[PATCH 5.10 032/121] ax25: improve the incomplete fix to avoid UAF and NPD bugs

Greg Kroah-Hartman
2022-02-21T09:10:19Z

From: Duoming Zhou <duoming@zju.edu.cn>

[ Upstream commit 4e0f718daf97d47cf7dec122da1be970f145c809 ]

The previous commit 1ade48d0c27d ("ax25: NPD bug when detaching
AX25 device") introduce lock_sock() into ax25_kill_by_device to
prevent NPD bug. But the concurrency NPD or UAF bug will occur,
when lock_sock() or release_sock() dereferences the ax25_cb->sock.

The NULL pointer dereference bug can be shown as below:

ax25_kill_by_device() | ax25_release()
| ax25_destroy_socket()
| ax25_cb_del()
... | ...
| ax25->sk=NULL;
lock_sock(s->sk); //(1) |
s->ax25_dev = NULL; | ...
release_sock(s->sk); //(2) |
... |

The root cause is that the sock is set to null before dereference
site (1) or (2). Therefore, this patch extracts the ax25_cb->sock
in advance, and uses ax25_list_lock to protect it, which can synchronize
with ax25_cb_del() and ensure the value of sock is not null before
dereference sites.

The concurrency UAF bug can be shown as below:

ax25_kill_by_device() | ax25_release()
| ax25_destroy_socket()
... | ...
| sock_put(sk); //FREE
lock_sock(s->sk); //(1) |
s->ax25_dev = NULL; | ...
release_sock(s->sk); //(2) |
... |

The root cause is that the sock is released before dereference
site (1) or (2). Therefore, this patch uses sock_hold() to increase
the refcount of sock and uses ax25_list_lock to protect it, which
can synchronize with ax25_cb_del() in ax25_destroy_socket() and
ensure the sock wil not be released before dereference sites.

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ax25/af_ax25.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 5e84dce5ff7ae..23bd26057a828 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -77,6 +77,7 @@ static void ax25_kill_by_device(struct net_device *dev)
{
ax25_dev *ax25_dev;
ax25_cb *s;
+ struct sock *sk;

if ((ax25_dev = ax25_dev_ax25dev(dev)) == NULL)
return;
@@ -85,13 +86,15 @@ static void ax25_kill_by_device(struct net_device *dev)
again:
ax25_for_each(s, &ax25_list) {
if (s->ax25_dev == ax25_dev) {
+ sk = s->sk;
+ sock_hold(sk);
spin_unlock_bh(&ax25_list_lock);
- lock_sock(s->sk);
+ lock_sock(sk);
s->ax25_dev = NULL;
- release_sock(s->sk);
+ release_sock(sk);
ax25_disconnect(s, ENETUNREACH);
spin_lock_bh(&ax25_list_lock);
-
+ sock_put(sk);
/* The entry could have been deleted from the
* list meanwhile and thus the next pointer is
* no longer valid. Play it safe and restart
--
2.34.1