On Mon, Mar 9, 2026 at 4:07 AM Jiayuan Chen <[email protected]> wrote: > > syzkaller reported a kernel panic [1] with the following crash stack: > > Call Trace: > BUG: unable to handle page fault for address: ffff8ebd08580000 > PF: supervisor write access in kernel mode > PF: error_code(0x0002) - not-present page > PGD 11f201067 P4D 11f201067 PUD 0 > Oops: Oops: 0002 [#1] SMP PTI > CPU: 2 UID: 0 PID: 451 Comm: test_progs Not tainted 6.19.0+ #161 PREEMPT_RT > RIP: 0010:bond_rr_gen_slave_id+0x90/0xd0 > RSP: 0018:ffffd3f4815f3448 EFLAGS: 00010246 > RAX: 0000000000000001 RBX: 0000000000000001 RCX: ffff8ebc8728b17e > RDX: 0000000000000000 RSI: ffffd3f4815f3538 RDI: ffff8ebc8abcce40 > RBP: ffffd3f4815f3460 R08: 0000000000000000 R09: 0000000000000000 > R10: 0000000000000000 R11: 0000000000000000 R12: ffffd3f4815f3538 > R13: ffff8ebc8abcce40 R14: ffff8ebc8728b17f R15: ffff8ebc8728b170 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: ffff8ebd08580000 CR3: 000000010a808006 CR4: 0000000000770ef0 > PKRU: 55555554 > Call Trace: > <TASK> > bond_xdp_get_xmit_slave+0xc0/0x240 > xdp_master_redirect+0x74/0xc0 > bpf_prog_run_generic_xdp+0x2f2/0x3f0 > do_xdp_generic+0x1fd/0x3d0 > __netif_receive_skb_core.constprop.0+0x30d/0x1220 > __netif_receive_skb_list_core+0xfc/0x250 > netif_receive_skb_list_internal+0x20c/0x3d0 > ? eth_type_trans+0x137/0x160 > netif_receive_skb_list+0x25/0x140 > xdp_test_run_batch.constprop.0+0x65b/0x6e0 > bpf_test_run_xdp_live+0x1ec/0x3b0 > bpf_prog_test_run_xdp+0x49d/0x6e0 > __sys_bpf+0x446/0x27b0 > __x64_sys_bpf+0x1a/0x30 > x64_sys_call+0x146c/0x26e0 > do_syscall_64+0xd3/0x1510 > entry_SYSCALL_64_after_hwframe+0x76/0x7e
Please, can you always provide symbols in such traces ? You can use scripts/decode_stacktrace.sh to make the trace really nice, instead of ugly. > > Problem Description > > bond_rr_gen_slave_id() dereferences bond->rr_tx_counter without a NULL > check. rr_tx_counter is a per-CPU counter only allocated in bond_open() > when the bond mode is round-robin. If the bond device was never brought > up, rr_tx_counter remains NULL. > > The XDP redirect path can reach this code even when the bond is not up: > bpf_master_redirect_enabled_key is a global static key, so when any bond > device has native XDP attached, the XDP_TX -> xdp_master_redirect() > interception is enabled for all bond slaves system-wide. > > Solution > > Patch 1: Add a NULL check with unlikely() in bond_rr_gen_slave_id() before > dereferencing rr_tx_counter. When rr_tx_counter is NULL (bond was never > opened), fall back to get_random_u32() for slave selection. The existing > allocation in bond_open() is kept, with WRITE_ONCE() added to pair with > the READ_ONCE() in the NULL check. > Patch 2: Add a selftest that reproduces the above scenario. > > Changes since v4: > https://lore.kernel.org/netdev/[email protected]/ > - Reverted unconditional alloc in bond_init(); instead add a NULL check > with unlikely()/READ_ONCE() in bond_rr_gen_slave_id() and WRITE_ONCE() > in bond_open(), avoiding memory waste for non-RR modes > (Suggested by Nikolay Aleksandrov, patch by Jay Vosburgh) > > Changes since v3: > https://lore.kernel.org/netdev/[email protected]/T/#t > - Added code comment and commit log explaining why rr_tx_counter is > allocated unconditionally for all modes (Suggested by Jay Vosburgh) > > Changes since v2: > https://lore.kernel.org/netdev/[email protected]/T/#t > - Moved allocation from bond_create_init() helper into bond_init() > (ndo_init), which is the natural single point covering both creation > paths and also handles post-creation mode changes to round-robin > > Changes since v1: > https://lore.kernel.org/netdev/[email protected]/T/#t > - Moved the guard for NULL rr_tx_counter from xdp_master_redirect() > into the bonding subsystem itself > (Suggested by Sebastian Andrzej Siewior <[email protected]>) > > [1] https://syzkaller.appspot.com/bug?extid=80e046b8da2820b6ba73 > > Jiayuan Chen (2): > bonding: fix null-ptr-deref in bond_rr_gen_slave_id() > selftests/bpf: add test for xdp_master_redirect with bond not up > > drivers/net/bonding/bond_main.c | 9 +- > .../selftests/bpf/prog_tests/xdp_bonding.c | 101 +++++++++++++++++- > 2 files changed, 106 insertions(+), 4 deletions(-) > > -- > 2.43.0 >
