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

[Paolo Abeni <pabeni@redhat.com>]

On 3/19/26 12:23 AM, Xiang Mei wrote:
> 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);

The additional clone is prone allocation failure (atomic scope) and adds
additional overhead. Is really needed? Would something alike the
following work instead of the current bond_is_last_slave() check?

		if (i + 1 == slaves_count)

/P