On 6/3/26 3:45 PM, Leon Hwang wrote:
> Since there're 4 bytes padding at the end of struct bpf_map_info, they
> won't be checked by bpf_check_uarg_tail_zero().
>
> pahole -C bpf_map_info ./vmlinux
> struct bpf_map_info {
> ...
> __u64 hash __attribute__((__aligned__(8))); /*
> 88 8 */
> __u32 hash_size; /* 96 4 */
>
> /* size: 104, cachelines: 2, members: 18 */
> /* padding: 4 */
> /* forced alignments: 1 */
> /* last cacheline: 40 bytes */
> } __attribute__((__aligned__(8)));
>
> If a future kernel extension adds a new 4-byte field, older userspace
> programs allocating this structure on the stack might inadvertently pass
> uninitialized stack garbage into the new field, permanently breaking
> backward compatibility. -- sashiko [1]
>
> Fix it by changing sizeof(info) to
> offsetofend(struct bpf_map_info, hash_size).
>
> [1] https://lore.kernel.org/bpf/[email protected]/
>
> Fixes: ea2e6467ac36 ("bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD")
> Signed-off-by: Leon Hwang <[email protected]>
> ---
Acked-by: Mykyta Yatsenko <[email protected]>
> kernel/bpf/syscall.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 625a4366fe6d..1007fb7c87e9 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5400,10 +5400,11 @@ static int bpf_map_get_info_by_fd(struct file *file,
> {
> struct bpf_map_info __user *uinfo = u64_to_user_ptr(attr->info.info);
> struct bpf_map_info info;
> - u32 info_len = attr->info.info_len;
> + u32 info_len = attr->info.info_len, len;
> int err;
>
> - err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info),
> info_len);
> + len = offsetofend(struct bpf_map_info, hash_size);
> + err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), len, info_len);
> if (err)
> return err;
> info_len = min_t(u32, sizeof(info), info_len);
