linux-ext4 - [PATCH] handle ext3 directory corruption better

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Message-ID: <453920B1.1030602@redhat.com>
Date:	Fri, 20 Oct 2006 14:17:05 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	ext4 development <linux-ext4@...r.kernel.org>
Subject: [PATCH] handle ext3 directory corruption better

(as previously discussed on the ext4 list)

I've been using Steve Grubb's purely evil "fsfuzzer" tool, at
http://people.redhat.com/sgrubb/files/fsfuzzer-0.4.tar.gz

basically it makes a filesystem, splats some random bits over it,
then tries to mount it and do some simple filesystem actions.

At best, the filesystem catches the corruption gracefully.
At worst, things spin out of control.

As you might guess, we found a couple places in ext3 where things 
spin out of control :)

First, we had a corrupted directory that was never checked
for consistency... it was corrupt, and pointed to another bad "entry"
of length 0.  The for() loop looped forever, since the length
of ext3_next_entry(de) was 0, and we kept looking at the same
pointer over and over and over and over... I modeled this check
and subsequent action on what is done for other directory types
in ext3_readdir...

(adding this check adds some computational expense; I am testing 
a followup patch to reduce the number of times we check and re-check
these directory entries, in all cases.  Thanks for the idea, Andreas).

Next we had a root directory inode which had a corrupted size, claimed
to be > 200M on a 4M filesystem.  There was only really 1 block in the 
directory, but because the size was so large, readdir kept coming back 
for more, spewing thousands of printk's along the way.

Per Andreas' suggestion, if we're in this read error condition and we're
trying to read an offset which is greater than i_blocks worth of bytes,
stop trying, and break out of the loop.

With these two changes fsfuzz test survives quite well on ext3.

Signed-off-by: Eric Sandeen <sandeen@...hat.com>

Index: linux-2.6.18/fs/ext3/namei.c
===================================================================
--- linux-2.6.18.orig/fs/ext3/namei.c
+++ linux-2.6.18/fs/ext3/namei.c
@@ -551,6 +551,15 @@ static int htree_dirblock_to_tree(struct
 					   dir->i_sb->s_blocksize -
 					   EXT3_DIR_REC_LEN(0));
 	for (; de < top; de = ext3_next_entry(de)) {
+		if (!ext3_check_dir_entry("htree_dirblock_to_tree", dir, de, bh,
+					(block<<EXT3_BLOCK_SIZE_BITS(dir->i_sb))
+						+((char *)de - bh->b_data))) {
+			/* On error, skip the f_pos to the next block. */
+			dir_file->f_pos = (dir_file->f_pos |
+					(dir->i_sb->s_blocksize - 1)) + 1;
+			brelse (bh);
+			return count;
+		}
 		ext3fs_dirhash(de->name, de->name_len, hinfo);
 		if ((hinfo->hash < start_hash) ||
 		    ((hinfo->hash == start_hash) &&
Index: linux-2.6.18/fs/ext3/dir.c
===================================================================
--- linux-2.6.18.orig/fs/ext3/dir.c
+++ linux-2.6.18/fs/ext3/dir.c
@@ -151,6 +151,9 @@ static int ext3_readdir(struct file * fi
 			ext3_error (sb, "ext3_readdir",
 				"directory #%lu contains a hole at offset %lu",
 				inode->i_ino, (unsigned long)filp->f_pos);
+			/* corrupt size?  Maybe no more blocks to read */
+			if (filp->f_pos > inode->i_blocks << 9)
+				break;
 			filp->f_pos += sb->s_blocksize - offset;
 			continue;
 		}

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