linux-kernel - [PATCH bpf-next 1/3] bpf: verifier: Do not extract constant map keys for irrelevant maps

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Message-ID: <ebbf8edf871a6543425b75bb659400221bd28275.1738439839.git.dxu@dxuuu.xyz>
Date: Sat,  1 Feb 2025 12:58:01 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: daniel@...earbox.net,
	andrii@...nel.org,
	ast@...nel.org
Cc: john.fastabend@...il.com,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
	song@...nel.org,
	yonghong.song@...ux.dev,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com,
	jolsa@...nel.org,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	mhartmay@...ux.ibm.com,
	iii@...ux.ibm.com
Subject: [PATCH bpf-next 1/3] bpf: verifier: Do not extract constant map keys for irrelevant maps

Previously, we were trying to extract constant map keys for all
bpf_map_lookup_elem(), regardless of map type. This is an issue if the
map has a u64 key and the value is very high, as it can be interpreted
as a negative signed value. This in turn is treated as an error value by
check_func_arg() which causes a valid program to be incorrectly
rejected.

Fix by only extracting constant map keys for relevant maps. See next
commit for an example via selftest.

Reported-by: Marc Hartmayer <mhartmay@...ux.ibm.com>
Reported-by: Ilya Leoshkevich <iii@...ux.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@...ux.ibm.com>
Signed-off-by: Daniel Xu <dxu@...uu.xyz>
---
 kernel/bpf/verifier.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 9971c03adfd5..e9176a5ce215 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9206,6 +9206,8 @@ static s64 get_constant_map_key(struct bpf_verifier_env *env,
 	return reg->var_off.value;
 }
 
+static bool can_elide_value_nullness(enum bpf_map_type type);
+
 static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 			  struct bpf_call_arg_meta *meta,
 			  const struct bpf_func_proto *fn,
@@ -9354,9 +9356,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		err = check_helper_mem_access(env, regno, key_size, BPF_READ, false, NULL);
 		if (err)
 			return err;
-		meta->const_map_key = get_constant_map_key(env, reg, key_size);
-		if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
-			return meta->const_map_key;
+		if (can_elide_value_nullness(meta->map_ptr->map_type)) {
+			meta->const_map_key = get_constant_map_key(env, reg, key_size);
+			if (meta->const_map_key < 0 && meta->const_map_key != -EOPNOTSUPP)
+				return meta->const_map_key;
+		}
 		break;
 	case ARG_PTR_TO_MAP_VALUE:
 		if (type_may_be_null(arg_type) && register_is_null(reg))
-- 
2.47.1