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Fri, 03 Apr 2026 15:13:42 -0700 (PDT) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Fri, 03 Apr 2026 18:13:41 -0400 Message-Id: To: "Weiming Shi" , "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" , , "Xiang Mei" Subject: Re: [PATCH bpf] bpf: reject negative CO-RE accessor indices in bpf_core_parse_spec() From: "Emil Tsalapatis" X-Mailer: aerc 0.21.0-0-g5549850facc2 References: <20260403035111.4031084-2-bestswngs@gmail.com> In-Reply-To: <20260403035111.4031084-2-bestswngs@gmail.com> On Thu Apr 2, 2026 at 11:51 PM EDT, Weiming Shi wrote: > 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 >=3D 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, 19= 96), 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 algor= ithm") > Reported-by: Xiang Mei > igned-off-by: Weiming Shi As mentioned by the bot, there is a missing S here. For the patch itself: 1) Please add a selftest for this. The offending CO:RE program should work. 2) Judging from your description, the code below just band-aids the problem= on the libbpf side. This sounds like a bounds checking issue directly in the kernel. > --- > 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) !=3D 1) > return -EINVAL; > + if (access_idx < 0) > + return -EINVAL; > if (spec->raw_len =3D=3D BPF_CORE_SPEC_MAX_LEN) > return -E2BIG; > spec_str +=3D parsed_len; > -- > 2.43.0