Re: [PATCH net] net/packet: fix a race in packet_bind() and packet_notifier()

From: Eric Dumazet <edumazet@google.com>

syzbot reported crashes [1] and provided a C repro easing bug hunting.

When/if packet_do_bind() calls __unregister_prot_hook() and releases
po->bind_lock, another thread can run packet_notifier() and process an
NETDEV_UP event.

This calls register_prot_hook() and hook again the socket right before
first thread was able to grab again po->bind_lock.

Fixes this issue by adding po->frozen bit :

It is set and cleared by __unregister_prot_hook() if po->bind_lock
needs to be released temporarily.

It is tested in register_prot_hook() to prevent the race condition.

[1]
dev_remove_pack: ffff8801bf16fa80 not found
------------[ cut here ]------------
kernel BUG at net/core/dev.c:7945!  ( BUG_ON(!list_empty(&dev->ptype_all)); )
invalid opcode: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
device syz0 entered promiscuous mode
CPU: 0 PID: 3161 Comm: syzkaller404108 Not tainted 4.14.0+ #190
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
task: ffff8801cc57a500 task.stack: ffff8801cc588000
RIP: 0010:netdev_run_todo+0x772/0xae0 net/core/dev.c:7945
RSP: 0018:ffff8801cc58f598 EFLAGS: 00010293
RAX: ffff8801cc57a500 RBX: dffffc0000000000 RCX: ffffffff841f75b2
RDX: 0000000000000000 RSI: 1ffff100398b1ede RDI: ffff8801bf1f8810
device syz0 entered promiscuous mode
RBP: ffff8801cc58f898 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801bf1f8cd8
R13: ffff8801cc58f870 R14: ffff8801bf1f8780 R15: ffff8801cc58f7f0
FS:  0000000001716880(0000) GS:ffff8801db400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020b13000 CR3: 0000000005e25000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:106
 tun_detach drivers/net/tun.c:670 [inline]
 tun_chr_close+0x49/0x60 drivers/net/tun.c:2845
 __fput+0x333/0x7f0 fs/file_table.c:210
 ____fput+0x15/0x20 fs/file_table.c:244
 task_work_run+0x199/0x270 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x9bb/0x1ae0 kernel/exit.c:865
 do_group_exit+0x149/0x400 kernel/exit.c:968
 SYSC_exit_group kernel/exit.c:979 [inline]
 SyS_exit_group+0x1d/0x20 kernel/exit.c:977
 entry_SYSCALL_64_fastpath+0x1f/0x96
RIP: 0033:0x44ad19


Fixes: 30f7ea1c2b5f ("packet: race condition in packet_bind")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <redacted>
Cc: Francesco Ruggeri <redacted>
---
 net/packet/af_packet.c |    5 ++++-
 net/packet/internal.h  |    1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 737092ca9b4eed464b6c0907d85b679ae4da6046..64382200ab9b0701a510f16a098257ccd7ac5ff5 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c

@@ -336,7 +336,7 @@ static void register_prot_hook(struct sock *sk)
 {
        struct packet_sock *po = pkt_sk(sk);

-       if (!po->running) {
+       if (!po->running && !po->frozen) {

                if (po->fanout)
                        __fanout_link(sk, po);
                else

@@ -368,9 +368,11 @@ static void __unregister_prot_hook(struct sock *sk, bool sync)
        __sock_put(sk);

        if (sync) {
+               po->frozen = 1;
                spin_unlock(&po->bind_lock);
                synchronize_net();
                spin_lock(&po->bind_lock);
+               po->frozen = 0;
        }
 }

@@ -3105,6 +3107,7 @@ static int packet_do_bind(struct sock *sk, const char *name, int ifindex,
                                                                 dev->ifindex);
                }

+               BUG_ON(po->running);
                po->num = proto;
                po->prot_hook.type = proto;

diff --git a/net/packet/internal.h b/net/packet/internal.h
index 562fbc155006374862e5bfdd78b65a7f46210bea..e039af6e71d650e3beb2936af6cbdd167313499e 100644
--- a/net/packet/internal.h
+++ b/net/packet/internal.h

@@ -114,6 +114,7 @@ struct packet_sock {
        spinlock_t              bind_lock;
        struct mutex            pg_vec_lock;
        unsigned int            running:1,      /* prot_hook is attached*/
+                               frozen:1,
                                auxdata:1,
                                origdev:1,
                                has_vnet_hdr:1;