On 2/17/24 4:03 AM, Kees Cook wrote:
On Fri, Feb 16, 2024 at 06:27:08PM -0600, Gustavo A. R. Silva wrote:
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@xxxxxxx>
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?
Thanks,
Daniel
