peerlookup: take lock before checking hash in replace operation

Eric reported that syzkaller found a race of this variety:

CPU 1                                       CPU 2
-------------------------------------------|---------------------------------------
wg_index_hashtable_replace(old, ...)       |
  if (hlist_unhashed(&old->index_hash))    |
                                           | wg_index_hashtable_remove(old)
                                           |   hlist_del_init_rcu(&old->index_hash)
				           |     old->index_hash.pprev = NULL
  hlist_replace_rcu(&old->index_hash, ...) |
    *old->index_hash.pprev                 |

The table->lock of the hash table is supposed to protect the bucket
heads, not the entires, but actually, since all the mutator functions
are already taking it, it makes sense to take it too for the test to
hlist_unhashed, so that it no longer races with deletions. This is fine
because, as Eric pointed out, the case of being unhashed is already the
unlikely case, so this won't add common contention. And comparing
instructions, this basically doesn't make much of a difference other
than pushing and popping %r13, used by the new `bool ret`.

The syzkaller crash is as follows:

  general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  CPU: 0 PID: 7395 Comm: kworker/0:3 Not tainted 5.9.0-rc4-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Workqueue: wg-kex-wg1 wg_packet_handshake_receive_worker
  RIP: 0010:hlist_replace_rcu include/linux/rculist.h:505 [inline]
  RIP: 0010:wg_index_hashtable_replace+0x176/0x330 drivers/net/wireguard/peerlookup.c:174
  Code: 00 fc ff df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 44 01 00 00 48 b9 00 00 00 00 00 fc ff df 48 8b 45 10 48 89 c6 48 c1 ee 03 <80> 3c 0e 00 0f 85 06 01 00 00 48 85 d2 4c 89 28 74 47 e8 a3 4f b5
  RSP: 0018:ffffc90006a97bf8 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff888050ffc4f8 RCX: dffffc0000000000
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88808e04e010
  RBP: ffff88808e04e000 R08: 0000000000000001 R09: ffff8880543d0000
  R10: ffffed100a87a000 R11: 000000000000016e R12: ffff8880543d0000
  R13: ffff88808e04e008 R14: ffff888050ffc508 R15: ffff888050ffc500
  FS:  0000000000000000(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00000000f5505db0 CR3: 0000000097cf7000 CR4: 00000000001526f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
  wg_noise_handshake_begin_session+0x752/0xc9a drivers/net/wireguard/noise.c:820
  wg_receive_handshake_packet drivers/net/wireguard/receive.c:183 [inline]
  wg_packet_handshake_receive_worker+0x33b/0x730 drivers/net/wireguard/receive.c:220
  process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
  worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
  kthread+0x3b5/0x4a0 kernel/kthread.c:292
  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
  Modules linked in:
  ---[ end trace 0d737db78b72da84 ]---
  RIP: 0010:hlist_replace_rcu include/linux/rculist.h:505 [inline]
  RIP: 0010:wg_index_hashtable_replace+0x176/0x330 drivers/net/wireguard/peerlookup.c:174
  Code: 00 fc ff df 48 89 f9 48 c1 e9 03 80 3c 01 00 0f 85 44 01 00 00 48 b9 00 00 00 00 00 fc ff df 48 8b 45 10 48 89 c6 48 c1 ee 03 <80> 3c 0e 00 0f 85 06 01 00 00 48 85 d2 4c 89 28 74 47 e8 a3 4f b5
  RSP: 0018:ffffc90006a97bf8 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff888050ffc4f8 RCX: dffffc0000000000
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88808e04e010
  RBP: ffff88808e04e000 R08: 0000000000000001 R09: ffff8880543d0000
  R10: ffffed100a87a000 R11: 000000000000016e R12: ffff8880543d0000
  R13: ffff88808e04e008 R14: ffff888050ffc508 R15: ffff888050ffc500
  FS:  0000000000000000(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00000000f5505db0 CR3: 0000000097cf7000 CR4: 00000000001526f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Author: zx2c4

src/peerlookup.c

@@ -167,9 +167,13 @@ bool wg_index_hashtable_replace(struct index_hashtable *table,
 				struct index_hashtable_entry *old,
 				struct index_hashtable_entry *new)
 {
-	if (unlikely(hlist_unhashed(&old->index_hash)))
-		return false;
+	bool ret;
+
 	spin_lock_bh(&table->lock);
+	ret = !hlist_unhashed(&old->index_hash);
+	if (unlikely(!ret))
+		goto out;
+
 	new->index = old->index;
 	hlist_replace_rcu(&old->index_hash, &new->index_hash);
 
@@ -180,8 +184,9 @@ bool wg_index_hashtable_replace(struct index_hashtable *table,
 	 * simply gets dropped, which isn't terrible.
 	 */
 	INIT_HLIST_NODE(&old->index_hash);
+out:
 	spin_unlock_bh(&table->lock);
-	return true;
+	return ret;
 }
 
 void wg_index_hashtable_remove(struct index_hashtable *table,