Re: [PATCH v1 net] af_unix: Call sk_diag_fill() under the bucket lock.

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2022-11-22 at 12:58 -0800, Kuniyuki Iwashima wrote:
> Wei Chen reported sk->sk_socket can be NULL in sk_user_ns(). [0][1]
> 
> It seems that syzbot was dumping an AF_UNIX socket while closing it,
> and there is a race below.
> 
>   unix_release_sock               unix_diag_handler_dump
>   |                               `- unix_diag_get_exact
>   |                                  |- unix_lookup_by_ino
>   |                                  |  |- spin_lock(&net->unx.table.locks[i])
>   |                                  |  |- sock_hold
>   |                                  |  `- spin_unlock(&net->unx.table.locks[i])
>   |- unix_remove_socket(net, sk)     |     /* after releasing this lock,
>   |  /* from here, sk cannot be      |      * there is no guarantee that
>   |   * seen in the hash table.      |      * sk is not SOCK_DEAD.
>   |   */                             |      */
>   |                                  |
>   |- unix_state_lock(sk)             |
>   |- sock_orphan(sk)                 `- sk_diag_fill
>   |  |- sock_set_flag(sk, SOCK_DEAD)    `- sk_diag_dump_uid
>   |  `- sk_set_socket(sk, NULL)            `- sk_user_ns
>   `- unix_state_unlock(sk)                   `- sk->sk_socket->file->f_cred->user_ns
>                                                 /* NULL deref here */
> 
> After releasing the bucket lock, we cannot guarantee that the found
> socket is still alive.  Then, we have to check the SOCK_DEAD flag
> under unix_state_lock() and keep holding it unless we access the socket.
> 
> In this case, however, we cannot acquire unix_state_lock() in
> unix_lookup_by_ino() because we lock it later in sk_diag_dump_peer(),
> resulting in deadlock.
> 
> Instead, we do not release the bucket lock; then, we can safely access
> sk->sk_socket later in sk_user_ns(), and there is no deadlock scenario.
> We are already using this strategy in unix_diag_dump().
> 
> Note we have to call nlmsg_new() before unix_lookup_by_ino() not to
> change the flag from GFP_KERNEL to GFP_ATOMIC.
> 
> [0]: https://lore.kernel.org/netdev/CAO4mrfdvyjFpokhNsiwZiP-wpdSD0AStcJwfKcKQdAALQ9_2Qw@mail.gmail.com/
> [1]:
> BUG: kernel NULL pointer dereference, address: 0000000000000270
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 12bbce067 P4D 12bbce067 PUD 12bc40067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP
> CPU: 0 PID: 27942 Comm: syz-executor.0 Not tainted 6.1.0-rc5-next-20221118 #2
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> rel-1.13.0-48-gd9c812dda519-prebuilt.qemu.org 04/01/2014
> RIP: 0010:sk_user_ns include/net/sock.h:920 [inline]
> RIP: 0010:sk_diag_dump_uid net/unix/diag.c:119 [inline]
> RIP: 0010:sk_diag_fill+0x77d/0x890 net/unix/diag.c:170
> Code: 89 ef e8 66 d4 2d fd c7 44 24 40 00 00 00 00 49 8d 7c 24 18 e8
> 54 d7 2d fd 49 8b 5c 24 18 48 8d bb 70 02 00 00 e8 43 d7 2d fd <48> 8b
> 9b 70 02 00 00 48 8d 7b 10 e8 33 d7 2d fd 48 8b 5b 10 48 8d
> RSP: 0018:ffffc90000d67968 EFLAGS: 00010246
> RAX: ffff88812badaa48 RBX: 0000000000000000 RCX: ffffffff840d481d
> RDX: 0000000000000465 RSI: 0000000000000000 RDI: 0000000000000270
> RBP: ffffc90000d679a8 R08: 0000000000000277 R09: 0000000000000000
> R10: 0001ffffffffffff R11: 0001c90000d679a8 R12: ffff88812ac03800
> R13: ffff88812c87c400 R14: ffff88812ae42210 R15: ffff888103026940
> FS:  00007f08b4e6f700(0000) GS:ffff88813bc00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000270 CR3: 000000012c58b000 CR4: 00000000003506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  unix_diag_get_exact net/unix/diag.c:285 [inline]
>  unix_diag_handler_dump+0x3f9/0x500 net/unix/diag.c:317
>  __sock_diag_cmd net/core/sock_diag.c:235 [inline]
>  sock_diag_rcv_msg+0x237/0x250 net/core/sock_diag.c:266
>  netlink_rcv_skb+0x13e/0x250 net/netlink/af_netlink.c:2564
>  sock_diag_rcv+0x24/0x40 net/core/sock_diag.c:277
>  netlink_unicast_kernel net/netlink/af_netlink.c:1330 [inline]
>  netlink_unicast+0x5e9/0x6b0 net/netlink/af_netlink.c:1356
>  netlink_sendmsg+0x739/0x860 net/netlink/af_netlink.c:1932
>  sock_sendmsg_nosec net/socket.c:714 [inline]
>  sock_sendmsg net/socket.c:734 [inline]
>  ____sys_sendmsg+0x38f/0x500 net/socket.c:2476
>  ___sys_sendmsg net/socket.c:2530 [inline]
>  __sys_sendmsg+0x197/0x230 net/socket.c:2559
>  __do_sys_sendmsg net/socket.c:2568 [inline]
>  __se_sys_sendmsg net/socket.c:2566 [inline]
>  __x64_sys_sendmsg+0x42/0x50 net/socket.c:2566
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
> RIP: 0033:0x4697f9
> Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 48 89 f8 48
> 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d
> 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f08b4e6ec48 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 000000000077bf80 RCX: 00000000004697f9
> RDX: 0000000000000000 RSI: 00000000200001c0 RDI: 0000000000000003
> RBP: 00000000004d29e9 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000077bf80
> R13: 0000000000000000 R14: 000000000077bf80 R15: 00007ffdb36bc6c0
>  </TASK>
> Modules linked in:
> CR2: 0000000000000270

I *think* that the root cause of the above splat could be
different/simpler. In unix_diag_get_exact():

	
	rep = nlmsg_new(sizeof(struct unix_diag_msg) + extra_len, GFP_KERNEL);
        if (!rep)
                goto out;

	// rep->sk is NULL now.
        err = sk_diag_fill(sk, rep, req, NETLINK_CB(in_skb).portid,
                           nlh->nlmsg_seq, 0, req->udiag_ino);

and sk_diag_fill() will splats deterministically. Note that the user
space don't trigger that code path usually, but it can be reproduced
with a sligthly modified 'ss' version.


Thanks,

Paolo