Re: [PATCH] fs/ntfs3: fix negative shift size in true_sectors_per_clst()

[Konstantin Komarov <almaz.alexandrovich@paragon-software.com>]

On 9/20/22 18:59, Tetsuo Handa wrote:
> syzbot is reporting shift-out-of-bounds in true_sectors_per_clst() [1], for
> commit a3b774342fa752a5 ("fs/ntfs3: validate BOOT sectors_per_clusters")
> did not address that (0 - boot->sectors_per_clusters) < 0 because "u8" was
> chosen for type of boot->sectors_per_clusters because 0x80 needs to be
> positive in order to support 64K clusters. Use "s8" cast in order to make
> sure that (0 - (s8) boot->sectors_per_clusters) > 0.
> 
> Link: https://syzkaller.appspot.com/bug?extid=1631f09646bc214d2e76 [1]
> Reported-by: syzbot <syzbot+1631f09646bc214d2e76@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Tested-by: syzbot <syzbot+1631f09646bc214d2e76@syzkaller.appspotmail.com>
> Fixes: a3b774342fa752a5 ("fs/ntfs3: validate BOOT sectors_per_clusters")
> 
> diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
> index 47012c9bf505..c7ffd21fb255 100644
> --- a/fs/ntfs3/super.c
> +++ b/fs/ntfs3/super.c
> @@ -672,7 +672,7 @@ static u32 true_sectors_per_clst(const struct NTFS_BOOT *boot)
>   	if (boot->sectors_per_clusters <= 0x80)
>   		return boot->sectors_per_clusters;
>   	if (boot->sectors_per_clusters >= 0xf4) /* limit shift to 2MB max */
> -		return 1U << (0 - boot->sectors_per_clusters);
> +		return 1U << (0 - (s8) boot->sectors_per_clusters);
>   	return -EINVAL;
>   }
>   

Hello
Thanks for patch, but there was already a similar patch by Shigeru Yoshida,
so I chose it.
Sorry about that, thanks again for your work.