bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

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] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

Signed-off-by Kees Cook <[email protected]>

---
Cc: Alexei Starovoitov <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Yonghong Song <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: KP Singh <[email protected]>
Cc: Stanislav Fomichev <[email protected]>
Cc: Hao Luo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mykola Lysenko <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Haowen Bai <[email protected]>
Cc: [email protected]
Cc: [email protected]

v2- clarify commit log, add more failure examples
v1- https://lore.kernel.org/all/[email protected]/

---
  include/uapi/linux/bpf.h                         | 2 +-
  tools/testing/selftests/bpf/progs/map_ptr_kern.c | 2 +-
  2 files changed, 2 insertions(+), 2 deletions(-)

include/uapi/linux/bpf.h +1 -1

index 754e68ca8744..359dd8a429c1 100644
@@ -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 {

tools/testing/selftests/bpf/progs/map_ptr_kern.c +1 -1

index 3325da17ec81..1d476c6ae284 100644
@@ -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;
  };
 

Quoted 52 lines · Kees Cook

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().

Quoted 3 lines

Link https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

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

Quoted 53 lines · Gustavo A. R. Silva

Quoted 52 lines · Kees Cook

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().

Quoted 5 lines

Quoted 3 lines

Link https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

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

Yup, but that's why I included the run-time splat above too. :)

Quoted 54 lines · Kees Cook

Quoted 53 lines · Gustavo A. R. Silva

Quoted 52 lines · Kees Cook

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().

Quoted 7 lines

Quoted 5 lines

Quoted 3 lines

Link https://lore.kernel.org/all/202206281009.4332AA33@keescook/ [1] open here

Reported-by Mark Rutland <[email protected]>

Closes https://paste.debian.net/hidden/ca500597/

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

Yup, but that's why I included the run-time splat above too. :)

I don't quite follow, this basically undoes 3024d95a4c52 ("bpf: Partially revert flexible-array member replacement") again with the small change that this 'fixes' up the BPF selftest to not embed struct bpf_lpm_trie_key.

Outside of BPF selftests though aren't we readding the same error that we fixed earlier for BPF programs in the wild which embed struct bpf_lpm_trie_key into their key structure?