Re: [PATCH] exfat: fix out-of-bounds in exfat_nls_to_ucs2()

[Namjae Jeon <linkinjeon@kernel.org>]

On Mon, Oct 6, 2025 at 8:45 PM Jeongjun Park <aha310510@gmail.com> wrote:
>
Hi Jeongjun,
> After the loop that converts characters to ucs2 ends, the variable i
> may be greater than or equal to len. However, when checking whether the
> last byte of p_cstring is NULL, the variable i is used as is, resulting
> in an out-of-bounds read if i >= len.
>
> Therefore, to prevent this, we need to modify the function to check
> whether i is less than len, and if i is greater than or equal to len,
> to check p_cstring[len - 1] byte.
I think we need to pass FSLABEL_MAX - 1 to exfat_nls_to_utf16, not FSLABEL_MAX.
Can you check it and update the patch?
Thanks.
>
> Cc: <stable@vger.kernel.org>
> Reported-by: syzbot+98cc76a76de46b3714d4@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> Fixes: 370e812b3ec1 ("exfat: add nls operations")
> Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> ---
>  fs/exfat/nls.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/exfat/nls.c b/fs/exfat/nls.c
> index 8243d94ceaf4..a52f3494eb20 100644
> --- a/fs/exfat/nls.c
> +++ b/fs/exfat/nls.c
> @@ -616,7 +616,7 @@ static int exfat_nls_to_ucs2(struct super_block *sb,
>                 unilen++;
>         }
>
> -       if (p_cstring[i] != '\0')
> +       if (p_cstring[min(i, len - 1)] != '\0')
>                 lossy |= NLS_NAME_OVERLEN;
>
>         *uniname = '\0';
> --