[PATCH 6.12.y] net: bonding: fix use-after-free in bond_xmit_broadcast()

[Chenglong Tang <chenglongtang@google.com>]

commit 2884bf72fb8f03409e423397319205de48adca16 upstream.

bond_xmit_broadcast() reuses the original skb for the last slave
(determined by bond_is_last_slave()) and clones it for others.
Concurrent slave enslave/release can mutate the slave list during
RCU-protected iteration, changing which slave is "last" mid-loop.
This causes the original skb to be double-consumed (double-freed).

Replace the racy bond_is_last_slave() check with a simple index
comparison (i + 1 == slaves_count) against the pre-snapshot slave
count taken via READ_ONCE() before the loop.  This preserves the
zero-copy optimization for the last slave while making the "last"
determination stable against concurrent list mutations.

The UAF can trigger the following crash:

==================================================================
BUG: KASAN: slab-use-after-free in skb_clone
Read of size 8 at addr ffff888100ef8d40 by task exploit/147

CPU: 1 UID: 0 PID: 147 Comm: exploit Not tainted 7.0.0-rc3+ #4 PREEMPTLAZY
Call Trace:
 <TASK>
 dump_stack_lvl (lib/dump_stack.c:123)
 print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
 kasan_report (mm/kasan/report.c:597)
 skb_clone (include/linux/skbuff.h:1724 include/linux/skbuff.h:1792 include/linux/skbuff.h:3396 net/core/skbuff.c:2108)
 bond_xmit_broadcast (drivers/net/bonding/bond_main.c:5334)
 bond_start_xmit (drivers/net/bonding/bond_main.c:5567 drivers/net/bonding/bond_main.c:5593)
 dev_hard_start_xmit (include/linux/netdevice.h:5325 include/linux/netdevice.h:5334 net/core/dev.c:3871 net/core/dev.c:3887)
 __dev_queue_xmit (include/linux/netdevice.h:3601 net/core/dev.c:4838)
 ip6_finish_output2 (include/net/neighbour.h:540 include/net/neighbour.h:554 net/ipv6/ip6_output.c:136)
 ip6_finish_output (net/ipv6/ip6_output.c:208 net/ipv6/ip6_output.c:219)
 ip6_output (net/ipv6/ip6_output.c:250)
 ip6_send_skb (net/ipv6/ip6_output.c:1985)
 udp_v6_send_skb (net/ipv6/udp.c:1442)
 udpv6_sendmsg (net/ipv6/udp.c:1733)
 __sys_sendto (net/socket.c:730 net/socket.c:742 net/socket.c:2206)
 __x64_sys_sendto (net/socket.c:2209)
 do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
 </TASK>

Allocated by task 147:

Freed by task 147:

The buggy address belongs to the object at ffff888100ef8c80
 which belongs to the cache skbuff_head_cache of size 224
The buggy address is located 192 bytes inside of
 freed 224-byte region [ffff888100ef8c80, ffff888100ef8d60)

Memory state around the buggy address:
 ffff888100ef8c00: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888100ef8c80: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888100ef8d00: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
                                                    ^
 ffff888100ef8d80: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
 ffff888100ef8e00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Fixes: 4e5bd03ae346 ("net: bonding: fix bond_xmit_broadcast return value error bug")
Reported-by: Weiming Shi <bestswngs@gmail.com>
Change-Id: I2349f4953b5760b7f4a12da583aa779c19c4b59c
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Link: https://patch.msgid.link/20260326075553.3960562-1-xmei5@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
[Kevin Berry <kpberry@google.com>: fixed merge conflicts and adapted
to 6.12 struct]
Signed-off-by: Chenglong Tang <chenglongtang@google.com>
---
 drivers/net/bonding/bond_main.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 2ac455a9d1bb..fb8d7fec27ee 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5346,23 +5346,33 @@ static netdev_tx_t bond_3ad_xor_xmit(struct sk_buff *skb,
 	return bond_tx_drop(dev, skb);
 }
 
-/* in broadcast mode, we send everything to all usable interfaces. */
+/* in broadcast mode, we send everything to all or usable slave interfaces.
+ * under rcu_read_lock when this function is called.
+ */
 static netdev_tx_t bond_xmit_broadcast(struct sk_buff *skb,
-				       struct net_device *bond_dev)
+				       struct net_device *bond_dev,
+				       bool all_slaves)
 {
 	struct bonding *bond = netdev_priv(bond_dev);
-	struct slave *slave = NULL;
-	struct list_head *iter;
+	struct bond_up_slave *slaves;
 	bool xmit_suc = false;
 	bool skb_used = false;
+	int slaves_count, i;
 
-	bond_for_each_slave_rcu(bond, slave, iter) {
+	if (all_slaves)
+		slaves = rcu_dereference(bond->all_slaves);
+	else
+		slaves = rcu_dereference(bond->usable_slaves);
+
+	slaves_count = slaves ? READ_ONCE(slaves->count) : 0;
+	for (i = 0; i < slaves_count; i++) {
+		struct slave *slave = slaves->arr[i];
 		struct sk_buff *skb2;
 
 		if (!(bond_slave_is_up(slave) && slave->link == BOND_LINK_UP))
 			continue;
 
-		if (bond_is_last_slave(bond, slave)) {
+		if (i + 1 == slaves_count) {
 			skb2 = skb;
 			skb_used = true;
 		} else {
@@ -5597,7 +5607,7 @@ static netdev_tx_t __bond_start_xmit(struct sk_buff *skb, struct net_device *dev
 	case BOND_MODE_XOR:
 		return bond_3ad_xor_xmit(skb, dev);
 	case BOND_MODE_BROADCAST:
-		return bond_xmit_broadcast(skb, dev);
+		return bond_xmit_broadcast(skb, dev, true);
 	case BOND_MODE_ALB:
 		return bond_alb_xmit(skb, dev);
 	case BOND_MODE_TLB:
-- 
2.54.0.rc0.605.g598a273b03-goog