[PATCH v2 06/25] ext4: aovid use-after-free in ext4_ext_insert_extent()

[libaokun]

From: Baokun Li <libaokun1@xxxxxxxxxx>

As Ojaswin mentioned in Link, in ext4_ext_insert_extent(), if the path is
reallocated in ext4_ext_create_new_leaf(), we'll use the stale path and
cause UAF. Below is a sample trace with dummy values:

ext4_ext_insert_extent
  path = *ppath = 2000
  ext4_ext_create_new_leaf(ppath)
    ext4_find_extent(ppath)
      path = *ppath = 2000
      if (depth > path[0].p_maxdepth)
            kfree(path = 2000);
            *ppath = path = NULL;
      path = kcalloc() = 3000
      *ppath = 3000;
      return path;
  /* here path is still 2000, UAF! */
  eh = path[depth].p_hdr

==================================================================
BUG: KASAN: slab-use-after-free in ext4_ext_insert_extent+0x26d4/0x3330
Read of size 8 at addr ffff8881027bf7d0 by task kworker/u36:1/179
CPU: 3 UID: 0 PID: 179 Comm: kworker/u6:1 Not tainted 6.11.0-rc2-dirty #866
Call Trace:
 <TASK>
 ext4_ext_insert_extent+0x26d4/0x3330
 ext4_ext_map_blocks+0xe22/0x2d40
 ext4_map_blocks+0x71e/0x1700
 ext4_do_writepages+0x1290/0x2800
[...]

Allocated by task 179:
 ext4_find_extent+0x81c/0x1f70
 ext4_ext_map_blocks+0x146/0x2d40
 ext4_map_blocks+0x71e/0x1700
 ext4_do_writepages+0x1290/0x2800
 ext4_writepages+0x26d/0x4e0
 do_writepages+0x175/0x700
[...]

Freed by task 179:
 kfree+0xcb/0x240
 ext4_find_extent+0x7c0/0x1f70
 ext4_ext_insert_extent+0xa26/0x3330
 ext4_ext_map_blocks+0xe22/0x2d40
 ext4_map_blocks+0x71e/0x1700
 ext4_do_writepages+0x1290/0x2800
 ext4_writepages+0x26d/0x4e0
 do_writepages+0x175/0x700
[...]
==================================================================

So use *ppath to update the path to avoid the above problem.

Reported-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
Closes: https://lore.kernel.org/r/ZqyL6rmtwl6N4MWR@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 10809df84a4d ("ext4: teach ext4_ext_find_extent() to realloc path if necessary")
Cc: stable@xxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 fs/ext4/extents.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 5879aef159d8..91c6586afcca 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2116,6 +2116,7 @@ int ext4_ext_insert_extent(handle_t *handle, struct inode *inode,
 				       ppath, newext);
 	if (err)
 		goto cleanup;
+	path = *ppath;
 	depth = ext_depth(inode);
 	eh = path[depth].p_hdr;
 
-- 
2.39.2