[PATCH] net: pktgen: fix proc entry use-after-free

[Chengfeng Ye <nicoyip.dev@gmail.com>]

pktgen_change_name() replaces pkt_dev->entry while holding t->if_lock.
pktgen_remove_device() removes the same entry before
_rem_dev_from_if_list() takes that lock.

This allows the following interleaving:

  CPU 0 (NETDEV_CHANGENAME)       CPU 1 (kpktgend)
  if_lock(t)
  proc_remove(pkt_dev->entry)
                                  proc_remove(pkt_dev->entry)
  pkt_dev->entry = proc_create_data(...)
  if_unlock(t)

The kthread can pass the stale proc_dir_entry to proc_remove() after the
rename path has freed it. A reproducer with a widened race window reports:

  BUG: KASAN: slab-use-after-free in proc_remove+0x78/0x80
  Read of size 8 at addr ffff8881478fea70 by task kpktgend_0/67
  Call Trace:
   proc_remove+0x78/0x80
   pktgen_remove_device.isra.0+0x11c/0x4c0
   pktgen_thread_worker+0x1214/0x6bc0
   kthread+0x2c6/0x3b0
  Allocated by task 95:
   __proc_create+0x204/0x790
   proc_create_data+0x72/0xe0
   pktgen_thread_write+0xd61/0x1510
  Freed by task 28:
   kmem_cache_free+0xcb/0x3d0
   proc_free_inode+0x5b/0x80
   rcu_core+0x50a/0x1850
  The buggy address belongs to the object at ffff8881478fea00
   which belongs to the cache proc_dir_entry of size 192

Move proc_remove() into the if_lock-protected list removal helper. Keep it
before list_del_rcu() to preserve the ordering required by add_device().
The rename path must then finish replacing the entry before removal, or
it observes that the device is no longer on the list.

Fixes: 39df232f1a9b ("[PKTGEN]: fix device name handling")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/core/pktgen.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index 8e185b318288..ee64f3012321 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -3972,6 +3972,7 @@ static void _rem_dev_from_if_list(struct pktgen_thread *t,
 	struct pktgen_dev *p;
 
 	if_lock(t);
+	proc_remove(pkt_dev->entry);
 	list_for_each_safe(q, n, &t->if_list) {
 		p = list_entry(q, struct pktgen_dev, list);
 		if (p == pkt_dev)
@@ -4001,9 +4002,6 @@ static int pktgen_remove_device(struct pktgen_thread *t,
 	 * list to determine if interface already exist, avoid race
 	 * with proc_create_data()
 	 */
-	proc_remove(pkt_dev->entry);
-
-	/* And update the thread if_list */
 	_rem_dev_from_if_list(t, pkt_dev);
 
 #ifdef CONFIG_XFRM
-- 
2.43.0