Re: [PATCH bpf-next 1/3] bpf: Check tail zero of bpf_map_info

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

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/20260513224823.6494FC19425@smtp.kernel.org/
> 
> Fixes: ea2e6467ac36 ("bpf: Return hashes of maps in BPF_OBJ_GET_INFO_BY_FD")
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>

>  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);