it fits in sk_rcvbuf, or if nothing else is charged to the socket. A reader with a small receive buffer that still has data charged to its socket when a dump starts gets ENOBUFS, even when a minimum size skb would fit. The dump stays marked as running and every recvmsg() retries it, so a reader that peeks before reading, as libnl does, never consumes the data at the head of the queue and spins forever. "iw dev wlan0 scan" hits this: its socket has an 8KiB buffer, it joins the nl80211 multicast groups, and it dumps the scan results as soon as it has read the NEW_SCAN_RESULTS notification. When it preempts the sender, the notification is still charged to its socket until netlink_broadcast() returns, the dump fails to start and iw spins at 100% CPU. Retry with alloc_min_size before failing the dump. Fixes: ae8f160e7eb2 ("netlink: Fix wraparounds of sk->sk_rmem_alloc.") Signed-off-by: Jhonata Poma-Hansen --- net/netlink/af_netlink.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c index a904dc3bc..b750e79fe 100644 --- a/net/netlink/af_netlink.c +++ b/net/netlink/af_netlink.c @@ -2318,10 +2318,21 @@ static int netlink_dump(struct sock *sk, bool lock_taken) if (!skb) goto errout_skb; +retry: rcvbuf = READ_ONCE(sk->sk_rcvbuf); rmem = atomic_add_return(skb->truesize, &sk->sk_rmem_alloc); if (rmem != skb->truesize && rmem >= rcvbuf) { atomic_sub(skb->truesize, &sk->sk_rmem_alloc); + /* A skb sized after max_recvmsg_len may not fit next to data + * still charged to the socket, while a minimum size one does. + */ + if (alloc_size > alloc_min_size) { + kfree_skb(skb); + alloc_size = alloc_min_size; + skb = alloc_skb(alloc_size, GFP_KERNEL); + if (skb) + goto retry; + } goto errout_skb; } -- 2.54.0