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
Link: https://lore.kernel.org/all/686491d6.a70a0220.3b7e22.20ea.GAE@google.com/T/
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 <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/cd09afbfd7bef10bbc432d72693f78ffdc1e8ee5.1751463262.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 525a21dbcce3a7c672562c51367b414d774e87b6..52e36fd23f4042ce77d13d21abc23858db20af4a 100644 (file)
--- 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;
        }