[PATCH net-next] bpf: fix a possible NULL deref in bpf_map_offload_map_alloc()

[Eric Dumazet <edumazet@google.com>]

Call bpf_dev_offload_check() before netdev_lock_ops().

This is needed if attr->map_ifindex is not valid.

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000197: 0000 [#1] PREEMPT SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000cb8-0x0000000000000cbf]
 RIP: 0010:netdev_need_ops_lock include/linux/netdevice.h:2792 [inline]
 RIP: 0010:netdev_lock_ops include/linux/netdevice.h:2803 [inline]
 RIP: 0010:bpf_map_offload_map_alloc+0x19a/0x910 kernel/bpf/offload.c:533
Call Trace:
 <TASK>
  map_create+0x946/0x11c0 kernel/bpf/syscall.c:1455
  __sys_bpf+0x6d3/0x820 kernel/bpf/syscall.c:5777
  __do_sys_bpf kernel/bpf/syscall.c:5902 [inline]
  __se_sys_bpf kernel/bpf/syscall.c:5900 [inline]
  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5900
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83

Fixes: 97246d6d21c2 ("net: hold netdev instance lock during ndo_bpf")
Reported-by: syzbot+0c7bfd8cf3aecec92708@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/67caa2b1.050a0220.15b4b9.0077.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 kernel/bpf/offload.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
index a10153c3be2df675af49f0ad9c70b64b83593f45..4f707cfe7f103c8a480d534cddcc75e958f0bd57 100644
--- a/kernel/bpf/offload.c
+++ b/kernel/bpf/offload.c
@@ -530,11 +530,12 @@ struct bpf_map *bpf_map_offload_map_alloc(union bpf_attr *attr)
 	bpf_map_init_from_attr(&offmap->map, attr);
 	rtnl_lock();
 	offmap->netdev = __dev_get_by_index(net, attr->map_ifindex);
-	netdev_lock_ops(offmap->netdev);
-	down_write(&bpf_devs_lock);
 	err = bpf_dev_offload_check(offmap->netdev);
 	if (err)
-		goto err_unlock;
+		goto err_unlock_rtnl;
+
+	netdev_lock_ops(offmap->netdev);
+	down_write(&bpf_devs_lock);
 
 	ondev = bpf_offload_find_netdev(offmap->netdev);
 	if (!ondev) {
@@ -556,6 +557,7 @@ struct bpf_map *bpf_map_offload_map_alloc(union bpf_attr *attr)
 err_unlock:
 	up_write(&bpf_devs_lock);
 	netdev_unlock_ops(offmap->netdev);
+err_unlock_rtnl:
 	rtnl_unlock();
 	bpf_map_area_free(offmap);
 	return ERR_PTR(err);
-- 
2.49.0.rc0.332.g42c0ae87b1-goog