Re: [PATCH v2] ext4: check for overlapping extents in ext4_valid_extent_entries()

[Eryu Guan <guaneryu@gmail.com>]

On Mon, Oct 21, 2013 at 06:06:23PM +0200, Lukáš Czerner wrote:
> On Sun, 20 Oct 2013, Eryu Guan wrote:
...
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index c9ebcb9..855b11d 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -387,11 +387,21 @@ static int ext4_valid_extent_entries(struct inode *inode,
> >  	if (depth == 0) {
> >  		/* leaf entries */
> >  		struct ext4_extent *ext = EXT_FIRST_EXTENT(eh);
> > +		ext4_lblk_t block = 0;
> > +		ext4_lblk_t prev = 0;
> > +		int len = 0;
> >  		while (entries) {
> >  			if (!ext4_valid_extent(inode, ext))
> >  				return 0;
> > +
> > +			/* Check for overlapping extents */
> > +			block = le32_to_cpu(ext->ee_block);
> > +			len = ext4_ext_get_actual_len(ext);
> > +			if ((block <= prev) && prev)
> 
> Both ext4_valid_extent() and ext4_valid_extent_idx() are setting
> s_last_error_block in the case of error. Maybe we should to the same
> here ? Note that the block saved in that variable is physical, not
> logical.

I think that makes sense, it's better to keep the consistency.

But it seems that the s_last_error_block will eventually be
overwritten by ext4_error_inode() in __ext4_ext_check() ?

> 
> Also I am curious what happens when one of the extents is corrupted
> in such a way that it crosses the 16TB boundary ? In this case the
> check would not recognise that since prev will underflow, but maybe
> something else catches that ?

Do you mean that a previous (ee_block + len - 1) could cross the 2**32
boundary? I think we can add another check in ext4_valid_extent() for
this situation.

I update the patch to a v3 version, could you please review again?

Thanks a lot!

Eryu Guan

---

From 467025c05bce3ee44e607887bc7cb74ff1bfefcb Mon Sep 17 00:00:00 2001
From: Eryu Guan <guaneryu@gmail.com>
Date: Thu, 17 Oct 2013 23:57:22 +0800
Subject: [PATCH v3] ext4: check for overlapping extents in
 ext4_valid_extent_entries()

A corrupted ext4 may have out of order leaf extents, i.e.

extent: lblk 0--1023, len 1024, pblk 9217, flags: LEAF UNINIT
extent: lblk 1000--2047, len 1024, pblk 10241, flags: LEAF UNINIT
             ^^^^ overlap with previous extent

Reading such extent could hit BUG_ON() in ext4_es_cache_extent().

	BUG_ON(end < lblk);

The problem is that __read_extent_tree_block() tries to cache holes as
well but assumes 'lblk' is greater than 'prev' and passes underflowed
length to ext4_es_cache_extent(). Fix it by checking for overlapping
extents in ext4_valid_extent_entries().

I hit this when fuzz testing ext4, and am able to reproduce it by
modifying the on-disk extent by hand.

Also add the check for (ee_block + len - 1) in ext4_valid_extent() to
make sure the value is not overflow.

Ran xfstests on patched ext4 and no regression.

Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Lukáš Czerner <lczerner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
---
v3: Address comments from Lukas
 - set s_last_error_block when there's overlapping extents found
 - check for (ee_block + len - 1) in ext4_valid_extent(), value should
   not overflow
v2:
 - check for overlapping extents explicitly not hide the corruption

 fs/ext4/extents.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index c9ebcb9..85d977f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -360,8 +360,10 @@ static int ext4_valid_extent(struct inode *inode, struct ext4_extent *ext)
 {
 	ext4_fsblk_t block = ext4_ext_pblock(ext);
 	int len = ext4_ext_get_actual_len(ext);
+	ext4_lblk_t lblock = le32_to_cpu(ext->ee_block);
+	ext4_lblk_t last = lblock + len - 1;
 
-	if (len == 0)
+	if (lblock > last)
 		return 0;
 	return ext4_data_block_valid(EXT4_SB(inode->i_sb), block, len);
 }
@@ -387,11 +389,26 @@ static int ext4_valid_extent_entries(struct inode *inode,
 	if (depth == 0) {
 		/* leaf entries */
 		struct ext4_extent *ext = EXT_FIRST_EXTENT(eh);
+		struct ext4_super_block *es = EXT4_SB(inode->i_sb)->s_es;
+		ext4_fsblk_t pblock = 0;
+		ext4_lblk_t lblock = 0;
+		ext4_lblk_t prev = 0;
+		int len = 0;
 		while (entries) {
 			if (!ext4_valid_extent(inode, ext))
 				return 0;
+
+			/* Check for overlapping extents */
+			lblock = le32_to_cpu(ext->ee_block);
+			len = ext4_ext_get_actual_len(ext);
+			if ((lblock <= prev) && prev) {
+				pblock = ext4_ext_pblock(ext);
+				es->s_last_error_block = cpu_to_le64(pblock);
+				return 0;
+			}
 			ext++;
 			entries--;
+			prev = lblock + len - 1;
 		}
 	} else {
 		struct ext4_extent_idx *ext_idx = EXT_FIRST_INDEX(eh);
-- 
1.8.3.1



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