On 6/3/26 3:45 PM, Leon Hwang wrote:
> Since there're 4 bytes padding at the end of struct bpf_prog_info, they
> won't be checked by bpf_check_uarg_tail_zero().
>
> pahole -C bpf_prog_info ./vmlinux
> struct bpf_prog_info {
> ...
> __u32 attach_btf_obj_id; /* 220 4 */
> __u32 attach_btf_id; /* 224 4 */
>
> /* size: 232, cachelines: 4, members: 38 */
> /* sum members: 224 */
> /* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 31 bits */
> /* padding: 4 */
> /* forced alignments: 9 */
> /* 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_prog_info, attach_btf_id).
>
> [1] https://lore.kernel.org/bpf/[email protected]/
>
> Fixes: b79c9fc9551b ("bpf: implement BPF_PROG_QUERY for BPF_LSM_CGROUP")
> 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 1007fb7c87e9..0c86a70d31c1 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -5115,10 +5115,11 @@ static int bpf_prog_get_info_by_fd(struct file *file,
> u32 info_len = attr->info.info_len;
> struct bpf_prog_kstats stats;
> char __user *uinsns;
> - u32 ulen;
> + u32 ulen, len;
> int err;
>
> - err = bpf_check_uarg_tail_zero(USER_BPFPTR(uinfo), sizeof(info),
> info_len);
> + len = offsetofend(struct bpf_prog_info, attach_btf_id);
> + 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);
