Re: [PATCH bpf] bpf: reject negative CO-RE accessor indices in bpf_core_parse_spec()

["Emil Tsalapatis" <emil@etsalapatis.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 >= 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:
>   <TASK>
>   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)
>   </TASK>
>
> 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 <xmei5@asu.edu>
> igned-off-by: Weiming Shi <bestswngs@gmail.com>

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) != 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