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

Daniel Borkmann
2024-02-19T17:49:14Z

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@arm.com>
>>> 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