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Date: Sun, 9 Jun 2024 12:53:20 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <mhal@...x.co>
CC: <cong.wang@...edance.com>, <davem@...emloft.net>, <edumazet@...gle.com>,
<kuba@...nel.org>, <kuni1840@...il.com>, <kuniyu@...zon.com>,
<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v2 net 01/15] af_unix: Set sk->sk_state under unix_state_lock() for truly disconencted peer.
From: Michal Luczaj <mhal@...x.co>
Date: Sun, 9 Jun 2024 13:28:34 +0200
> On 6/4/24 18:52, Kuniyuki Iwashima wrote:
> > When a SOCK_DGRAM socket connect()s to another socket, the both sockets'
> > sk->sk_state are changed to TCP_ESTABLISHED so that we can register them
> > to BPF SOCKMAP. (...)
>
> Speaking of af_unix and sockmap, SOCK_STREAM has a tiny window for
> bpf(BPF_MAP_UPDATE_ELEM) and unix_stream_connect() to race: when
> sock_map_sk_state_allowed() passes (sk_state == TCP_ESTABLISHED), but
> unix_peer(sk) in unix_stream_bpf_update_proto() _still_ returns NULL:
>
> T0 bpf T1 connect
> ====== ==========
>
> WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)
> sock_map_sk_state_allowed(sk)
> ...
> sk_pair = unix_peer(sk)
> sock_hold(sk_pair)
> sock_hold(newsk)
> smp_mb__after_atomic()
> unix_peer(sk) = newsk
> unix_state_unlock(sk)
>
> With mdelay(1) stuffed in unix_stream_connect():
>
> [ 902.277593] BUG: kernel NULL pointer dereference, address: 0000000000000080
> [ 902.277633] #PF: supervisor write access in kernel mode
> [ 902.277661] #PF: error_code(0x0002) - not-present page
> [ 902.277688] PGD 107191067 P4D 107191067 PUD 10f63c067 PMD 0
> [ 902.277716] Oops: Oops: 0002 [#23] PREEMPT SMP NOPTI
> [ 902.277742] CPU: 2 PID: 1505 Comm: a.out Tainted: G D 6.10.0-rc1+ #130
> [ 902.277769] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
> [ 902.277793] RIP: 0010:unix_stream_bpf_update_proto+0xa1/0x150
>
> Setting TCP_ESTABLISHED _after_ unix_peer() fixes the issue, so how about
> something like
>
> @@ -1631,12 +1631,13 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
> /* Set credentials */
> copy_peercred(sk, other);
>
> - sock->state = SS_CONNECTED;
> - WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED);
> sock_hold(newsk);
> + smp_mb__after_atomic(); /* sock_hold() does an atomic_inc() */
> + WRITE_ONCE(unix_peer(sk), newsk);
> + smp_wmb(); /* ensure peer is set before sk_state */
>
> - smp_mb__after_atomic(); /* sock_hold() does an atomic_inc() */
> - unix_peer(sk) = newsk;
> + sock->state = SS_CONNECTED;
> + WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED);
>
> unix_state_unlock(sk);
>
> @@ -180,7 +180,8 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
> * be a single matching destroy operation.
> */
> if (!psock->sk_pair) {
> - sk_pair = unix_peer(sk);
> + smp_rmb();
> + sk_pair = READ_ONCE(unix_peer(sk));
> sock_hold(sk_pair);
> psock->sk_pair = sk_pair;
> }
>
> This should keep things ordered and lockless... I hope.
sock_map_update_elem() assumes that the socket is protected
by lock_sock(), but AF_UNIX uses it only for the general path.
So, I think we should fix sock_map_sk_state_allowed() and
then use smp_store_release()/smp_load_acquire() rather than
smp_[rw]mb() for unix_peer(sk).
Could you test this with the mdelay(1) change ?
Note that we need not touch sock->state. I have a patch for
net-next that removes sock->state uses completely from AF_UNIX
as we don't use it. Even unix_seq_show() depends on sk->sk_state.
