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From:   Anant Thazhemadam <anant.thazhemadam@gmail.com>
To:     Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>
Cc:     linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org,
        Anant Thazhemadam <anant.thazhemadam@gmail.com>,
        syzbot+ca9a785f8ac472085994@syzkaller.appspotmail.com
Subject: [PATCH] fs: f2fs: fix potential shift-out-of-bounds error in sanity_check_raw_super()
Date:   Wed,  9 Dec 2020 23:43:22 +0530
Message-Id: <20201209181322.27932-1-anant.thazhemadam@gmail.com>
MIME-Version: 1.0
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In sanity_check_raw_super(), if
1 << le32_to_cpu(raw_super->log_blocksize) != F2FS_BLKSIZE, then the
block size is deemed to be invalid.

syzbot triggered a shift-out-of-bounds bug by assigning a value of 59 to
le32_to_cpu(raw_super->log_blocksize).
Although the value assigned itself isn't of much significance, this goes
to show that even if the block size is invalid,
le32_to_cpu(raw_super->log_blocksize) can be potentially evaluated to a
value for which the shift exponent becomes too large for the unsigned
int.

Since 1 << le32_to_cpu(raw_super->log_blocksize) must be = 4096 for a
valid block size, le32_to_cpu(raw_super->log_blocksize) must equal 12.
Replacing the existing check with the more direct sanity check
resolves this bug.

Reported-by: syzbot+ca9a785f8ac472085994@syzkaller.appspotmail.com
Tested-by: syzbot+ca9a785f8ac472085994@syzkaller.appspotmail.com
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@gmail.com>
---
 fs/f2fs/super.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 33808c397580..4bc7372af43f 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2775,7 +2775,6 @@ static int sanity_check_raw_super(struct f2fs_sb_info *sbi,
 	block_t total_sections, blocks_per_seg;
 	struct f2fs_super_block *raw_super = (struct f2fs_super_block *)
 					(bh->b_data + F2FS_SUPER_OFFSET);
-	unsigned int blocksize;
 	size_t crc_offset = 0;
 	__u32 crc = 0;
 
@@ -2802,10 +2801,8 @@ static int sanity_check_raw_super(struct f2fs_sb_info *sbi,
 	}
 
 	/* Currently, support only 4KB block size */
-	blocksize = 1 << le32_to_cpu(raw_super->log_blocksize);
-	if (blocksize != F2FS_BLKSIZE) {
-		f2fs_info(sbi, "Invalid blocksize (%u), supports only 4KB",
-			  blocksize);
+	if (le32_to_cpu(raw_super->log_blocksize) != 12) {
+		f2fs_info(sbi, "Invalid blocksize. Only 4KB supported");
 		return -EFSCORRUPTED;
 	}
 
-- 
2.25.1