On 2024/7/27 18:36, Ojaswin Mujoo wrote:
On Wed, Jul 10, 2024 at 12:06:39PM +0800, libaokun@xxxxxxxxxxxxxxx wrote:
From: Baokun Li <libaokun1@xxxxxxxxxx>
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 #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: dfe5080939ea ("ext4: drop EXT4_EX_NOFREE_ON_ERR from rest of extents handling code")
Cc: stable@xxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
fs/ext4/extents.c | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 6e5b5baf3aa6..3a70a0739af8 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3252,6 +3252,25 @@ 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;
+
Hi Baokun, nice catch!
I was just wondering if it makes more sense to only update path if we
encounter an error:
err = ext4_ext_insert_extent(handle, inode, ppath, &newex, flags);
if (err != -ENOSPC && err != -EDQUOT && err != -ENOMEM)
goto out;
else if (err < 0) {
...
path = ext4_find_extent(inode, ee_block, ppath, flags | EXT4_EX_NOFAIL);
}
I believe that's the only time we'd end up with the use after free issue
and this way we can avoid the (maybe tiny) performance overhead of calling
the extra find extent. What are your thoughts?
regards,
ojaswin
Hi Ojaswin,
We're already there now, updating the path only on recoverable,
non-critical errors (-ENOSPC, -EDQUOT, and -ENOMEM).
Thanks,
Baokun
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) {
@@ -3304,7 +3323,7 @@ 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;
}
--
2.39.2
--
With Best Regards,
Baokun Li
