Hello,

On Tue, 12 Jul 2022 13:00:49 +0200 Paolo Abeni wrote:

On Mon, 2022-07-11 at 09:31 +0800, Duoming Zhou wrote:

quoted

When the link layer connection is broken, the rose->neighbour is
set to null. But rose->neighbour could be used by rose_connection()
and rose_release() later, because there is no synchronization among
them. As a result, the null-ptr-deref bugs will happen.

One of the null-ptr-deref bugs is shown below:

    (thread 1)                  |        (thread 2)
                                |  rose_connect
rose_kill_by_neigh              |    lock_sock(sk)
  spin_lock_bh(&rose_list_lock) |    if (!rose->neighbour)
  rose->neighbour = NULL;//(1)  |
                                |    rose->neighbour->use++;//(2)

The rose->neighbour is set to null in position (1) and dereferenced
in position (2).

The KASAN report triggered by POC is shown below:

KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
...
RIP: 0010:rose_connect+0x6c2/0xf30
RSP: 0018:ffff88800ab47d60 EFLAGS: 00000206
RAX: 0000000000000005 RBX: 000000000000002a RCX: 0000000000000000
RDX: ffff88800ab38000 RSI: ffff88800ab47e48 RDI: ffff88800ab38309
RBP: dffffc0000000000 R08: 0000000000000000 R09: ffffed1001567062
R10: dfffe91001567063 R11: 1ffff11001567061 R12: 1ffff11000d17cd0
R13: ffff8880068be680 R14: 0000000000000002 R15: 1ffff11000d17cd0
...
Call Trace:
  <TASK>
  ? __local_bh_enable_ip+0x54/0x80
  ? selinux_netlbl_socket_connect+0x26/0x30
  ? rose_bind+0x5b0/0x5b0
  __sys_connect+0x216/0x280
  __x64_sys_connect+0x71/0x80
  do_syscall_64+0x43/0x90
  entry_SYSCALL_64_after_hwframe+0x46/0xb0

This patch adds lock_sock() in rose_kill_by_neigh() in order to
synchronize with rose_connect() and rose_release(). Then, changing
type of 'neighbour->use' from unsigned short to atomic_t in order to
mitigate race conditions caused by holding different socket lock while
updating 'neighbour->use'.

Meanwhile, this patch adds sock_hold() protected by rose_list_lock
that could synchronize with rose_remove_socket() in order to mitigate
UAF bug caused by lock_sock() we add.

What's more, there is no need using rose_neigh_list_lock to protect
rose_kill_by_neigh(). Because we have already used rose_neigh_list_lock
to protect the state change of rose_neigh in rose_link_failed(), which
is well synchronized.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Duoming Zhou <redacted>
---
Changes in v6:
  - Change sk_for_each() to sk_for_each_safe().
  - Change type of 'neighbour->use' from unsigned short to atomic_t.

 include/net/rose.h    |  2 +-
 net/rose/af_rose.c    | 19 +++++++++++++------
 net/rose/rose_in.c    | 12 ++++++------
 net/rose/rose_route.c | 24 ++++++++++++------------
 net/rose/rose_timer.c |  2 +-
 5 files changed, 33 insertions(+), 26 deletions(-)

diff --git a/include/net/rose.h b/include/net/rose.h
index 0f0a4ce0fee..d5ddebc556d 100644
--- a/include/net/rose.h
+++ b/include/net/rose.h

@@ -95,7 +95,7 @@ struct rose_neigh {
 	ax25_cb			*ax25;
 	struct net_device		*dev;
 	unsigned short		count;
-	unsigned short		use;
+	atomic_t		use;
 	unsigned int		number;
 	char			restarted;
 	char			dce_mode;

diff --git a/net/rose/af_rose.c b/net/rose/af_rose.c
index bf2d986a6bc..54e7b76c4f3 100644
--- a/net/rose/af_rose.c
+++ b/net/rose/af_rose.c

@@ -163,16 +163,23 @@ static void rose_remove_socket(struct sock *sk)
 void rose_kill_by_neigh(struct rose_neigh *neigh)
 {
 	struct sock *s;
+	struct hlist_node *tmp;
 
 	spin_lock_bh(&rose_list_lock);
-	sk_for_each(s, &rose_list) {
+	sk_for_each_safe(s, tmp, &rose_list) {
 		struct rose_sock *rose = rose_sk(s);
 
+		sock_hold(s);
+		spin_unlock_bh(&rose_list_lock);
+		lock_sock(s);
 		if (rose->neighbour == neigh) {
 			rose_disconnect(s, ENETUNREACH, ROSE_OUT_OF_ORDER, 0);
-			rose->neighbour->use--;
+			atomic_dec(&rose->neighbour->use);
 			rose->neighbour = NULL;
 		}
+		release_sock(s);
+		sock_put(s);

I'm sorry, this does not work. At this point both 's' and 'tmp' sockets
can be freed and reused. Both iterators are not valid anymore when you
acquire the 'rose_list_lock' later.

Thank you for your time and reply! But I think both 's' and 'tmp' can not
be freed and reused in rose_kill_by_neigh(). Because rose_remove_socket()
calls sk_del_node_init() which is protected by rose_list_lock to delete the
socket node from the hlist and if sk->sk_refcnt equals to 1, the socket will
be deallocated.

static void rose_remove_socket(struct sock *sk)
{
	spin_lock_bh(&rose_list_lock);
	sk_del_node_init(sk);
	spin_unlock_bh(&rose_list_lock);
}

https://elixir.bootlin.com/linux/v5.19-rc6/source/net/rose/af_rose.c#L152

Both 's' and 'tmp' in rose_kill_by_neigh() is also protected by rose_list_lock.

If the socket is deleted from the hlist, sk_for_each_safe() could not find
the socket and the UAF bug could be prevented. 

If the socket could be found by sk_for_each_safe(), we use sock_hold(s)
to increase the refcount of the socket. As a result, the UAF bugs could
be prevented.

Best regards,
Duoming Zhou