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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 2/16/24 17:55, Kees Cook wrote:
> 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/

mmh... this URL expires: 2024-05-15

> Signed-off-by: Kees Cook <keescook@chromium.org>

Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Thanks!
--
Gustavo

> ---
> 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;
>   };
>