linux-kernel - [PATCH 4.9 022/177] ext4: fix crash when a directorys i

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Message-Id: <20171218152910.992688209@linuxfoundation.org>
Date:   Mon, 18 Dec 2017 16:47:25 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Chandan Rajendra <chandan@...ux.vnet.ibm.com>
Subject: [PATCH 4.9 022/177] ext4: fix crash when a directorys i_size is too small

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Chandan Rajendra <chandan@...ux.vnet.ibm.com>

commit 9d5afec6b8bd46d6ed821aa1579634437f58ef1f upstream.

On a ppc64 machine, when mounting a fuzzed ext2 image (generated by
fsfuzzer) the following call trace is seen,

VFS: brelse: Trying to free free buffer
WARNING: CPU: 1 PID: 6913 at /root/repos/linux/fs/buffer.c:1165 .__brelse.part.6+0x24/0x40
.__brelse.part.6+0x20/0x40 (unreliable)
.ext4_find_entry+0x384/0x4f0
.ext4_lookup+0x84/0x250
.lookup_slow+0xdc/0x230
.walk_component+0x268/0x400
.path_lookupat+0xec/0x2d0
.filename_lookup+0x9c/0x1d0
.vfs_statx+0x98/0x140
.SyS_newfstatat+0x48/0x80
system_call+0x58/0x6c

This happens because the directory that ext4_find_entry() looks up has
inode->i_size that is less than the block size of the filesystem. This
causes 'nblocks' to have a value of zero. ext4_bread_batch() ends up not
reading any of the directory file's blocks. This renders the entries in
bh_use[] array to continue to have garbage data. buffer_uptodate() on
bh_use[0] can then return a zero value upon which brelse() function is
invoked.

This commit fixes the bug by returning -ENOENT when the directory file
has no associated blocks.

Reported-by: Abdul Haleem <abdhalee@...ux.vnet.ibm.com>
Signed-off-by: Chandan Rajendra <chandan@...ux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/ext4/namei.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1417,6 +1417,10 @@ static struct buffer_head * ext4_find_en
 			       "falling back\n"));
 	}
 	nblocks = dir->i_size >> EXT4_BLOCK_SIZE_BITS(sb);
+	if (!nblocks) {
+		ret = NULL;
+		goto cleanup_and_exit;
+	}
 	start = EXT4_I(dir)->i_dir_start_lookup;
 	if (start >= nblocks)
 		start = 0;