net: appletalk: Fix use-after-free in AARP proxy probe

The AARP proxy‐probe routine (aarp_proxy_probe_network) sends a probe,
releases the aarp_lock, sleeps, then re-acquires the lock.  During that
window an expire timer thread (__aarp_expire_timer) can remove and
kfree() the same entry, leading to a use-after-free.

race condition:

         cpu 0                          |            cpu 1
    atalk_sendmsg()                     |   atif_proxy_probe_device()
    aarp_send_ddp()                     |   aarp_proxy_probe_network()
    mod_timer()                         |   lock(aarp_lock) // LOCK!!
    timeout around 200ms                |   alloc(aarp_entry)
    and then call                       |   proxies[hash] = aarp_entry
    aarp_expire_timeout()               |   aarp_send_probe()
                                        |   unlock(aarp_lock) // UNLOCK!!
    lock(aarp_lock) // LOCK!!           |   msleep(100);
    __aarp_expire_timer(&proxies[ct])   |
    free(aarp_entry)                    |
    unlock(aarp_lock) // UNLOCK!!       |
                                        |   lock(aarp_lock) // LOCK!!
                                        |   UAF aarp_entry !!

==================================================================
BUG: KASAN: slab-use-after-free in aarp_proxy_probe_network+0x560/0x630 net/appletalk/aarp.c:493
Read of size 4 at addr ffff8880123aa360 by task repro/13278

CPU: 3 UID: 0 PID: 13278 Comm: repro Not tainted 6.15.2 linux4sam#3 PREEMPT(full)
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1b0 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:408 [inline]
 print_report+0xc1/0x630 mm/kasan/report.c:521
 kasan_report+0xca/0x100 mm/kasan/report.c:634
 aarp_proxy_probe_network+0x560/0x630 net/appletalk/aarp.c:493
 atif_proxy_probe_device net/appletalk/ddp.c:332 [inline]
 atif_ioctl+0xb58/0x16c0 net/appletalk/ddp.c:857
 atalk_ioctl+0x198/0x2f0 net/appletalk/ddp.c:1818
 sock_do_ioctl+0xdc/0x260 net/socket.c:1190
 sock_ioctl+0x239/0x6a0 net/socket.c:1311
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:906 [inline]
 __se_sys_ioctl fs/ioctl.c:892 [inline]
 __x64_sys_ioctl+0x194/0x200 fs/ioctl.c:892
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcb/0x250 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

Allocated:
 aarp_alloc net/appletalk/aarp.c:382 [inline]
 aarp_proxy_probe_network+0xd8/0x630 net/appletalk/aarp.c:468
 atif_proxy_probe_device net/appletalk/ddp.c:332 [inline]
 atif_ioctl+0xb58/0x16c0 net/appletalk/ddp.c:857
 atalk_ioctl+0x198/0x2f0 net/appletalk/ddp.c:1818

Freed:
 kfree+0x148/0x4d0 mm/slub.c:4841
 __aarp_expire net/appletalk/aarp.c:90 [inline]
 __aarp_expire_timer net/appletalk/aarp.c:261 [inline]
 aarp_expire_timeout+0x480/0x6e0 net/appletalk/aarp.c:317

The buggy address belongs to the object at ffff8880123aa300
 which belongs to the cache kmalloc-192 of size 192
The buggy address is located 96 bytes inside of
 freed 192-byte region [ffff8880123aa300, ffff8880123aa3c0)

Memory state around the buggy address:
 ffff8880123aa200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8880123aa280: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
>ffff8880123aa300: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                       ^
 ffff8880123aa380: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
 ffff8880123aa400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Kito Xu (veritas501) <hxzene@gmail.com>
Link: https://patch.msgid.link/20250717012843.880423-1-hxzene@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: veritas501

net/appletalk/aarp.c

@@ -35,6 +35,7 @@
 #include <linux/seq_file.h>
 #include <linux/export.h>
 #include <linux/etherdevice.h>
+#include <linux/refcount.h>
 
 int sysctl_aarp_expiry_time = AARP_EXPIRY_TIME;
 int sysctl_aarp_tick_time = AARP_TICK_TIME;
@@ -44,6 +45,7 @@ int sysctl_aarp_resolve_time = AARP_RESOLVE_TIME;
 /* Lists of aarp entries */
 /**
  *	struct aarp_entry - AARP entry
+ *	@refcnt: Reference count
  *	@last_sent: Last time we xmitted the aarp request
  *	@packet_queue: Queue of frames wait for resolution
  *	@status: Used for proxy AARP
@@ -55,6 +57,7 @@ int sysctl_aarp_resolve_time = AARP_RESOLVE_TIME;
  *	@next: Next entry in chain
  */
 struct aarp_entry {
+	refcount_t			refcnt;
 	/* These first two are only used for unresolved entries */
 	unsigned long		last_sent;
 	struct sk_buff_head	packet_queue;
@@ -79,6 +82,17 @@ static DEFINE_RWLOCK(aarp_lock);
 /* Used to walk the list and purge/kick entries.  */
 static struct timer_list aarp_timer;
 
+static inline void aarp_entry_get(struct aarp_entry *a)
+{
+	refcount_inc(&a->refcnt);
+}
+
+static inline void aarp_entry_put(struct aarp_entry *a)
+{
+	if (refcount_dec_and_test(&a->refcnt))
+		kfree(a);
+}
+
 /*
  *	Delete an aarp queue
  *
@@ -87,7 +101,7 @@ static struct timer_list aarp_timer;
 static void __aarp_expire(struct aarp_entry *a)
 {
 	skb_queue_purge(&a->packet_queue);
-	kfree(a);
+	aarp_entry_put(a);
 }
 
 /*
@@ -380,9 +394,11 @@ static void aarp_purge(void)
 static struct aarp_entry *aarp_alloc(void)
 {
 	struct aarp_entry *a = kmalloc(sizeof(*a), GFP_ATOMIC);
+	if (!a)
+		return NULL;
 
-	if (a)
-		skb_queue_head_init(&a->packet_queue);
+	refcount_set(&a->refcnt, 1);
+	skb_queue_head_init(&a->packet_queue);
 	return a;
 }
 
@@ -477,6 +493,7 @@ int aarp_proxy_probe_network(struct atalk_iface *atif, struct atalk_addr *sa)
 	entry->dev = atif->dev;
 
 	write_lock_bh(&aarp_lock);
+	aarp_entry_get(entry);
 
 	hash = sa->s_node % (AARP_HASH_SIZE - 1);
 	entry->next = proxies[hash];
@@ -502,6 +519,7 @@ int aarp_proxy_probe_network(struct atalk_iface *atif, struct atalk_addr *sa)
 		retval = 1;
 	}
 
+	aarp_entry_put(entry);
 	write_unlock_bh(&aarp_lock);
 out:
 	return retval;