Re: [PATCH bpf v3 1/2] bpf: reject negative CO-RE accessor indices in bpf_core_parse_spec()

[Paul Chaignon <paul.chaignon@gmail.com>]

On Sun, Apr 05, 2026 at 12:12:20AM +0800, 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 because C integer
> promotion converts the __u16 btf_vlen result to int, making the
> comparison (int)(-1) >= (int)(N) false for any positive N.
> 
> 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 deterministically
> during BPF_PROG_LOAD on any system with CONFIG_DEBUG_INFO_BTF=y
> (default on major distributions).  The bug is reachable with CAP_BPF:
> 
>  BUG: unable to handle page fault for address: ffffed11818b6626
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  Oops: Oops: 0000 [#1] SMP KASAN NOPTI
>  CPU: 0 UID: 0 PID: 85 Comm: poc Not tainted 7.0.0-rc6 #18 PREEMPT(full)
>  RIP: 0010:bpf_core_parse_spec (tools/lib/bpf/relo_core.c:354)
>  RAX: 00000000ffffffff
>  Call Trace:
>   <TASK>
>   bpf_core_calc_relo_insn (tools/lib/bpf/relo_core.c:1321)
>   bpf_core_apply (kernel/bpf/btf.c:9507)
>   check_core_relo (kernel/bpf/verifier.c:19475)
>   bpf_check (kernel/bpf/verifier.c:26031)
>   bpf_prog_load (kernel/bpf/syscall.c:3089)
>   __sys_bpf (kernel/bpf/syscall.c:6228)
>   </TASK>
> 
> CO-RE accessor indices are inherently non-negative (struct member index,
> array element index, or enumerator index), so reject them immediately
> after parsing.
> 
> Fixes: ddc7c3042614 ("libbpf: implement BPF CO-RE offset relocation algorithm")
> Reported-by: Xiang Mei <xmei5@asu.edu>
> Signed-off-by: Weiming Shi <bestswngs@gmail.com>

Acked-by: Paul Chaignon <paul.chaignon@gmail.com>

> ---
> v3: added selftest 
> 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
> 
>