On 06.06.2018 16:16, Paolo Abeni wrote:
> KCM removes the packets from sk_receive_queue in requeue_rx_msgs()
>
> without acquiring any lock. Moreover, in R() when the MSG_PEEK
> flag is not present, the skb is peeked and dequeued with two
> separate, non-atomic, calls.
>
> The above create room for races, which SYZBOT has been able to
> exploit, causing list corruption and kernel oops:
>
> kasan: CONFIG_KASAN_INLINE enabled
> kasan: GPF could be caused by NULL-ptr deref or user memory access
> general protection fault: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
> (ftrace buffer empty)
> Modules linked in:
> CPU: 0 PID: 8484 Comm: syz-executor919 Not tainted 4.17.0-rc7+ #74
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: 0010:__skb_unlink include/linux/skbuff.h:1844 [inline]
> RIP: 0010:skb_unlink+0xc1/0x160 net/core/skbuff.c:2921
> RSP: 0018:ffff8801d012f6f0 EFLAGS: 00010002
> RAX: 0000000000000286 RBX: ffff8801d6e073c0 RCX: 0000000000000001
> RDX: dffffc0000000000 RSI: 0000000000000004 RDI: 0000000000000008
> RBP: ffff8801d012f718 R08: ffffed0038bb3b6d R09: ffffed0038bb3b6c
> R10: ffffed0038bb3b6c R11: ffff8801c5d9db63 R12: 0000000000000000
> R13: 0000000000000000 R14: ffff8801c5d9db60 R15: ffff8801d012fce0
> FS: 0000000000ab7880(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000020e5b000 CR3: 00000001c31fb000 CR4: 00000000001406f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
> kcm_recvmsg+0x48d/0x590 net/kcm/kcmsock.c:1160
> sock_recvmsg_nosec+0x8c/0xb0 net/socket.c:802
> ___sys_recvmsg+0x2b6/0x680 net/socket.c:2279
> __sys_recvmmsg+0x2f9/0xb80 net/socket.c:2391
> do_sys_recvmmsg+0xe4/0x190 net/socket.c:2472
> __do_sys_recvmmsg net/socket.c:2485 [inline]
> __se_sys_recvmmsg net/socket.c:2481 [inline]
> __x64_sys_recvmmsg+0xbe/0x150 net/socket.c:2481
> do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x4417a9
> RSP: 002b:00007ffe27282838 EFLAGS: 00000206 ORIG_RAX: 000000000000012b
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004417a9
> RDX: 00000000040000f7 RSI: 00000000200002c0 RDI: 0000000000000006
> RBP: 0000000000000000 R08: 0000000020000200 R09: 00007ffe272829f8
> R10: 0000000000000060 R11: 0000000000000206 R12: 00000000000001f3
> R13: 000000000001f871 R14: 0000000000000000 R15: 0000000000000000
> Code: 00 00 00 49 8d 7d 08 4c 8b 63 08 48 ba 00 00 00 00 00 fc ff df 48 c7
> 43 08 00 00 00 00 48 89 f9 48 c7 03 00 00 00 00 48 c1 e9 03 <80> 3c 11 00
> 75 5b 4c 89 e1 4d 89 65 08 48 ba 00 00 00 00 00 fc
> RIP: __skb_unlink include/linux/skbuff.h:1844 [inline] RSP: ffff8801d012f6f0
> RIP: skb_unlink+0xc1/0x160 net/core/skbuff.c:2921 RSP: ffff8801d012f6f0
>
> To fix the above, we need to use the locked version of the socket dequeue
> helper in requeue_rx_msgs() and kcm_wait_data is changed to dequeue
> the available skb when not peeking.
>
> RFC -> v1:
> - use skb_dequeue(), as suggested by Tom
> - explicitly close the race between skb_peek and skb_unlink
>
> Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> Reported-and-tested-by: syzbot+278279efdd2730dd1...@syzkaller.appspotmail.com
> Signed-off-by: Paolo Abeni <pab...@redhat.com>
> ---
> This is an RFC, since I'm really new to this area, anyway the syzport
> reported success in testing the proposed fix.
