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

[Pali Rohár]

Hello!

On Monday 06 October 2025 20:45:07 Jeongjun Park wrote:
> After the loop that converts characters to ucs2 ends, the variable i 
> may be greater than or equal to len.

It is really possible to have "i" greater than len? Because I do not see
from the code how such thing could happen.

I see only a case when i is equal to len (which is also overflow).

My understanding:
while-loop condition ensures that i cannot be greater than len and i is
increased by exfat_convert_char_to_ucs2() function which has upper bound
of "len-i". So value of i can be increased maximally by (len-i) which
could lead to maximal value of i to be just "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.
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Reported-by: syzbot+98cc76a76de46b3714d4@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=98cc76a76de46b3714d4
> Fixes: 370e812b3ec1 ("exfat: add nls operations")
> Signed-off-by: Jeongjun Park <aha310510@xxxxxxxxx>
> ---
>  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')

What about "if (i < len)" condition instead?

The p_cstring is the nul term string and my understanding is that the
"p_cstring[i] != '\0'" is checking that i is at position of strlen()+1.
So should not be "if (i < len)" the same check without need to
dereference the p_cstring?

>  		lossy |= NLS_NAME_OVERLEN;
>  
>  	*uniname = '\0';
> --