linux-kernel - Forwarded: [PATCH] ext4: fix BUG_ON in ext4_es_cache

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Message-ID: <68d8fbd1.a00a0220.102ee.0028.GAE@google.com>
Date: Sun, 28 Sep 2025 02:11:45 -0700
From: syzbot <syzbot+038b7bf43423e132b308@...kaller.appspotmail.com>
To: linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Forwarded: [PATCH] ext4: fix BUG_ON in ext4_es_cache_extent due to
 out-of-order extents

For archival purposes, forwarding an incoming command email to
linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com.

***

Subject: [PATCH] ext4: fix BUG_ON in ext4_es_cache_extent due to out-of-order extents
Author: kartikey406@...il.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master


syzbot reported a BUG_ON in ext4_es_cache_extent() triggered when
opening a verity file on a corrupted ext4 filesystem mounted without
a journal.

The issue occurs when the extent tree contains out-of-order extents,
which can happen in a corrupted filesystem. In ext4_cache_extents(),
when processing an extent that has a lower logical block number than
the previous extent's end, the calculation of the hole size underflows:

  If prev = 4352 and lblk = 1280:
  lblk - prev = 1280 - 4352 = -3072 (as signed)
  = 4294964224 (as unsigned)

This underflowed value is passed as 'len' to ext4_es_cache_extent(),
where the end block calculation overflows:

  end = lblk + len - 1 = 4352 + 4294964224 - 1 = 1279

Then the BUG_ON(end < lblk) triggers because 1279 < 4352.

Fix this by adding a check in ext4_cache_extents() to ensure extents
are properly ordered before attempting to cache holes. If out-of-order
extents are detected, report the filesystem corruption and return.

Reported-by: syzbot+038b7bf43423e132b308@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=038b7bf43423e132b308
Fixes: a86c6181109a ("ext4: cache extent hole in extent status tree for ext4_da_map_blocks()")
Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
---
---
 fs/ext4/extents.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index ca5499e9412b..2e630b500a31 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -535,10 +535,17 @@ static void ext4_cache_extents(struct inode *inode,
 		ext4_lblk_t lblk = le32_to_cpu(ex->ee_block);
 		int len = ext4_ext_get_actual_len(ex);
 
-		if (prev && (prev != lblk))
-			ext4_es_cache_extent(inode, prev, lblk - prev, ~0,
-					     EXTENT_STATUS_HOLE);
-
+		if (prev && (prev != lblk)) {
+			if (lblk > prev) {
+				ext4_es_cache_extent(inode, prev, lblk - prev, ~0,
+				     EXTENT_STATUS_HOLE);
+			} else {
+				/* Extents out of order - corrupted? */
+				EXT4_ERROR_INODE(inode, "corrupted extent tree: lblk %u < prev %u",
+					 lblk, prev);
+				return;
+			}
+		}
 		if (ext4_ext_is_unwritten(ex))
 			status = EXTENT_STATUS_UNWRITTEN;
 		ext4_es_cache_extent(inode, lblk, len,
-- 
2.43.0