> This is very likely a scenario where the upcoming skb->prev,next -> list_head
> conversion would have helped a lot, thanks to list poisoning and list debug
> ---
> net/kcm/kcmsock.c | 19 ++++++++++++-------
> 1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> index d3601d421571..dd2d02bb35ae 100644
> --- a/net/kcm/kcmsock.c
> +++ b/net/kcm/kcmsock.c
> @@ -223,7 +223,7 @@ static void requeue_rx_msgs(struct kcm_mux *mux, struct
> sk_buff_head *head)
> struct sk_buff *skb;
> struct kcm_sock *kcm;
>
> - while ((skb = __skb_dequeue(head))) {
> + while ((skb = skb_dequeue(head))) {
I try to find how the patch protects against the following race:
requeue_rx_msgs() kcm_recvmsg()
skb = skb_dequeue() skb = kcm_wait_data(peek = true)
... ...
free skb ...
... skb_copy_datagram_msg(skb) <--- Use after free?
Isn't there possible a use-after-free?
Thanks,
Kirill
> /* Reset destructor to avoid calling kcm_rcv_ready */
> skb->destructor = sock_rfree;
> skb_orphan(skb);
> @@ -1080,12 +1080,17 @@ static int kcm_sendmsg(struct socket *sock, struct
> msghdr *msg, size_t len)
> return err;
> }
>
> -static struct sk_buff *kcm_wait_data(struct sock *sk, int flags,
> +static struct sk_buff *kcm_wait_data(struct sock *sk, int flags, bool peek,
> long timeo, int *err)
> {
> struct sk_buff *skb;
>
> - while (!(skb = skb_peek(&sk->sk_receive_queue))) {
> + for (;; ) {
> + skb = peek ? skb_peek(&sk->sk_receive_queue) :
> + skb_dequeue(&sk->sk_receive_queue);
> + if (skb)
> + break;
> +
> if (sk->sk_err) {
> *err = sock_error(sk);
> return NULL;
> @@ -1116,6 +1121,7 @@ static int kcm_recvmsg(struct socket *sock, struct
> msghdr *msg,
> {
> struct sock *sk = sock->sk;
> struct kcm_sock *kcm = kcm_sk(sk);
> + bool peek = flags & MSG_PEEK;
> int err = 0;
> long timeo;
> struct strp_msg *stm;
> @@ -1126,7 +1132,7 @@ static int kcm_recvmsg(struct socket *sock, struct
> msghdr *msg,
>
> lock_sock(sk);
>
> - skb = kcm_wait_data(sk, flags, timeo, &err);
> + skb = kcm_wait_data(sk, flags, peek, timeo, &err);
> if (!skb)
> goto out;
>
> @@ -1142,7 +1148,7 @@ static int kcm_recvmsg(struct socket *sock, struct
> msghdr *msg,
> goto out;
>
> copied = len;
> - if (likely(!(flags & MSG_PEEK))) {
> + if (likely(!peek)) {
> KCM_STATS_ADD(kcm->stats.rx_bytes, copied);
> if (copied < stm->full_len) {
> if (sock->type == SOCK_DGRAM) {
> @@ -1157,7 +1163,6 @@ static int kcm_recvmsg(struct socket *sock, struct
> msghdr *msg,
> /* Finished with message */
> msg->msg_flags |= MSG_EOR;
> KCM_STATS_INCR(kcm->stats.rx_msgs);
> - skb_unlink(skb, &sk->sk_receive_queue);
> kfree_skb(skb);
> }
> }
> @@ -1186,7 +1191,7 @@ static ssize_t kcm_splice_read(struct socket *sock,
> loff_t *ppos,
>
> lock_sock(sk);
>
> - skb = kcm_wait_data(sk, flags, timeo, &err);
> + skb = kcm_wait_data(sk, flags, true, timeo, &err);
> if (!skb)
> goto err_out;
>
>
