Re: net/kcm: WARNING in kcm_write_msgs

From: Cong Wang
Date: Mon Feb 06 2017 - 18:46:56 EST


On Mon, Feb 6, 2017 at 4:43 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> [resending as plain text]
>
> Hello,
>
> The following program triggers WARNING in kcm_write_msgs:
>
> WARNING: CPU: 3 PID: 2936 at net/kcm/kcmsock.c:627
> kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
> CPU: 3 PID: 2936 Comm: a.out Not tainted 4.10.0-rc6+ #209
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:15 [inline]
> dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
> panic+0x1fb/0x412 kernel/panic.c:179
> __warn+0x1c4/0x1e0 kernel/panic.c:539
> warn_slowpath_null+0x2c/0x40 kernel/panic.c:582
> kcm_write_msgs+0x12e3/0x1b90 net/kcm/kcmsock.c:627
> kcm_sendmsg+0x163a/0x2200 net/kcm/kcmsock.c:1029
> sock_sendmsg_nosec net/socket.c:635 [inline]
> sock_sendmsg+0xca/0x110 net/socket.c:645
> sock_write_iter+0x326/0x600 net/socket.c:848
> new_sync_write fs/read_write.c:499 [inline]
> __vfs_write+0x483/0x740 fs/read_write.c:512
> vfs_write+0x187/0x530 fs/read_write.c:560
> SYSC_write fs/read_write.c:607 [inline]
> SyS_write+0xfb/0x230 fs/read_write.c:599
> entry_SYSCALL_64_fastpath+0x1f/0xc2
[...]
> syscall(__NR_write, sock2, 0x208aaf27ul, 0x0ul);

Looks like len == 0 case is not handled correctly in kcm_sendmsg().
The attached patch fixes it, but I am not sure if it is correct in all
cases yet, the logic is complicated.
diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 7e08a4d..64f0e85 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -929,23 +929,25 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
goto out_error;
}

- /* New message, alloc head skb */
- head = alloc_skb(0, sk->sk_allocation);
- while (!head) {
- kcm_push(kcm);
- err = sk_stream_wait_memory(sk, &timeo);
- if (err)
- goto out_error;
-
+ if (msg_data_left(msg)) {
+ /* New message, alloc head skb */
head = alloc_skb(0, sk->sk_allocation);
- }
+ while (!head) {
+ kcm_push(kcm);
+ err = sk_stream_wait_memory(sk, &timeo);
+ if (err)
+ goto out_error;

- skb = head;
+ head = alloc_skb(0, sk->sk_allocation);
+ }

- /* Set ip_summed to CHECKSUM_UNNECESSARY to avoid calling
- * csum_and_copy_from_iter from skb_do_copy_data_nocache.
- */
- skb->ip_summed = CHECKSUM_UNNECESSARY;
+ skb = head;
+
+ /* Set ip_summed to CHECKSUM_UNNECESSARY to avoid calling
+ * csum_and_copy_from_iter from skb_do_copy_data_nocache.
+ */
+ skb->ip_summed = CHECKSUM_UNNECESSARY;
+ }

start:
while (msg_data_left(msg)) {
@@ -1018,10 +1020,12 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
if (eor) {
bool not_busy = skb_queue_empty(&sk->sk_write_queue);

- /* Message complete, queue it on send buffer */
- __skb_queue_tail(&sk->sk_write_queue, head);
- kcm->seq_skb = NULL;
- KCM_STATS_INCR(kcm->stats.tx_msgs);
+ if (head) {
+ /* Message complete, queue it on send buffer */
+ __skb_queue_tail(&sk->sk_write_queue, head);
+ kcm->seq_skb = NULL;
+ KCM_STATS_INCR(kcm->stats.tx_msgs);
+ }

if (msg->msg_flags & MSG_BATCH) {
kcm->tx_wait_more = true;