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Message-ID: <e0173c17-3837-a619-4bcc-7a0ba4843cc4@infradead.org>
Date:   Thu, 22 Sep 2022 17:38:53 -0700
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Subject: Re: [PATCH] fs/ntfs3: fix negative shift size in
 true_sectors_per_clst()
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Konstantin Komarov <almaz.alexandrovich@paragon-software.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Namjae Jeon <linkinjeon@kernel.org>,
        Shigeru Yoshida <syoshida@redhat.com>
Cc:     syzbot <syzbot+1631f09646bc214d2e76@syzkaller.appspotmail.com>,
        syzkaller-bugs@googlegroups.com, ntfs3@lists.linux.dev,
        LKML <linux-kernel@vger.kernel.org>
References: <000000000000f8b5ef05dd25b963@google.com>
 <4b37f037-3b10-b4e4-0644-73441c8fa0af@I-love.SAKURA.ne.jp>
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From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <4b37f037-3b10-b4e4-0644-73441c8fa0af@I-love.SAKURA.ne.jp>
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Hi,

On 9/20/22 08:59, 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;
>  }
>  

I slightly prefer the earlier patch:

https://lore.kernel.org/all/20220823144625.1627736-1-syoshida@redhat.com/

but it appears that the NTFS3 maintainer is MIA again. :(

-- 
~Randy