Re: [PATCH] nfc: llcp: Fix race in handling llcp_devices

[Dongliang Mu]

On Tue, Nov 29, 2022 at 5:46 PM 'Wang ShaoBo' via syzkaller-bugs
<syzkaller-bugs@xxxxxxxxxxxxxxxx> wrote:
>
> There are multiple path operate llcp_devices list without protection:
>
>          CPU0                        CPU1
>
> nfc_unregister_device()        nfc_register_device()
>  nfc_llcp_unregister_device()    nfc_llcp_register_device() //no lock
>     ...                            list_add(local->list, llcp_devices)
>     local_release()
>       list_del(local->list)
>
>         CPU2
> ...
>  nfc_llcp_find_local()
>    list_for_each_entry(,&llcp_devices,)
>
> So reach race condition if two of the three occur simultaneously like
> following crash report, although there is no reproduction script in
> syzbot currently, our artificially constructed use cases can also
> reproduce it:
>
> list_del corruption. prev->next should be ffff888060ce7000, but was ffff88802a0ad000. (prev=ffffffff8e536240)
> ------------[ cut here ]------------
> kernel BUG at lib/list_debug.c:59!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 0 PID: 16622 Comm: syz-executor.5 Not tainted 6.1.0-rc6-next-20221125-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
> RIP: 0010:__list_del_entry_valid.cold+0x12/0x72 lib/list_debug.c:59
> Code: f0 ff 0f 0b 48 89 f1 48 c7 c7 60 96 a6 8a 4c 89 e6 e8 4b 29 f0 ff 0f 0b 4c 89 e1 48 89 ee 48 c7 c7 c0 98 a6 8a e8 37 29 f0 ff <0f> 0b 48 89 ee 48 c7 c7 a0 97 a6 8a e8 26 29 f0 ff 0f 0b 4c 89 e2
> RSP: 0018:ffffc900151afd58 EFLAGS: 00010282
> RAX: 000000000000006d RBX: 0000000000000001 RCX: 0000000000000000
> RDX: ffff88801e7eba80 RSI: ffffffff8166001c RDI: fffff52002a35f9d
> RBP: ffff888060ce7000 R08: 000000000000006d R09: 0000000000000000
> R10: 0000000080000000 R11: 0000000000000000 R12: ffffffff8e536240
> R13: ffff88801f3f3000 R14: ffff888060ce1000 R15: ffff888079d855f0
> FS:  0000555556f57400(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f095d5ad988 CR3: 000000002155a000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  __list_del_entry include/linux/list.h:134 [inline]
>  list_del include/linux/list.h:148 [inline]
>  local_release net/nfc/llcp_core.c:171 [inline]
>  kref_put include/linux/kref.h:65 [inline]
>  nfc_llcp_local_put net/nfc/llcp_core.c:181 [inline]
>  nfc_llcp_local_put net/nfc/llcp_core.c:176 [inline]
>  nfc_llcp_unregister_device+0xb8/0x260 net/nfc/llcp_core.c:1619
>  nfc_unregister_device+0x196/0x330 net/nfc/core.c:1179
>  virtual_ncidev_close+0x52/0xb0 drivers/nfc/virtual_ncidev.c:163
>  __fput+0x27c/0xa90 fs/file_table.c:320
>  task_work_run+0x16f/0x270 kernel/task_work.c:179
>  resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
>  exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
>  exit_to_user_mode_prepare+0x23c/0x250 kernel/entry/common.c:203
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
>  syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
>  do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> This patch add specific mutex lock llcp_devices_list_lock to ensure
> handling llcp_devices list safety.
>
> Fixes: 30cc4587659e ("NFC: Move LLCP code to the NFC top level diirectory")
> Reported-by: syzbot+81232c4a81a886e2b580@xxxxxxxxxxxxxxxxxxxxxxxxx

There is another syzbot bug report [1] contributed by the same bug. It
has the syz reproducer. A patch testing has been deployed on the
syzbot dashboard.

Besides, I have a doubt about this patch. This patch adds a mutex lock
to make list operations(add, delete, find etc.) exclusive. If the
following thread interleaving occurs, will this patch work?

CPU0                                         CPU2
nfc_llcp_unregister_device        nfc_llcp_unregister_device
    ......                                             nfc_llcp_find_local
    local_release                              ......
        list_del                                    ......
                                                       local_cleanup
                                                       local_llcp_local_put

If nfc_llcp_find_local executes before list deletion, it will still
lead to double free and UAF. Please correct me if I make any mistakes.
Thanks in advance.

[1] https://syzkaller.appspot.com/bug?id=41d9ed9b6dcd7b7c5611ed5eb64835b1a554e998

> Signed-off-by: Wang ShaoBo <bobo.shaobowang@xxxxxxxxxx>
> ---
>  net/nfc/llcp_core.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
> index 3364caabef8b..7deaecd9d3cd 100644
> --- a/net/nfc/llcp_core.c
> +++ b/net/nfc/llcp_core.c
> @@ -17,6 +17,7 @@
>  static u8 llcp_magic[3] = {0x46, 0x66, 0x6d};
>
>  static LIST_HEAD(llcp_devices);
> +static DEFINE_MUTEX(llcp_devices_list_lock);
>
>  static void nfc_llcp_rx_skb(struct nfc_llcp_local *local, struct sk_buff *skb);
>
> @@ -168,7 +169,9 @@ static void local_release(struct kref *ref)
>
>         local = container_of(ref, struct nfc_llcp_local, ref);
>
> +       mutex_lock(&llcp_devices_list_lock);
>         list_del(&local->list);
> +       mutex_unlock(&llcp_devices_list_lock);
>         local_cleanup(local);
>         kfree(local);
>  }
> @@ -282,9 +285,13 @@ struct nfc_llcp_local *nfc_llcp_find_local(struct nfc_dev *dev)
>  {
>         struct nfc_llcp_local *local;
>
> +       mutex_lock(&llcp_devices_list_lock);
>         list_for_each_entry(local, &llcp_devices, list)
> -               if (local->dev == dev)
> +               if (local->dev == dev) {
> +                       mutex_unlock(&llcp_devices_list_lock);
>                         return local;
> +               }
> +       mutex_unlock(&llcp_devices_list_lock);
>
>         pr_debug("No device found\n");
>
> @@ -1600,7 +1607,9 @@ int nfc_llcp_register_device(struct nfc_dev *ndev)
>         timer_setup(&local->sdreq_timer, nfc_llcp_sdreq_timer, 0);
>         INIT_WORK(&local->sdreq_timeout_work, nfc_llcp_sdreq_timeout_work);
>
> +       mutex_lock(&llcp_devices_list_lock);
>         list_add(&local->list, &llcp_devices);
> +       mutex_unlock(&llcp_devices_list_lock);
>
>         return 0;
>  }
> --
> 2.25.1
>
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