[PATCH] 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, when rose_timer_expiry() puts the neighbor, the base
member digipeat is set to NULL.

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
 call_timer_fn+0x19a/0x620 kernel/time/timer.c:1747

Fixes: dcb34659028f ("net: rose: split remove and free operations in rose_remove_neigh()")
Reported-by: syzbot+caa052a0958a9146870d@syzkaller.appspotmail.com
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
 include/net/rose.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/net/rose.h b/include/net/rose.h
index 2b5491bbf39a..9b0dc81a9589 100644
--- a/include/net/rose.h
+++ b/include/net/rose.h
@@ -163,6 +163,7 @@ static inline void rose_neigh_put(struct rose_neigh *rose_neigh)
 		if (rose_neigh->ax25)
 			ax25_cb_put(rose_neigh->ax25);
 		kfree(rose_neigh->digipeat);
+		rose_neigh->digipeat = NULL;
 		kfree(rose_neigh);
 	}
 }
-- 
2.43.0