CVE-2026-31419: net: bonding: fix use-after-free in bond_xmit_broadcast()

In the Linux kernel, the following vulnerability has been resolved:

net: bonding: fix use-after-free in bondxmitbroadcast()

bondxmitbroadcast() reuses the original skb for the last slave (determined by bondislastslave()) 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 bondislastslave() check with a simple index comparison (i + 1 == slavescount) against the pre-snapshot slave count taken via READONCE() 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 skbclone 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> dumpstacklvl (lib/dumpstack.c:123) printreport (mm/kasan/report.c:379 mm/kasan/report.c:482) kasanreport (mm/kasan/report.c:597) skbclone (include/linux/skbuff.h:1724 include/linux/skbuff.h:1792 include/linux/skbuff.h:3396 net/core/skbuff.c:2108) bondxmitbroadcast (drivers/net/bonding/bondmain.c:5334) bondstartxmit (drivers/net/bonding/bondmain.c:5567 drivers/net/bonding/bondmain.c:5593) devhardstartxmit (include/linux/netdevice.h:5325 include/linux/netdevice.h:5334 net/core/dev.c:3871 net/core/dev.c:3887) devqueuexmit (include/linux/netdevice.h:3601 net/core/dev.c:4838) ip6finishoutput2 (include/net/neighbour.h:540 include/net/neighbour.h:554 net/ipv6/ip6output.c:136) ip6finishoutput (net/ipv6/ip6output.c:208 net/ipv6/ip6output.c:219) ip6output (net/ipv6/ip6output.c:250) ip6sendskb (net/ipv6/ip6output.c:1985) udpv6sendskb (net/ipv6/udp.c:1442) udpv6sendmsg (net/ipv6/udp.c:1733) syssendto (net/socket.c:730 net/socket.c:742 net/socket.c:2206) x64syssendto (net/socket.c:2209) dosyscall64 (arch/x86/entry/syscall64.c:63 arch/x86/entry/syscall64.c:94) entrySYSCALL64afterhwframe (arch/x86/entry/entry64.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 skbuffheadcache 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 ==================================================================