[PATCH 5.10 65/93] af_unix: fix races in sk_peer_pid and sk_peer_cred accesses

Greg Kroah-Hartman
2021-10-04T13:24:16Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 35306eb23814444bd4021f8a1c3047d3cb0c8b2b ]

Jann Horn reported that SO_PEERCRED and SO_PEERGROUPS implementations
are racy, as af_unix can concurrently change sk_peer_pid and sk_peer_cred.

In order to fix this issue, this patch adds a new spinlock that needs
to be used whenever these fields are read or written.

Jann also pointed out that l2cap_sock_get_peer_pid_cb() is currently
reading sk->sk_peer_pid which makes no sense, as this field
is only possibly set by AF_UNIX sockets.
We will have to clean this in a separate patch.
This could be done by reverting b48596d1dc25 "Bluetooth: L2CAP: Add get_peer_pid callback"
or implementing what was truly expected.

Fixes: 109f6e39fa07 ("af_unix: Allow SO_PEERCRED to work across namespaces.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jann Horn <jannh@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/sock.h | 2 ++
net/core/sock.c | 32 ++++++++++++++++++++++++++------
net/unix/af_unix.c | 34 ++++++++++++++++++++++++++++------
3 files changed, 56 insertions(+), 12 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 3c7addf95150..cdca984f3630 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -479,8 +479,10 @@ struct sock {
u32 sk_ack_backlog;
u32 sk_max_ack_backlog;
kuid_t sk_uid;
+ spinlock_t sk_peer_lock;
struct pid *sk_peer_pid;
const struct cred *sk_peer_cred;
+
long sk_rcvtimeo;
ktime_t sk_stamp;
#if BITS_PER_LONG==32
diff --git a/net/core/sock.c b/net/core/sock.c
index d638c5361ed2..f9c835167391 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1255,6 +1255,16 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
}
EXPORT_SYMBOL(sock_setsockopt);

+static const struct cred *sk_get_peer_cred(struct sock *sk)
+{
+ const struct cred *cred;
+
+ spin_lock(&sk->sk_peer_lock);
+ cred = get_cred(sk->sk_peer_cred);
+ spin_unlock(&sk->sk_peer_lock);
+
+ return cred;
+}

static void cred_to_ucred(struct pid *pid, const struct cred *cred,
struct ucred *ucred)
@@ -1428,7 +1438,11 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
struct ucred peercred;
if (len > sizeof(peercred))
len = sizeof(peercred);
+
+ spin_lock(&sk->sk_peer_lock);
cred_to_ucred(sk->sk_peer_pid, sk->sk_peer_cred, &peercred);
+ spin_unlock(&sk->sk_peer_lock);
+
if (copy_to_user(optval, &peercred, len))
return -EFAULT;
goto lenout;
@@ -1436,20 +1450,23 @@ int sock_getsockopt(struct socket *sock, int level, int optname,

case SO_PEERGROUPS:
{
+ const struct cred *cred;
int ret, n;

- if (!sk->sk_peer_cred)
+ cred = sk_get_peer_cred(sk);
+ if (!cred)
return -ENODATA;

- n = sk->sk_peer_cred->group_info->ngroups;
+ n = cred->group_info->ngroups;
if (len < n * sizeof(gid_t)) {
len = n * sizeof(gid_t);
+ put_cred(cred);
return put_user(len, optlen) ? -EFAULT : -ERANGE;
}
len = n * sizeof(gid_t);

- ret = groups_to_user((gid_t __user *)optval,
- sk->sk_peer_cred->group_info);
+ ret = groups_to_user((gid_t __user *)optval, cred->group_info);
+ put_cred(cred);
if (ret)
return ret;
goto lenout;
@@ -1788,9 +1805,10 @@ static void __sk_destruct(struct rcu_head *head)
sk->sk_frag.page = NULL;
}

- if (sk->sk_peer_cred)
- put_cred(sk->sk_peer_cred);
+ /* We do not need to acquire sk->sk_peer_lock, we are the last user. */
+ put_cred(sk->sk_peer_cred);
put_pid(sk->sk_peer_pid);
+
if (likely(sk->sk_net_refcnt))
put_net(sock_net(sk));
sk_prot_free(sk->sk_prot_creator, sk);
@@ -3000,6 +3018,8 @@ void sock_init_data(struct socket *sock, struct sock *sk)

sk->sk_peer_pid = NULL;
sk->sk_peer_cred = NULL;
+ spin_lock_init(&sk->sk_peer_lock);
+
sk->sk_write_pending = 0;
sk->sk_rcvlowat = 1;
sk->sk_rcvtimeo = MAX_SCHEDULE_TIMEOUT;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index d5c0ae34b1e4..b7edca89e0ba 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -593,20 +593,42 @@ static void unix_release_sock(struct sock *sk, int embrion)

static void init_peercred(struct sock *sk)
{
- put_pid(sk->sk_peer_pid);
- if (sk->sk_peer_cred)
- put_cred(sk->sk_peer_cred);
+ const struct cred *old_cred;
+ struct pid *old_pid;
+
+ spin_lock(&sk->sk_peer_lock);
+ old_pid = sk->sk_peer_pid;
+ old_cred = sk->sk_peer_cred;
sk->sk_peer_pid = get_pid(task_tgid(current));
sk->sk_peer_cred = get_current_cred();
+ spin_unlock(&sk->sk_peer_lock);
+
+ put_pid(old_pid);
+ put_cred(old_cred);
}

static void copy_peercred(struct sock *sk, struct sock *peersk)
{
- put_pid(sk->sk_peer_pid);
- if (sk->sk_peer_cred)
- put_cred(sk->sk_peer_cred);
+ const struct cred *old_cred;
+ struct pid *old_pid;
+
+ if (sk < peersk) {
+ spin_lock(&sk->sk_peer_lock);
+ spin_lock_nested(&peersk->sk_peer_lock, SINGLE_DEPTH_NESTING);
+ } else {
+ spin_lock(&peersk->sk_peer_lock);
+ spin_lock_nested(&sk->sk_peer_lock, SINGLE_DEPTH_NESTING);
+ }
+ old_pid = sk->sk_peer_pid;
+ old_cred = sk->sk_peer_cred;
sk->sk_peer_pid = get_pid(peersk->sk_peer_pid);
sk->sk_peer_cred = get_cred(peersk->sk_peer_cred);
+
+ spin_unlock(&sk->sk_peer_lock);
+ spin_unlock(&peersk->sk_peer_lock);
+
+ put_pid(old_pid);
+ put_cred(old_cred);
}

static int unix_listen(struct socket *sock, int backlog)
--
2.33.0