[RFC] ext4: don't remove already removed extent

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Syzbot has hit the following bug on current and all older kernels:
BUG: KASAN: out-of-bounds in ext4_ext_rm_leaf fs/ext4/extents.c:2736 [inline]
BUG: KASAN: out-of-bounds in ext4_ext_remove_space+0x2482/0x4d90 fs/ext4/extents.c:2958
Read of size 18446744073709551508 at addr ffff888073aea078 by task syz-executor420/6443

On investigation, I've found that eh->eh_entries is zero, ex is
referring to last entry and EXT_LAST_EXTENT(eh) is referring to first.
Hence EXT_LAST_EXTENT(eh) - ex becomes negative and causes the wrong
buffer read.

element: FFFF8882F8F0D06C       <----- ex
element: FFFF8882F8F0D060
element: FFFF8882F8F0D054
element: FFFF8882F8F0D048
element: FFFF8882F8F0D03C
element: FFFF8882F8F0D030
element: FFFF8882F8F0D024
element: FFFF8882F8F0D018
element: FFFF8882F8F0D00C	<------  EXT_FIRST_EXTENT(eh)
header:  FFFF8882F8F0D000	<------  EXT_LAST_EXTENT(eh) and eh

Cc: stable@vger.kernel.org
Reported-by: syzbot+6e5f2db05775244c73b7@syzkaller.appspotmail.com
Closes: https://groups.google.com/g/syzkaller-bugs/c/G6zS-LKgDW0/m/63MgF6V7BAAJ
Fixes: d583fb87a3ff ("ext4: punch out extents")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
---
This patch is only fixing the local issue. There may be bigger bug. Why
is ex set to last entry if the eh->eh_entries is 0. If any ext4
developer want to look at the bug, please don't hesitate.
---
 fs/ext4/extents.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e4115d338f101..7b7779b4cb87f 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2726,7 +2726,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
 		 * If the extent was completely released,
 		 * we need to remove it from the leaf
 		 */
-		if (num == 0) {
+		if (num == 0 && eh->eh_entries) {
 			if (end != EXT_MAX_BLOCKS - 1) {
 				/*
 				 * For hole punching, we need to scoot all the
-- 
2.40.1