Re: [PATCH v1 net 1/2] ipv4: fib: Don't dump dying fib_info in fib_leaf_notify().

[David Ahern <dsahern@kernel.org>]

On 6/10/26 12:17 AM, Kuniyuki Iwashima wrote:
> syzbot reported use-after-free in nsim_fib4_prepare_event(). [0]
> 
> The problem is that the following functions call fib_info_hold() /
> refcount_inc() while dumping fib_info under RCU, which is unsafe.
> 
>   * mlxsw_sp_router_fib4_event()
>   * rocker_router_fib_event()
>   * nsim_fib4_prepare_event()
> 
> refcount_inc_not_zero() must be used, but it would be too late
> there.
> 
> Let's guarantee the lifetime of fib_info in fib_leaf_notify().
> 
> Note that IPv6 does not need the corresponding change since
> fib6_table_dump() holds fib6_table.tb6_lock.
> 
> [0]:
> refcount_t: addition on 0; use-after-free.
> WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#0: kworker/u8:15/3420
> Modules linked in:
> CPU: 0 UID: 0 PID: 3420 Comm: kworker/u8:15 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
> Workqueue: netns cleanup_net
> RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
> Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 1b f1 22 fd 48 8d 3d 84 cb f1 0a 67 48 0f b9 3a eb 4a e8 08 f1 22 fd 48 8d 3d 81 cb f1 0a <67> 48 0f b9 3a eb 37 e8 f5 f0 22 fd 48 8d 3d 7e cb f1 0a 67 48 0f
> RSP: 0018:ffffc9000f2c7270 EFLAGS: 00010293
> RAX: ffffffff84a18858 RBX: 0000000000000002 RCX: ffff888032ff9ec0
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8f9353e0
> RBP: 0000000000000000 R08: ffff888032ff9ec0 R09: 0000000000000005
> R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880570cc000
> R13: dffffc0000000000 R14: ffff88802b40563c R15: ffff8880570cc000
> FS:  0000000000000000(0000) GS:ffff888126173000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fb1f4d5d000 CR3: 000000006072a000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  __refcount_add include/linux/refcount.h:-1 [inline]
>  __refcount_inc include/linux/refcount.h:366 [inline]
>  refcount_inc include/linux/refcount.h:383 [inline]
>  fib_info_hold include/net/ip_fib.h:629 [inline]
>  nsim_fib4_prepare_event drivers/net/netdevsim/fib.c:930 [inline]
>  nsim_fib_event_schedule_work drivers/net/netdevsim/fib.c:1000 [inline]
>  nsim_fib_event_nb+0x1055/0x1240 drivers/net/netdevsim/fib.c:1043
>  call_fib_notifier+0x45/0x80 net/core/fib_notifier.c:25
>  call_fib_entry_notifier net/ipv4/fib_trie.c:90 [inline]
>  fib_leaf_notify net/ipv4/fib_trie.c:2176 [inline]
>  fib_table_notify net/ipv4/fib_trie.c:2194 [inline]
>  fib_notify+0x36b/0x5e0 net/ipv4/fib_trie.c:2217
>  fib_net_dump net/core/fib_notifier.c:70 [inline]
>  register_fib_notifier+0x184/0x360 net/core/fib_notifier.c:108
>  nsim_fib_create+0x85d/0x9f0 drivers/net/netdevsim/fib.c:1596
>  nsim_dev_reload_create drivers/net/netdevsim/dev.c:1604 [inline]
>  nsim_dev_reload_up+0x374/0x7c0 drivers/net/netdevsim/dev.c:1058
>  devlink_reload+0x501/0x8d0 net/devlink/dev.c:475
>  devlink_pernet_pre_exit+0x1ff/0x420 net/devlink/core.c:558
>  ops_pre_exit_list net/core/net_namespace.c:161 [inline]
>  ops_undo_list+0x187/0x940 net/core/net_namespace.c:234
>  cleanup_net+0x56e/0x800 net/core/net_namespace.c:702
>  process_one_work kernel/workqueue.c:3314 [inline]
>  process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3397
>  worker_thread+0xa53/0xfc0 kernel/workqueue.c:3478
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> 
> Fixes: 0ae3eb7b4611 ("netdevsim: fib: Perform the route programming in a non-atomic context")
> Fixes: c3852ef7f2f8 ("ipv4: fib: Replay events when registering FIB notifier")
> Reported-by: syzbot+cb2aa2390ac024e25f5c@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/6a290011.39669fcc.33b062.00b1.GAE@google.com/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  include/net/ip_fib.h | 5 +++++
>  net/ipv4/fib_trie.c  | 4 ++++
>  2 files changed, 9 insertions(+)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>