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Fri, 03 Apr 2026 03:43:45 -0700 (PDT) Received: from SLSGDTSWING002.tail0ac356.ts.net ([129.126.109.177]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c76c6491157sm4525653a12.9.2026.04.03.03.43.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 03 Apr 2026 03:43:44 -0700 (PDT) From: Weiming Shi To: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko Cc: Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , bpf@vger.kernel.org, Xiang Mei , Weiming Shi Subject: [PATCH bpf v2] bpf: reject negative CO-RE accessor indices in bpf_core_parse_spec() Date: Fri, 3 Apr 2026 18:41:45 +0800 Message-ID: <20260403104144.4132126-2-bestswngs@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit CO-RE accessor strings are colon-separated indices that describe a path from a root BTF type to a target field, e.g. "0:1:2" walks through nested struct members. bpf_core_parse_spec() parses each component with sscanf("%d"), so negative values like -1 are silently accepted. The subsequent bounds checks (access_idx >= btf_vlen(t)) only guard the upper bound and always pass for negative values. When -1 reaches btf_member_bit_offset() it gets cast to u32 0xffffffff, producing an out-of-bounds read far past the members array. A crafted BPF program with a negative CO-RE accessor on any struct that exists in vmlinux BTF (e.g. task_struct) crashes the kernel during BPF_PROG_LOAD: BUG: unable to handle page fault for address: ffffed11818b6626 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 7f74e067 P4D 7f74e067 PUD 0 Oops: Oops: 0000 [#1] SMP KASAN NOPTI CPU: 0 UID: 0 PID: 85 Comm: poc Not tainted 7.0.0-rc6 #18 PREEMPT(full) Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:bpf_core_parse_spec (tools/lib/bpf/relo_core.c:348) RAX: 00000000ffffffff RBX: ffff88800c5b3128 RCX: 0000000000000000 Call Trace: bpf_core_calc_relo_insn (tools/lib/bpf/relo_core.c:1319) bpf_core_apply (kernel/bpf/btf.c:9507) bpf_check (kernel/bpf/verifier.c:26031) bpf_prog_load (kernel/bpf/syscall.c:3089) __sys_bpf (kernel/bpf/syscall.c:6228) __x64_sys_bpf (kernel/bpf/syscall.c:6339) do_syscall_64 (arch/x86/entry/syscall_64.c:94) CO-RE accessor indices are inherently non-negative (field index, array index, or enumerator index), so reject them after parsing. Fixes: ddc7c3042614 ("libbpf: implement BPF CO-RE offset relocation algorithm") Reported-by: Xiang Mei Signed-off-by: Weiming Shi --- v2: fix typo Signed-off-by tag (missing leading 'S') tools/lib/bpf/relo_core.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/tools/lib/bpf/relo_core.c b/tools/lib/bpf/relo_core.c index 6eea5edba58a..0ccc8f548cba 100644 --- a/tools/lib/bpf/relo_core.c +++ b/tools/lib/bpf/relo_core.c @@ -292,6 +292,8 @@ int bpf_core_parse_spec(const char *prog_name, const struct btf *btf, ++spec_str; if (sscanf(spec_str, "%d%n", &access_idx, &parsed_len) != 1) return -EINVAL; + if (access_idx < 0) + return -EINVAL; if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN) return -E2BIG; spec_str += parsed_len; -- 2.43.0