[PATCH 4.9 061/102] net: check before dereferencing netdev_ops during busy poll

[Greg Kroah-Hartman]

From: Josh Elsasser <jelsasser@xxxxxxxxxxx>

init_dummy_netdev() leaves its netdev_ops pointer zeroed. This leads
to a NULL pointer dereference when sk_busy_loop fires against an iwlwifi
wireless adapter and checks napi->dev->netdev_ops->ndo_busy_poll.

Avoid this by ensuring napi->dev->netdev_ops is valid before following
the pointer, avoiding the following panic when busy polling on a dummy
netdev:

  BUG: unable to handle kernel NULL pointer dereference at 00000000000000c8
  IP: [<ffffffff817b4b72>] sk_busy_loop+0x92/0x2f0
  Call Trace:
   [<ffffffff815a3134>] ? uart_write_room+0x74/0xf0
   [<ffffffff817964a9>] sock_poll+0x99/0xa0
   [<ffffffff81223142>] do_sys_poll+0x2e2/0x520
   [<ffffffff8118d3fc>] ? get_page_from_freelist+0x3bc/0xa30
   [<ffffffff810ada22>] ? update_curr+0x62/0x140
   [<ffffffff811ea671>] ? __slab_free+0xa1/0x2a0
   [<ffffffff811ea671>] ? __slab_free+0xa1/0x2a0
   [<ffffffff8179dbb1>] ? skb_free_head+0x21/0x30
   [<ffffffff81221bd0>] ? poll_initwait+0x50/0x50
   [<ffffffff811eaa36>] ? kmem_cache_free+0x1c6/0x1e0
   [<ffffffff815a4884>] ? uart_write+0x124/0x1d0
   [<ffffffff810bd1cd>] ? remove_wait_queue+0x4d/0x60
   [<ffffffff810bd224>] ? __wake_up+0x44/0x50
   [<ffffffff81582731>] ? tty_write_unlock+0x31/0x40
   [<ffffffff8158c5c6>] ? tty_ldisc_deref+0x16/0x20
   [<ffffffff81584820>] ? tty_write+0x1e0/0x2f0
   [<ffffffff81587e50>] ? process_echoes+0x80/0x80
   [<ffffffff8120c17b>] ? __vfs_write+0x2b/0x130
   [<ffffffff8120d09a>] ? vfs_write+0x15a/0x1a0
   [<ffffffff81223455>] SyS_poll+0x75/0x100
   [<ffffffff819a6524>] entry_SYSCALL_64_fastpath+0x24/0xcf

Commit 79e7fff47b7b ("net: remove support for per driver ndo_busy_poll()")
indirectly fixed this upstream in linux-4.11 by removing the offending
pointer usage. No other users of napi->dev touch its netdev_ops.

Fixes: ce6aea93f751 ("net: network drivers no longer need to implement ndo_busy_poll()") # 4.9.y
Signed-off-by: Josh Elsasser <jelsasser@xxxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Tested-by: Matteo Croce <mcroce@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---

No changes since V2[1], resent as per discussiond on -stable[2]. I hope
this is the correct way to send net fixes for older LTS releases, I'm
going off of the latest netdev FAQ:

   For earlier stable releases, each stable branch maintainer is supposed
   to take care of them. If you find any patch is missing from an earlier
   stable branch, please notify stable@xxxxxxxxxxxxxxx with either a commit
   ID or a formal patch backported, and CC Dave and other relevant networking
   developers.

[1]: https://patchwork.ozlabs.org/patch/884986/
[2]: https://lore.kernel.org/stable/CAGnkfhx3ykbEsW+=FtpMFWU=_Vnie7RpPYWpWqa1S1HPMXj9kw@xxxxxxxxxxxxxx/


 net/core/dev.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)


--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5083,7 +5083,10 @@ bool sk_busy_loop(struct sock *sk, int n
 		goto out;
 
 	/* Note: ndo_busy_poll method is optional in linux-4.5 */
-	busy_poll = napi->dev->netdev_ops->ndo_busy_poll;
+	if (napi->dev->netdev_ops)
+		busy_poll = napi->dev->netdev_ops->ndo_busy_poll;
+	else
+		busy_poll = NULL;
 
 	do {
 		rc = 0;