ext4: fix slab-use-after-free in ext4_split_extent_at()

We hit the following use-after-free:

==================================================================
BUG: KASAN: slab-use-after-free in ext4_split_extent_at+0xba8/0xcc0
Read of size 2 at addr ffff88810548ed08 by task kworker/u20:0/40
CPU: 0 PID: 40 Comm: kworker/u20:0 Not tainted 6.9.0-dirty torvalds#724
Call Trace:
 <TASK>
 kasan_report+0x93/0xc0
 ext4_split_extent_at+0xba8/0xcc0
 ext4_split_extent.isra.0+0x18f/0x500
 ext4_split_convert_extents+0x275/0x750
 ext4_ext_handle_unwritten_extents+0x73e/0x1580
 ext4_ext_map_blocks+0xe20/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]

Allocated by task 40:
 __kmalloc_noprof+0x1ac/0x480
 ext4_find_extent+0xf3b/0x1e70
 ext4_ext_map_blocks+0x188/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]

Freed by task 40:
 kfree+0xf1/0x2b0
 ext4_find_extent+0xa71/0x1e70
 ext4_ext_insert_extent+0xa22/0x3260
 ext4_split_extent_at+0x3ef/0xcc0
 ext4_split_extent.isra.0+0x18f/0x500
 ext4_split_convert_extents+0x275/0x750
 ext4_ext_handle_unwritten_extents+0x73e/0x1580
 ext4_ext_map_blocks+0xe20/0x2dc0
 ext4_map_blocks+0x724/0x1700
 ext4_do_writepages+0x12d6/0x2a70
[...]
==================================================================

The flow of issue triggering is as follows:

ext4_split_extent_at
  path = *ppath
  ext4_ext_insert_extent(ppath)
    ext4_ext_create_new_leaf(ppath)
      ext4_find_extent(orig_path)
        path = *orig_path
        read_extent_tree_block
          // return -ENOMEM or -EIO
        ext4_free_ext_path(path)
          kfree(path)
        *orig_path = NULL
  a. If err is -ENOMEM:
  ext4_ext_dirty(path + path->p_depth)
  // path use-after-free !!!
  b. If err is -EIO and we have EXT_DEBUG defined:
  ext4_ext_show_leaf(path)
    eh = path[depth].p_hdr
    // path also use-after-free !!!

So when trying to zeroout or fix the extent length, call ext4_find_extent()
to update the path.

In addition we use *ppath directly as an ext4_ext_show_leaf() input to
avoid possible use-after-free when EXT_DEBUG is defined, and to avoid
unnecessary path updates.

Fixes: dfe5080 ("ext4: drop EXT4_EX_NOFREE_ON_ERR from rest of extents handling code")
Cc: stable@kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20240822023545.1994557-4-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/extents.c

@@ -3251,6 +3251,25 @@ static int ext4_split_extent_at(handle_t *handle,
 	if (err != -ENOSPC && err != -EDQUOT && err != -ENOMEM)
 		goto out;
 
+	/*
+	 * Update path is required because previous ext4_ext_insert_extent()
+	 * may have freed or reallocated the path. Using EXT4_EX_NOFAIL
+	 * guarantees that ext4_find_extent() will not return -ENOMEM,
+	 * otherwise -ENOMEM will cause a retry in do_writepages(), and a
+	 * WARN_ON may be triggered in ext4_da_update_reserve_space() due to
+	 * an incorrect ee_len causing the i_reserved_data_blocks exception.
+	 */
+	path = ext4_find_extent(inode, ee_block, ppath,
+				flags | EXT4_EX_NOFAIL);
+	if (IS_ERR(path)) {
+		EXT4_ERROR_INODE(inode, "Failed split extent on %u, err %ld",
+				 split, PTR_ERR(path));
+		return PTR_ERR(path);
+	}
+	depth = ext_depth(inode);
+	ex = path[depth].p_ext;
+	*ppath = path;
+
 	if (EXT4_EXT_MAY_ZEROOUT & split_flag) {
 		if (split_flag & (EXT4_EXT_DATA_VALID1|EXT4_EXT_DATA_VALID2)) {
 			if (split_flag & EXT4_EXT_DATA_VALID1) {
@@ -3303,7 +3322,7 @@ static int ext4_split_extent_at(handle_t *handle,
 	ext4_ext_dirty(handle, inode, path + path->p_depth);
 	return err;
 out:
-	ext4_ext_show_leaf(inode, path);
+	ext4_ext_show_leaf(inode, *ppath);
 	return err;
 }