Re: [PATCH] fs/ntfs3: validate BOOT sectors_per_clusters

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On Fri, Apr 29, 2022 at 10:27:11AM -0700, Randy Dunlap wrote:
> When the NTFS BOOT sectors_per_clusters field is > 0x80,
> it represents a shift value. First change its sign to positive
> and then make sure that the shift count is not too large.
> This prevents negative shift values and shift values that are
> larger than the field size.
> 
> Prevents this UBSAN error:
> 
>  UBSAN: shift-out-of-bounds in ../fs/ntfs3/super.c:673:16
>  shift exponent -192 is negative
> 
> Fixes: 82cae269cfa9 ("fs/ntfs3: Add initialization of super block")
> Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Reported-by: syzbot+1631f09646bc214d2e76@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: Konstantin Komarov <almaz.alexandrovich@xxxxxxxxxxxxxxxxxxxx>
> Cc: ntfs3@xxxxxxxxxxxxxxx
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kari Argillander <kari.argillander@xxxxxxxxxxxxxxxxxxxx>
> Cc: Namjae Jeon <linkinjeon@xxxxxxxxxx>
> ---
>  fs/ntfs3/super.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> --- linux-next-20220428.orig/fs/ntfs3/super.c
> +++ linux-next-20220428/fs/ntfs3/super.c
> @@ -670,7 +670,8 @@ static u32 true_sectors_per_clst(const s
>  {
>  	return boot->sectors_per_clusters <= 0x80
>  		       ? boot->sectors_per_clusters
> -		       : (1u << (0 - boot->sectors_per_clusters));
> +		       : -(s8)boot->sectors_per_clusters > 31 ? -1
> +		       : (1u << -(s8)boot->sectors_per_clusters);
>  }

This hurts my brain.  Can we do instead:

	if (boot->sectors_per_clusters <= 0x80)
		return boot->sectors_per_clusters;
	if (boot->sectors_per_clusters < 0xA0)
		return 1U << (boot->sectors_per_clusters - 0x80);
	return -1;

>  /*
> @@ -713,7 +714,7 @@ static int ntfs_init_from_boot(struct su
>  
>  	/* cluster size: 512, 1K, 2K, 4K, ... 2M */
>  	sct_per_clst = true_sectors_per_clst(boot);
> -	if (!is_power_of_2(sct_per_clst))
> +	if ((int)sct_per_clst < 0 || !is_power_of_2(sct_per_clst))
>  		goto out;

Do we need this change?  Presumably -1 is not a power of 2 ...



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