netdev - [PATCH bpf-next 2/4] bpf: verifier: explain opcode check in check_ld

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Date:   Fri, 20 May 2022 19:37:26 +0800
From:   Shung-Hsi Yu <shung-hsi.yu@...e.com>
To:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
CC:     Shung-Hsi Yu <shung-hsi.yu@...e.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>
Subject: [PATCH bpf-next 2/4] bpf: verifier: explain opcode check in check_ld_imm()

The BPF_SIZE check in the beginning of check_ld_imm() actually guard
against program with JMP instructions that goes to the second
instruction of BPF_LD_IMM64, but may be easily dismissed as an simple
opcode check that's duplicating the effort of bpf_opcode_in_insntable().

Add comment to better reflect the importance of the check.

Signed-off-by: Shung-Hsi Yu <shung-hsi.yu@...e.com>
---
 kernel/bpf/verifier.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 79a2695ee2e2..133929751f80 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -9921,6 +9921,10 @@ static int check_ld_imm(struct bpf_verifier_env *env, struct bpf_insn *insn)
 	struct bpf_map *map;
 	int err;
 
+	/* checks that this is not the second part of BPF_LD_IMM64, which is
+	 * skipped over during opcode check, but a JMP with invalid offset may
+	 * cause check_ld_imm() to be called upon it.
+	 */
 	if (BPF_SIZE(insn->code) != BPF_DW) {
 		verbose(env, "invalid BPF_LD_IMM insn\n");
 		return -EINVAL;
-- 
2.36.1