Re: [PATCH net v6] net: rose: fix null-ptr-deref caused by rose_kill_by_neigh

From: Paolo Abeni
Date: Tue Jul 12 2022 - 07:01:16 EST

On Mon, 2022-07-11 at 09:31 +0800, Duoming Zhou wrote:
> 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 <duoming@xxxxxxxxxx>
> ---
> 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.

I really think you should resort to something similar to the following
(completelly untested, just to give an idea). In any case it would be
better to split this change in 2 separate patches: the first patch
replaces 'int use;' with an antomic_t and the 2nd one addresses the
race you describe above.

---
diff --git a/net/rose/af_rose.c b/net/rose/af_rose.c
index bf2d986a6bc3..27b1027aaedf 100644
--- a/net/rose/af_rose.c
+++ b/net/rose/af_rose.c
@@ -156,25 +156,45 @@ static void rose_remove_socket(struct sock *sk)
spin_unlock_bh(&rose_list_lock);
}

+static DEFINE_MUTEX(kill_lock);
+
/*
* Kill all bound sockets on a broken link layer connection to a
* particular neighbour.
*/
void rose_kill_by_neigh(struct rose_neigh *neigh)
{
- struct sock *s;
+ HLIST_HEAD(rose_list_copy);
+ struct sock *s, *tmp;
+
+ mutex_lock(&kill_lock);

spin_lock_bh(&rose_list_lock);
sk_for_each(s, &rose_list) {
+ sock_hold(s);
+ /* sk_bind_node is apparently unused by rose. Alternatively
+ * you can add another hlist_node to rose_sock and use it here
+ */
+ sk_add_bind_node(s, &rose_list_copy);
+ }
+ spin_unlock_bh(&rose_list_lock);
+
+ hlist_for_each_entry_safe(s, tmp, &rose_list_copy, sk_bind_node) {
struct rose_sock *rose = rose_sk(s);

+ __sk_del_bind_node(s);
+ 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);
}
- spin_unlock_bh(&rose_list_lock);
+
+ mutex_unlock(&kill_lock);
}

/*
---
/P