[PATCH V3] net: rose: Prevent the use of freed digipeat

[Lizhi Xu <lizhi.xu@windriver.com>]

There is no synchronization between the two timers, rose_t0timer_expiry
and rose_timer_expiry.
rose_timer_expiry() puts the neighbor when the rose state is ROSE_STATE_2.
However, rose_t0timer_expiry() does initiate a restart request on the
neighbor.
When rose_t0timer_expiry() accesses the released neighbor member digipeat,
a UAF is triggered.

To avoid this UAF, defer the put operation to rose_t0timer_expiry() and
stop restarting t0timer after putting the neighbor.

When putting the neighbor, set the neighbor to NULL. Setting neighbor to
NULL prevents rose_t0timer_expiry() from restarting t0timer.

syzbot reported a slab-use-after-free Read in ax25_find_cb.
BUG: KASAN: slab-use-after-free in ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
Read of size 1 at addr ffff888059c704c0 by task syz.6.2733/17200
Call Trace:
 ax25_find_cb+0x3b8/0x3f0 net/ax25/af_ax25.c:237
 ax25_send_frame+0x157/0xb60 net/ax25/ax25_out.c:55
 rose_send_frame+0xcc/0x2c0 net/rose/rose_link.c:106
 rose_transmit_restart_request+0x1b8/0x240 net/rose/rose_link.c:198
 rose_t0timer_expiry+0x1d/0x150 net/rose/rose_link.c:83

Freed by task 17183:
 kfree+0x2b8/0x6d0 mm/slub.c:6826
 rose_neigh_put include/net/rose.h:165 [inline]
 rose_timer_expiry+0x537/0x630 net/rose/rose_timer.c:183

Fixes: d860d1faa6b2 ("net: rose: convert 'use' field to refcount_t")
Reported-by: syzbot+caa052a0958a9146870d@syzkaller.appspotmail.com
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
V1 -> V2: Putting the neighbor stops t0timer from automatically starting
V2 -> V3: add rose_neigh_putex for set rose neigh to NULL

 include/net/rose.h   | 12 ++++++++++++
 net/rose/rose_link.c |  5 +++++
 2 files changed, 17 insertions(+)

diff --git a/include/net/rose.h b/include/net/rose.h
index 2b5491bbf39a..33de310ba778 100644
--- a/include/net/rose.h
+++ b/include/net/rose.h
@@ -167,6 +167,18 @@ static inline void rose_neigh_put(struct rose_neigh *rose_neigh)
 	}
 }
 
+static inline void rose_neigh_putex(struct rose_neigh **roseneigh)
+{
+	struct rose_neigh *rose_neigh = *roseneigh;
+	if (refcount_dec_and_test(&rose_neigh->use)) {
+		if (rose_neigh->ax25)
+			ax25_cb_put(rose_neigh->ax25);
+		kfree(rose_neigh->digipeat);
+		kfree(rose_neigh);
+		*roseneigh = NULL;
+	}
+}
+
 /* af_rose.c */
 extern ax25_address rose_callsign;
 extern int  sysctl_rose_restart_request_timeout;
diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
index 7746229fdc8c..334c8cc0876d 100644
--- a/net/rose/rose_link.c
+++ b/net/rose/rose_link.c
@@ -43,6 +43,9 @@ void rose_start_ftimer(struct rose_neigh *neigh)
 
 static void rose_start_t0timer(struct rose_neigh *neigh)
 {
+	if (!neigh)
+		return;
+
 	timer_delete(&neigh->t0timer);
 
 	neigh->t0timer.function = rose_t0timer_expiry;
@@ -80,10 +83,12 @@ static void rose_t0timer_expiry(struct timer_list *t)
 {
 	struct rose_neigh *neigh = timer_container_of(neigh, t, t0timer);
 
+	rose_neigh_hold(neigh);
 	rose_transmit_restart_request(neigh);
 
 	neigh->dce_mode = 0;
 
+	rose_neigh_putex(&neigh);
 	rose_start_t0timer(neigh);
 }
 
-- 
2.43.0