---8<---
diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index d3dbb92153f2..67794d2c7498 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -549,7 +549,7 @@ static bool sock_map_sk_state_allowed(const struct sock *sk)
if (sk_is_tcp(sk))
return (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_LISTEN);
if (sk_is_stream_unix(sk))
- return (1 << sk->sk_state) & TCPF_ESTABLISHED;
+ return (1 << READ_ONCE(sk->sk_state)) & TCPF_ESTABLISHED;
return true;
}
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 80846279de9f..a558745c7d76 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1632,11 +1632,11 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
copy_peercred(sk, other);
sock->state = SS_CONNECTED;
- WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED);
sock_hold(newsk);
smp_mb__after_atomic(); /* sock_hold() does an atomic_inc() */
- unix_peer(sk) = newsk;
+ smp_store_release(&unix_peer(sk), newsk);
+ WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED);
unix_state_unlock(sk);
diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
index bd84785bf8d6..6d9ae8e63901 100644
--- a/net/unix/unix_bpf.c
+++ b/net/unix/unix_bpf.c
@@ -180,7 +180,7 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
* be a single matching destroy operation.
*/
if (!psock->sk_pair) {
- sk_pair = unix_peer(sk);
+ sk_pair = smp_load_acquire(&unix_peer(sk));
sock_hold(sk_pair);
psock->sk_pair = sk_pair;
}
---8<---
>
> Alternatively, maybe it would be better just to make BPF respect the unix
> state lock?
>
> @@ -180,6 +180,8 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
> * be a single matching destroy operation.
> */
> if (!psock->sk_pair) {
> + unix_state_lock(sk);
> sk_pair = unix_peer(sk);
> + unix_state_unlock(sk);
> sock_hold(sk_pair);
> psock->sk_pair = sk_pair;
>
> What do you think?
If we'd go this way, I'd change like this:
---8<---
diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
index bd84785bf8d6..1db42cfee70d 100644
--- a/net/unix/unix_bpf.c
+++ b/net/unix/unix_bpf.c
@@ -159,8 +159,6 @@ int unix_dgram_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool re
int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool restore)
{
- struct sock *sk_pair;
-
/* Restore does not decrement the sk_pair reference yet because we must
* keep the a reference to the socket until after an RCU grace period
* and any pending sends have completed.
@@ -180,9 +178,9 @@ int unix_stream_bpf_update_proto(struct sock *sk, struct sk_psock *psock, bool r
* be a single matching destroy operation.
*/
if (!psock->sk_pair) {
- sk_pair = unix_peer(sk);
- sock_hold(sk_pair);
- psock->sk_pair = sk_pair;
+ psock->sk_pair = unix_peer_get(sk);
+ if (WARN_ON_ONCE(!psock->sk_pair))
+ return -EINVAL;
}
unix_stream_bpf_check_needs_rebuild(psock->sk_proto);
---8<---
And the _last_ option would be..., no :)
---8<---
diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index b6eedf7650da..c7e31bc3e95e 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -94,8 +94,8 @@ struct unix_sock {
#define unix_sk(ptr) container_of_const(ptr, struct unix_sock, sk)
#define unix_peer(sk) (unix_sk(sk)->peer)
-#define unix_state_lock(s) spin_lock(&unix_sk(s)->lock)
-#define unix_state_unlock(s) spin_unlock(&unix_sk(s)->lock)
+#define unix_state_lock(s) lock_sock(s)
+#define unix_state_unlock(s) release_sock(s)
enum unix_socket_lock_class {
U_LOCK_NORMAL,
U_LOCK_SECOND, /* for double locking, see unix_state_double_lock(). */
@@ -108,7 +108,7 @@ enum unix_socket_lock_class {
static inline void unix_state_lock_nested(struct sock *sk,
enum unix_socket_lock_class subclass)
{
- spin_lock_nested(&unix_sk(sk)->lock, subclass);
+ lock_sock_nested(sk, subclass);
}
#define peer_wait peer_wq.wait
---8<---
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