linux-kernel - [PATCH] bpf: Reject BPF_MAP_TYPE_INSN_ARRAY in check_reg_const

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Message-ID: <20260107021037.289644-1-kartikey406@gmail.com>
Date: Wed,  7 Jan 2026 07:40:37 +0530
From: Deepanshu Kartikey <kartikey406@...il.com>
To: ast@...nel.org,
	daniel@...earbox.net,
	john.fastabend@...il.com,
	andrii@...nel.org,
	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,
	a.s.protopopov@...il.com
Cc: bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Deepanshu Kartikey <kartikey406@...il.com>,
	syzbot+2c29addf92581b410079@...kaller.appspotmail.com
Subject: [PATCH] bpf: Reject BPF_MAP_TYPE_INSN_ARRAY in check_reg_const_str()

BPF_MAP_TYPE_INSN_ARRAY maps store instruction pointers in their
ips array, not string data. The map_direct_value_addr callback for
this map type returns the address of the ips array, which is not
suitable for use as a constant string argument.

When a BPF program passes a pointer to an insn_array map value as
ARG_PTR_TO_CONST_STR (e.g., to bpf_snprintf), the verifier's
null-termination check in check_reg_const_str() operates on the
wrong memory region, and at runtime bpf_bprintf_prepare() can read
out of bounds searching for a null terminator.

Reject BPF_MAP_TYPE_INSN_ARRAY in check_reg_const_str() since this
map type is not designed to hold string data.

Reported-by: syzbot+2c29addf92581b410079@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=2c29addf92581b410079
Tested-by: syzbot+2c29addf92581b410079@...kaller.appspotmail.com
Fixes: 493d9e0d6083 ("bpf, x86: add support for indirect jumps")
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f0ca69f888fa..3135643d5695 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9609,6 +9609,11 @@ static int check_reg_const_str(struct bpf_verifier_env *env,
 	if (reg->type != PTR_TO_MAP_VALUE)
 		return -EINVAL;
 
+	if (map->map_type == BPF_MAP_TYPE_INSN_ARRAY) {
+		verbose(env, "R%d points to insn_array map which cannot be used as const string\n", regno);
+		return -EACCES;
+	}
+
 	if (!bpf_map_is_rdonly(map)) {
 		verbose(env, "R%d does not point to a readonly map'\n", regno);
 		return -EACCES;
-- 
2.43.0