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 <redacted>
---
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;
+       }
+}