linux-kernel - [PATCH] fs/ntfs3: fix negative shift size in true_sectors_per

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Message-ID: <4b37f037-3b10-b4e4-0644-73441c8fa0af@I-love.SAKURA.ne.jp>
Date:   Wed, 21 Sep 2022 00:59:27 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Konstantin Komarov <almaz.alexandrovich@...agon-software.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Namjae Jeon <linkinjeon@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>
Cc:     syzbot <syzbot+1631f09646bc214d2e76@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, ntfs3@...ts.linux.dev,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] fs/ntfs3: fix negative shift size in true_sectors_per_clst()

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@...kaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+1631f09646bc214d2e76@...kaller.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;
 }
 
-- 
2.18.4