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

[Weiming Shi <bestswngs@gmail.com>]

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>
---
 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