Re: [PATCH] fs/ntfs3: Avoid UBSAN error on true_sectors_per_clst()

[Shigeru Yoshida <syoshida@redhat.com>]

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On Tue, 23 Aug 2022 23:46:25 +0900, Shigeru Yoshida wrote:
> syzbot reported UBSAN error as below:
> 
> [   76.901829][ T6677] ================================================================================
> [   76.903908][ T6677] UBSAN: shift-out-of-bounds in fs/ntfs3/super.c:675:13
> [   76.905363][ T6677] shift exponent -247 is negative
> 
> This patch avoid this error.
> 
> Link: https://syzkaller.appspot.com/bug?id=b0299c09a14aababf0f1c862dd4ebc8ab9eb0179
> Fixes: a3b774342fa7 (fs/ntfs3: validate BOOT sectors_per_clusters)
> Cc: Author: Randy Dunlap <rdunlap@infradead.org>
> Reported-by: syzbot+35b87c668935bb55e666@syzkaller.appspotmail.com
> Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
> ---
>  fs/ntfs3/super.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ntfs3/super.c b/fs/ntfs3/super.c
> index 47012c9bf505..adc4f73722b7 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 << -(s8)boot->sectors_per_clusters;
>  	return -EINVAL;
>  }
>  
> -- 
> 2.37.2
>