From 65fdafd6765f677494ab0cb35b8237eb742a18e5 Mon Sep 17 00:00:00 2001 From: Paul Chaignon Date: Wed, 2 Jul 2025 15:54:54 +0200 Subject: [PATCH] bpf: Avoid warning on multiple referenced args in call The description of full helper calls in syzkaller [1] and the addition of kernel warnings in commit 0df1a55afa83 ("bpf: Warn on internal verifier errors") allowed syzbot to reach a verifier state that was thought to indicate a verifier bug [2]: 12: (85) call bpf_tcp_raw_gen_syncookie_ipv4#204 verifier bug: more than one arg with ref_obj_id R2 2 2 This error can be reproduced with the program from the previous commit: 0: (b7) r2 = 20 1: (b7) r3 = 0 2: (18) r1 = 0xffff92cee3cbc600 4: (85) call bpf_ringbuf_reserve#131 5: (55) if r0 == 0x0 goto pc+3 6: (bf) r1 = r0 7: (bf) r2 = r0 8: (85) call bpf_tcp_raw_gen_syncookie_ipv4#204 9: (95) exit bpf_tcp_raw_gen_syncookie_ipv4 expects R1 and R2 to be ARG_PTR_TO_FIXED_SIZE_MEM (with a size of at least sizeof(struct iphdr) for R1). R0 is a ring buffer payload of 20B and therefore matches this requirement. The verifier reaches the check on ref_obj_id while verifying R2 and rejects the program because the helper isn't supposed to take two referenced arguments. This case is a legitimate rejection and doesn't indicate a kernel bug, so we shouldn't log it as such and shouldn't emit a kernel warning. Link: https://github.com/google/syzkaller/pull/4313 [1] Link: https://lore.kernel.org/all/686491d6.a70a0220.3b7e22.20ea.GAE@google.com/T/ [2] Fixes: 457f44363a88 ("bpf: Implement BPF ring buffer and verifier support for it") Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors") Reported-by: syzbot+69014a227f8edad4d8c6@syzkaller.appspotmail.com Signed-off-by: Paul Chaignon Link: https://lore.kernel.org/r/cd09afbfd7bef10bbc432d72693f78ffdc1e8ee5.1751463262.git.paul.chaignon@gmail.com Signed-off-by: Alexei Starovoitov Signed-off-by: Andrii Nakryiko --- kernel/bpf/verifier.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 525a21dbcce3a..52e36fd23f404 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -9673,10 +9673,10 @@ skip_type_check: if (reg->ref_obj_id && base_type(arg_type) != ARG_KPTR_XCHG_DEST) { if (meta->ref_obj_id) { - verifier_bug(env, "more than one arg with ref_obj_id R%d %u %u", - regno, reg->ref_obj_id, - meta->ref_obj_id); - return -EFAULT; + verbose(env, "more than one arg with ref_obj_id R%d %u %u", + regno, reg->ref_obj_id, + meta->ref_obj_id); + return -EACCES; } meta->ref_obj_id = reg->ref_obj_id; } -- 2.47.2