Message-Id: <20190211090949.18560-1-jakub@cloudflare.com>
Date:   Mon, 11 Feb 2019 10:09:49 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     netdev@...r.kernel.org
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Marek Majkowski <marek@...udflare.com>
Subject: [PATCH net] sk_msg: Keep reference on socket file while psock lives

Backlog work for psock (sk_psock_backlog) might sleep while waiting for
memory to free up when sending packets. While sleeping, socket can
disappear from under our feet together with its wait queue because the
userspace has closed it.

This breaks an assumption in sk_stream_wait_memory, which expects the
wait queue to be still there when it wakes up resulting in a
use-after-free:

==================================================================
BUG: KASAN: use-after-free in remove_wait_queue+0x31/0x70
Write of size 8 at addr ffff888069a0c4e8 by task kworker/0:2/110

CPU: 0 PID: 110 Comm: kworker/0:2 Not tainted 5.0.0-rc2-00335-g28f9d1a3d4fe-dirty #14
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-2.fc27 04/01/2014
Workqueue: events sk_psock_backlog
Call Trace:
 print_address_description+0x6e/0x2b0
 ? remove_wait_queue+0x31/0x70
 kasan_report+0xfd/0x177
 ? remove_wait_queue+0x31/0x70
 ? remove_wait_queue+0x31/0x70
 remove_wait_queue+0x31/0x70
 sk_stream_wait_memory+0x4dd/0x5f0
 ? sk_stream_wait_close+0x1b0/0x1b0
 ? wait_woken+0xc0/0xc0
 ? tcp_current_mss+0xc5/0x110
 tcp_sendmsg_locked+0x634/0x15d0
 ? tcp_set_state+0x2e0/0x2e0
 ? __kasan_slab_free+0x1d1/0x230
 ? kmem_cache_free+0x70/0x140
 ? sk_psock_backlog+0x40c/0x4b0
 ? process_one_work+0x40b/0x660
 ? worker_thread+0x82/0x680
 ? kthread+0x1b9/0x1e0
 ? ret_from_fork+0x1f/0x30
 ? check_preempt_curr+0xaf/0x130
 ? iov_iter_kvec+0x5f/0x70
 ? kernel_sendmsg_locked+0xa0/0xe0
 skb_send_sock_locked+0x273/0x3c0
 ? skb_splice_bits+0x180/0x180
 ? start_thread+0xe0/0xe0
 ? update_min_vruntime.constprop.27+0x88/0xc0
 sk_psock_backlog+0xb3/0x4b0
 ? strscpy+0xbf/0x1e0
 process_one_work+0x40b/0x660
 worker_thread+0x82/0x680
 ? process_one_work+0x660/0x660
 kthread+0x1b9/0x1e0
 ? __kthread_create_on_node+0x250/0x250
 ret_from_fork+0x1f/0x30

Allocated by task 109:
 sock_alloc_inode+0x54/0x120
 alloc_inode+0x28/0xb0
 new_inode_pseudo+0x7/0x60
 sock_alloc+0x21/0xe0
 __sys_accept4+0xc2/0x330
 __x64_sys_accept+0x3b/0x50
 do_syscall_64+0xb2/0x3e0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 7:
 kfree+0x7f/0x140
 rcu_process_callbacks+0xe0/0x100
 __do_softirq+0xe5/0x29a

The buggy address belongs to the object at ffff888069a0c4e0
 which belongs to the cache kmalloc-64 of size 64
The buggy address is located 8 bytes inside of
 64-byte region [ffff888069a0c4e0, ffff888069a0c520)
The buggy address belongs to the page:
page:ffffea0001a68300 count:1 mapcount:0 mapping:ffff88806d4018c0 index:0x0
flags: 0x4000000000000200(slab)
raw: 4000000000000200 dead000000000100 dead000000000200 ffff88806d4018c0
raw: 0000000000000000 00000000002a002a 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888069a0c380: fb fb fb fb fc fc fc fc fb fb fb fb fb fb fb fb
 ffff888069a0c400: fc fc fc fc fb fb fb fb fb fb fb fb fc fc fc fc
>ffff888069a0c480: 00 00 00 00 00 00 00 00 fc fc fc fc fb fb fb fb
                                                          ^
 ffff888069a0c500: fb fb fb fb fc fc fc fc fb fb fb fb fb fb fb fb
 ffff888069a0c580: fc fc fc fc fb fb fb fb fb fb fb fb fc fc fc fc
==================================================================

Avoid it by keeping a reference to the socket file until the psock gets
destroyed.

While at it, rearrange the order of reference grabbing and
initialization to match the destructor in reverse.

Reported-by: Marek Majkowski <marek@...udflare.com>
Link: https://lore.kernel.org/netdev/CAJPywTLwgXNEZ2dZVoa=udiZmtrWJ0q5SuBW64aYs0Y1khXX3A@mail.gmail.com
Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
---
 net/core/skmsg.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 8c826603bf36..a38442b8580b 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -493,8 +493,13 @@ struct sk_psock *sk_psock_init(struct sock *sk, int node)
 	sk_psock_set_state(psock, SK_PSOCK_TX_ENABLED);
 	refcount_set(&psock->refcnt, 1);
 
-	rcu_assign_sk_user_data(sk, psock);
+	/* Hold on to socket wait queue. Backlog work waits on it for
+	 * memory when sending. We must cancel work before socket wait
+	 * queue can go away.
+	 */
+	get_file(sk->sk_socket->file);
 	sock_hold(sk);
+	rcu_assign_sk_user_data(sk, psock);
 
 	return psock;
 }
@@ -558,6 +563,7 @@ static void sk_psock_destroy_deferred(struct work_struct *gc)
 	if (psock->sk_redir)
 		sock_put(psock->sk_redir);
 	sock_put(psock->sk);
+	fput(psock->sk->sk_socket->file);
 	kfree(psock);
 }
 
-- 
2.17.2

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