[PATCH 4.4.y] net: check before dereferencing netdev_ops during busy poll

[PATCH 4.4.y] net: check before dereferencing netdev_ops during busy poll

[Josh Elsasser]

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: 8b80cda536ea ("net: rename include/net/ll_poll.h to include/net/busy_poll.h") # 4.4.y
Signed-off-by: Josh Elsasser <jelsasser@appneta.com>
---

This is a straightforward backport of the 4.9.y fix[1] for this crash, which doesn't
apply to the older LTS releases. Only build-tested on 4.4.y, as I don't have access
to wireless hardware and firmware that runs on older LTS kernels.

[1]: https://lore.kernel.org/stable/20190701234143.72631-1-jelsasser@appneta.com/T/#u

 include/net/busy_poll.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
index 1d67fb6b23a0..6d238506d49b 100644
--- a/include/net/busy_poll.h
+++ b/include/net/busy_poll.h
@@ -93,7 +93,7 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
 		goto out;
 
 	ops = napi->dev->netdev_ops;
-	if (!ops->ndo_busy_poll)
+	if (!ops || !ops->ndo_busy_poll)
 		goto out;
 
 	do {
-- 
2.20.1