[PATCH v2] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

[Kees Cook <keescook@chromium.org>]

Replace deprecated 0-length array in struct bpf_lpm_trie_key with
flexible array. Found with GCC 13:

../kernel/bpf/lpm_trie.c:207:51: warning: array subscript i is outside array bounds of 'const __u8[0]' {aka 'const unsigned char[]'} [-Warray-bounds=]
  207 |                                        *(__be16 *)&key->data[i]);
      |                                                   ^~~~~~~~~~~~~
../include/uapi/linux/swab.h:102:54: note: in definition of macro '__swab16'
  102 | #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
      |                                                      ^
../include/linux/byteorder/generic.h:97:21: note: in expansion of macro '__be16_to_cpu'
   97 | #define be16_to_cpu __be16_to_cpu
      |                     ^~~~~~~~~~~~~
../kernel/bpf/lpm_trie.c:206:28: note: in expansion of macro 'be16_to_cpu'
  206 |                 u16 diff = be16_to_cpu(*(__be16 *)&node->data[i]
^
      |                            ^~~~~~~~~~~
In file included from ../include/linux/bpf.h:7:
../include/uapi/linux/bpf.h:82:17: note: while referencing 'data'
   82 |         __u8    data[0];        /* Arbitrary size */
      |                 ^~~~

And found at run-time under CONFIG_FORTIFY_SOURCE:

  UBSAN: array-index-out-of-bounds in kernel/bpf/lpm_trie.c:218:49
  index 0 is out of range for type '__u8 [*]'

This includes fixing the selftest which was incorrectly using a
variable length struct as a header, identified earlier[1]. Avoid this
by just explicitly including the prefixlen member instead of struct
bpf_lpm_trie_key.

Note that it is not possible to simply remove the "data" member, as it
is referenced by userspace

cilium:
        struct egress_gw_policy_key in_key = {
                .lpm_key = { 32 + 24, {} },
                .saddr   = CLIENT_IP,
                .daddr   = EXTERNAL_SVC_IP & 0Xffffff,
        };

systemd:
	ipv6_map_fd = bpf_map_new(
			BPF_MAP_TYPE_LPM_TRIE,
			offsetof(struct bpf_lpm_trie_key, data) + sizeof(uint32_t)*4,
			sizeof(uint64_t),
			...

The only risk to UAPI would be if sizeof() were used directly on the
data member, which it does not seem to be. It is only used as a static
initializer destination and to find its location via offsetof().

Link: https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1]
Reported-by: Mark Rutland <mark.rutland@arm.com>
Closes: https://paste.debian.net/hidden/ca500597/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Cc: Song Liu <song@kernel.org>
Cc: Yonghong Song <yhs@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Stanislav Fomichev <sdf@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mykola Lysenko <mykolal@fb.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Haowen Bai <baihaowen@meizu.com>
Cc: bpf@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

v2- clarify commit log, add more failure examples
v1- https://lore.kernel.org/all/63e531e3.170a0220.3a46a.3262@mx.google.com/
---
 include/uapi/linux/bpf.h                         | 2 +-
 tools/testing/selftests/bpf/progs/map_ptr_kern.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 754e68ca8744..359dd8a429c1 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -80,7 +80,7 @@ struct bpf_insn {
 /* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
 struct bpf_lpm_trie_key {
 	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
-	__u8	data[0];	/* Arbitrary size */
+	__u8	data[];		/* Arbitrary size */
 };
 
 struct bpf_cgroup_storage_key {
diff --git a/tools/testing/selftests/bpf/progs/map_ptr_kern.c b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
index 3325da17ec81..1d476c6ae284 100644
--- a/tools/testing/selftests/bpf/progs/map_ptr_kern.c
+++ b/tools/testing/selftests/bpf/progs/map_ptr_kern.c
@@ -316,7 +316,7 @@ struct lpm_trie {
 } __attribute__((preserve_access_index));
 
 struct lpm_key {
-	struct bpf_lpm_trie_key trie_key;
+	__u32 prefixlen;
 	__u32 data;
 };
 
-- 
2.34.1