Re: [PATCH] configfs: Fix use-after-free issue in __configfs_open_file

[Chao Yu]

+Cc fsdevel

Ping,

Any comments one this patch?

On 2021/3/1 14:10, Chao Yu wrote:
> From: Daiyue Zhang <zhangdaiyue1@xxxxxxxxxx>
>
> Commit b0841eefd969 ("configfs: provide exclusion between IO and removals")
> uses ->frag_dead to mark the fragment state, thus no bothering with extra
> refcount on config_item when opening a file. The configfs_get_config_item
> was removed in __configfs_open_file, but not with config_item_put. So the
> refcount on config_item will lost its balance, causing use-after-free
> issues in some occasions like this:
>
> Test:
> 1. Mount configfs on /config with read-only items:
> drwxrwx--- 289 root   root            0 2021-04-01 11:55 /config
> drwxr-xr-x   2 root   root            0 2021-04-01 11:54 /config/a
> --w--w--w-   1 root   root         4096 2021-04-01 11:53 /config/a/1.txt
> ......
>
> 2. Then run:
> for file in /config
> do
> echo $file
> grep -R 'key' $file
> done
>
> 3. __configfs_open_file will be called in parallel, the first one
> got called will do:
> if (file->f_mode & FMODE_READ) {
> 	if (!(inode->i_mode & S_IRUGO))
> 		goto out_put_module;
> 			config_item_put(buffer->item);
> 				kref_put()
> 					package_details_release()
> 						kfree()
>
> the other one will run into use-after-free issues like this:
> BUG: KASAN: use-after-free in __configfs_open_file+0x1bc/0x3b0
> Read of size 8 at addr fffffff155f02480 by task grep/13096
> CPU: 0 PID: 13096 Comm: grep VIP: 00 Tainted: G        W       4.14.116-kasan #1
> TGID: 13096 Comm: grep
> Call trace:
> dump_stack+0x118/0x160
> kasan_report+0x22c/0x294
> __asan_load8+0x80/0x88
> __configfs_open_file+0x1bc/0x3b0
> configfs_open_file+0x28/0x34
> do_dentry_open+0x2cc/0x5c0
> vfs_open+0x80/0xe0
> path_openat+0xd8c/0x2988
> do_filp_open+0x1c4/0x2fc
> do_sys_open+0x23c/0x404
> SyS_openat+0x38/0x48
>
> Allocated by task 2138:
> kasan_kmalloc+0xe0/0x1ac
> kmem_cache_alloc_trace+0x334/0x394
> packages_make_item+0x4c/0x180
> configfs_mkdir+0x358/0x740
> vfs_mkdir2+0x1bc/0x2e8
> SyS_mkdirat+0x154/0x23c
> el0_svc_naked+0x34/0x38
>
> Freed by task 13096:
> kasan_slab_free+0xb8/0x194
> kfree+0x13c/0x910
> package_details_release+0x524/0x56c
> kref_put+0xc4/0x104
> config_item_put+0x24/0x34
> __configfs_open_file+0x35c/0x3b0
> configfs_open_file+0x28/0x34
> do_dentry_open+0x2cc/0x5c0
> vfs_open+0x80/0xe0
> path_openat+0xd8c/0x2988
> do_filp_open+0x1c4/0x2fc
> do_sys_open+0x23c/0x404
> SyS_openat+0x38/0x48
> el0_svc_naked+0x34/0x38
>
> To fix this issue, remove the config_item_put in
> __configfs_open_file to balance the refcount of config_item.
>
> Fixes: b0841eefd969 ("configfs: provide exclusion between IO and removals")
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Daiyue Zhang <zhangdaiyue1@xxxxxxxxxx>
> Signed-off-by: Yi Chen <chenyi77@xxxxxxxxxx>
> Signed-off-by: Ge Qiu <qiuge@xxxxxxxxxx>
> Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>
> ---
>   fs/configfs/file.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/fs/configfs/file.c b/fs/configfs/file.c
> index 1f0270229d7b..da8351d1e455 100644
> --- a/fs/configfs/file.c
> +++ b/fs/configfs/file.c
> @@ -378,7 +378,7 @@ static int __configfs_open_file(struct inode *inode, struct file *file, int type
>   
>   	attr = to_attr(dentry);
>   	if (!attr)
> -		goto out_put_item;
> +		goto out_free_buffer;
>   
>   	if (type & CONFIGFS_ITEM_BIN_ATTR) {
>   		buffer->bin_attr = to_bin_attr(dentry);
> @@ -391,7 +391,7 @@ static int __configfs_open_file(struct inode *inode, struct file *file, int type
>   	/* Grab the module reference for this attribute if we have one */
>   	error = -ENODEV;
>   	if (!try_module_get(buffer->owner))
> -		goto out_put_item;
> +		goto out_free_buffer;
>   
>   	error = -EACCES;
>   	if (!buffer->item->ci_type)
> @@ -435,8 +435,6 @@ static int __configfs_open_file(struct inode *inode, struct file *file, int type
>   
>   out_put_module:
>   	module_put(buffer->owner);
> -out_put_item:
> -	config_item_put(buffer->item);
>   out_free_buffer:
>   	up_read(&frag->frag_sem);
>   	kfree(buffer);