On Wed 10-07-24 12:06:39, 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>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> 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;
> +
> 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
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
