netlink: make sure we allow at least one dump skb

Commit under Fixes tightened up the memory accounting for Netlink
sockets. Looks like the accounting is too strict for some existing
use cases, Marek reported issues with nl80211 / WiFi iw CLI.

To reduce number of iterations Netlink dumps try to allocate
messages based on the size of the buffer passed to previous
recvmsg() calls. If user space uses a larger buffer in recvmsg()
than sk_rcvbuf we will allocate an skb we won't be able to queue.

Make sure we always allow at least one skb to be queued.
Same workaround is already present in netlink_attachskb().
Alternative would be to cap the allocation size to
  rcvbuf - rmem_alloc
but as I said, the workaround is already present in other places.

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Link: https://lore.kernel.org/9794af18-4905-46c6-b12c-365ea2f05858@samsung.com
Fixes: ae8f160e7eb2 ("netlink: Fix wraparounds of sk->sk_rmem_alloc.")
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250711001121.3649033-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2d107a8a75d993738ffee837ca774b60a06da12e..6332a0e06596758ab439a58dea9fa8175cfe9883 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2258,11 +2258,11 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
        struct netlink_ext_ack extack = {};
        struct netlink_callback *cb;
        struct sk_buff *skb = NULL;
+       unsigned int rmem, rcvbuf;
        size_t max_recvmsg_len;
        struct module *module;
        int err = -ENOBUFS;
        int alloc_min_size;
-       unsigned int rmem;
        int alloc_size;
 
        if (!lock_taken)
@@ -2294,8 +2294,9 @@ static int netlink_dump(struct sock *sk, bool lock_taken)
        if (!skb)
                goto errout_skb;
 
+       rcvbuf = READ_ONCE(sk->sk_rcvbuf);
        rmem = atomic_add_return(skb->truesize, &sk->sk_rmem_alloc);
-       if (rmem >= READ_ONCE(sk->sk_rcvbuf)) {
+       if (rmem != skb->truesize && rmem >= rcvbuf) {
                atomic_sub(skb->truesize, &sk->sk_rmem_alloc);
                goto errout_skb;
        }