[PATCH bpf v2 RESEND 1/2] bpf: reject dev-bound-only programs on other devices

[Weiming Shi <bestswngs@gmail.com>]

__bpf_offload_dev_match() falls back to comparing offdev pointers after an
exact netdev mismatch. Bound-only programs normally have NULL offdevs, so
unrelated netdevs compare equal. A bound-only program on an
offload-registered netdev can instead inherit a real offdev and match a
sibling port. With CAP_BPF and CAP_NET_ADMIN, a caller can use
bpf(BPF_LINK_CREATE) with a different target ifindex to run metadata kfuncs
specialized for the bound driver on the target driver's xdp_buff. Running a
veth-bound program on tun reads beyond tun's bare stack xdp_buff as a
veth_xdp_buff.

  Oops: general protection fault, probably for non-canonical address
  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:veth_xdp_rx_timestamp (drivers/net/veth.c:1673)
  Call Trace:
   ...
   tun_build_skb (drivers/net/tun.c:1739)
   tun_get_user (drivers/net/tun.c:1856)
   tun_chr_write_iter (drivers/net/tun.c:2091)
   vfs_write (fs/read_write.c:595 fs/read_write.c:687)
   ksys_write (fs/read_write.c:739)
   do_syscall_64 (arch/x86/entry/syscall_64.c:84)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception in interrupt

Restrict non-offloaded programs to exact netdev matches and retain the
shared-offdev fallback only for genuinely offloaded multi-port programs.

Cc: stable@vger.kernel.org
Fixes: 2b3486bc2d23 ("bpf: Introduce device-bound XDP programs")
Reported-by: <co+ac0a8c41de69121d@bugs.sh>
Link: https://lore.kernel.org/bpf/20260917161335.1020405-2-bestswngs@gmail.com/
Assisted-by: LLM
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
Resend:
- Start a new thread after the previous v2 was threaded under v1; no patch
  changes.

Changes in v2:
- Drop the redundant offdev NULL check.
- State the capability requirements instead of using attacker wording.
- Move the regression test to patch 2.

 kernel/bpf/offload.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/offload.c b/kernel/bpf/offload.c
index 0d6f55695..d85539981 100644
--- a/kernel/bpf/offload.c
+++ b/kernel/bpf/offload.c
@@ -698,6 +698,8 @@ static bool __bpf_offload_dev_match(struct bpf_prog *prog,
 		return false;
 	if (offload->netdev == netdev)
 		return true;
+	if (!bpf_prog_is_offloaded(prog->aux))
+		return false;
 
 	ondev1 = bpf_offload_find_netdev(offload->netdev);
 	ondev2 = bpf_offload_find_netdev(netdev);
-- 
2.55.0