[PATCH bpf v3 1/2] 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 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>
---
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