kcm: Fix unnecessary psock unreservation.

kcm_write_msgs() calls unreserve_psock() to release its hold on the
underlying TCP socket if it has run out of things to transmit, but if we
have nothing in the write queue on entry (e.g. because someone did a
zero-length sendmsg), we don't actually go into the transmission loop and
as a consequence don't call reserve_psock().

Fix this by skipping the call to unreserve_psock() if we didn't reserve a
psock.

Fixes: c31a25e1db48 ("kcm: Send multiple frags in one sendmsg()")
Reported-by: syzbot+dd1339599f1840e4cc65@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/000000000000a61ffe05fe0c3d08@google.com/
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: syzbot+dd1339599f1840e4cc65@syzkaller.appspotmail.com
cc: Tom Herbert <tom@herbertland.com>
cc: Tom Herbert <tom@quantonium.net>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Link: https://lore.kernel.org/r/20787.1686828722@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index d75d775e9462a8e0083ca3b60611dff8da3840d6..d0537c1c8cd79218572b560bd1a137a1d94cab10 100644 (file)
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -661,6 +661,7 @@ retry:
                                kcm_abort_tx_psock(psock, ret ? -ret : EPIPE,
                                                   true);
                                unreserve_psock(kcm);
+                               psock = NULL;
 
                                txm->started_tx = false;
                                kcm_report_tx_retry(kcm);
@@ -696,7 +697,8 @@ out:
        if (!head) {
                /* Done with all queued messages. */
                WARN_ON(!skb_queue_empty(&sk->sk_write_queue));
-               unreserve_psock(kcm);
+               if (psock)
+                       unreserve_psock(kcm);
        }
 
        /* Check if write space is available */