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

[Xiang Mei <xmei5@asu.edu>]

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 either double-consumed or freed.

Always clone the skb for every slave and unconditionally free the
original after the loop, removing the racy bond_is_last_slave()
optimization.

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>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
---
 drivers/net/bonding/bond_main.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 33f414d03ab91..8e57bee13e4ad 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -5310,7 +5310,6 @@ static netdev_tx_t bond_xmit_broadcast(struct sk_buff *skb,
 	struct bonding *bond = netdev_priv(bond_dev);
 	struct bond_up_slave *slaves;
 	bool xmit_suc = false;
-	bool skb_used = false;
 	int slaves_count, i;
 
 	if (all_slaves)
@@ -5326,24 +5325,18 @@ static netdev_tx_t bond_xmit_broadcast(struct sk_buff *skb,
 		if (!(bond_slave_is_up(slave) && slave->link == BOND_LINK_UP))
 			continue;
 
-		if (bond_is_last_slave(bond, slave)) {
-			skb2 = skb;
-			skb_used = true;
-		} else {
-			skb2 = skb_clone(skb, GFP_ATOMIC);
-			if (!skb2) {
-				net_err_ratelimited("%s: Error: %s: skb_clone() failed\n",
-						    bond_dev->name, __func__);
-				continue;
-			}
+		skb2 = skb_clone(skb, GFP_ATOMIC);
+		if (!skb2) {
+			net_err_ratelimited("%s: Error: %s: skb_clone() failed\n",
+					    bond_dev->name, __func__);
+			continue;
 		}
 
 		if (bond_dev_queue_xmit(bond, skb2, slave->dev) == NETDEV_TX_OK)
 			xmit_suc = true;
 	}
 
-	if (!skb_used)
-		dev_kfree_skb_any(skb);
+	dev_kfree_skb_any(skb);
 
 	if (xmit_suc)
 		return NETDEV_TX_OK;
-- 
2.43.0