linux-kernel - [PATCH bpf-next 1/2] bpf: Notify user if we ever hit a bpf

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Message-Id: <20210422235543.4007694-2-revest@chromium.org>
Date:   Fri, 23 Apr 2021 01:55:42 +0200
From:   Florent Revest <revest@...omium.org>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kpsingh@...nel.org, jackmanb@...gle.com,
        linux-kernel@...r.kernel.org, Florent Revest <revest@...omium.org>
Subject: [PATCH bpf-next 1/2] bpf: Notify user if we ever hit a bpf_snprintf verifier bug

In check_bpf_snprintf_call(), a map_direct_value_addr() of the fmt map
should never fail because it has already been checked by
ARG_PTR_TO_CONST_STR. But if it ever fails, it's better to error out
with an explicit debug message rather than silently fail.

Signed-off-by: Florent Revest <revest@...omium.org>
Reported-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/verifier.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 58730872f7e5..59799a9b014a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5940,8 +5940,10 @@ static int check_bpf_snprintf_call(struct bpf_verifier_env *env,
 	fmt_map_off = fmt_reg->off + fmt_reg->var_off.value;
 	err = fmt_map->ops->map_direct_value_addr(fmt_map, &fmt_addr,
 						  fmt_map_off);
-	if (err)
-		return err;
+	if (err) {
+		verbose(env, "verifier bug\n");
+		return -EFAULT;
+	}
 	fmt = (char *)(long)fmt_addr + fmt_map_off;
 
 	/* We are also guaranteed that fmt+fmt_map_off is NULL terminated, we
-- 
2.31.1.498.g6c1eba8ee3d-goog