Re: [PATCH] configfs: fix a race in configfs_lookup()

[Gong, Sishuai]

On Aug 23, 2021, at 1:08 PM, Christoph Hellwig <hch@xxxxxx> wrote:
> 
> On Mon, Aug 23, 2021 at 04:12:10PM +0000, Gong, Sishuai wrote:
>> On Aug 23, 2021, at 3:46 AM, Christoph Hellwig <hch@xxxxxx<mailto:hch@xxxxxx>> wrote:
>> 
>> On Fri, Aug 20, 2021 at 05:44:58PM -0400, sishuaigong wrote:
>> When configfs_lookup() is executing list_for_each_entry(),
>> it is possible that configfs_dir_lseek() is calling list_del().
>> Some unfortunate interleavings of them can cause a kernel NULL
>> pointer dereference error
>> 
>> Thread 1                  Thread 2
>> //configfs_dir_lseek()    //configfs_lookup()
>> list_del(&cursor->s_sibling);
>>                         list_for_each_entry(sd, ...)
>> 
>> Fix this bug by using list_for_each_entry_safe() instead.
>> 
>> I don't see how list_for_each_entry_safe would save you there.
>> You need a lock to sychronize the two, list_for_each_entry_safe
>> only ensures the next entry is looked up before iterating over
>> the current one.
>> Thanks for pointing that out!
>> 
>> It looks like config_lookup() should hold configfs_dirent_lock
>> when doing list_for_each_entry(), but configfs_attach_attr()
>> also needs to be changed since it might be called by
>> config_lookup() and then wait for configfs_dirent_lock,
>> which will cause a deadlock.
>> 
>> Do you think a future patch like this makes sense?
> 
> We can't hold a spinlock over inode allocation.  So it would have to be
> something like this:
> 
Thanks for your suggestion! I will come up with a second patch soon.
> diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
> index ac5e0c0e9181..48022e27664d 100644
> --- a/fs/configfs/dir.c
> +++ b/fs/configfs/dir.c
> @@ -417,44 +417,13 @@ static void configfs_remove_dir(struct config_item * item)
> 	dput(dentry);
> }
> 
> -
> -/* attaches attribute's configfs_dirent to the dentry corresponding to the
> - * attribute file
> - */
> -static int configfs_attach_attr(struct configfs_dirent * sd, struct dentry * dentry)
> -{
> -	struct configfs_attribute * attr = sd->s_element;
> -	struct inode *inode;
> -
> -	spin_lock(&configfs_dirent_lock);
> -	dentry->d_fsdata = configfs_get(sd);
> -	sd->s_dentry = dentry;
> -	spin_unlock(&configfs_dirent_lock);
> -
> -	inode = configfs_create(dentry, (attr->ca_mode & S_IALLUGO) | S_IFREG);
> -	if (IS_ERR(inode)) {
> -		configfs_put(sd);
> -		return PTR_ERR(inode);
> -	}
> -	if (sd->s_type & CONFIGFS_ITEM_BIN_ATTR) {
> -		inode->i_size = 0;
> -		inode->i_fop = &configfs_bin_file_operations;
> -	} else {
> -		inode->i_size = PAGE_SIZE;
> -		inode->i_fop = &configfs_file_operations;
> -	}
> -	d_add(dentry, inode);
> -	return 0;
> -}
> -
> static struct dentry * configfs_lookup(struct inode *dir,
> 				       struct dentry *dentry,
> 				       unsigned int flags)
> {
> -	struct configfs_dirent * parent_sd = dentry->d_parent->d_fsdata;
> -	struct configfs_dirent * sd;
> -	int found = 0;
> -	int err;
> +	struct configfs_dirent *parent_sd = dentry->d_parent->d_fsdata;
> +	struct configfs_dirent *sd;
> +	struct inode *inode = NULL;
> 
> 	/*
> 	 * Fake invisibility if dir belongs to a group/default groups hierarchy
> @@ -464,36 +433,46 @@ static struct dentry * configfs_lookup(struct inode *dir,
> 	 * not complete their initialization, since the dentries of the
> 	 * attributes won't be instantiated.
> 	 */
> -	err = -ENOENT;
> 	if (!configfs_dirent_is_ready(parent_sd))
> -		goto out;
> +		return ERR_PTR(-ENOENT);
> 
> +	spin_lock(&configfs_dirent_lock);
> 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
> -		if (sd->s_type & CONFIGFS_NOT_PINNED) {
> -			const unsigned char * name = configfs_get_name(sd);
> +		if ((sd->s_type & CONFIGFS_NOT_PINNED) &&
> +		    !strcmp(configfs_get_name(sd), dentry->d_name.name)) {
> +		    	struct configfs_attribute *attr = sd->s_element;
> +			umode_t mode = (attr->ca_mode & S_IALLUGO) | S_IFREG;
> 
> -			if (strcmp(name, dentry->d_name.name))
> -				continue;
> +			dentry->d_fsdata = configfs_get(sd);
> +			sd->s_dentry = dentry;
> +			spin_unlock(&configfs_dirent_lock);
> 
> -			found = 1;
> -			err = configfs_attach_attr(sd, dentry);
> -			break;
> +			inode = configfs_create(dentry, mode);
> +			if (IS_ERR(inode)) {
> +				configfs_put(sd);
> +				return ERR_CAST(inode);
This return statement from the original configfs_attach_attr() needs to be handled accordingly.
> +			}
> +			if (sd->s_type & CONFIGFS_ITEM_BIN_ATTR) {
> +				inode->i_size = 0;
> +				inode->i_fop = &configfs_bin_file_operations;
> +			} else {
> +				inode->i_size = PAGE_SIZE;
> +				inode->i_fop = &configfs_file_operations;
> +			}
> +			goto done;
> 		}
> 	}
> +	spin_unlock(&configfs_dirent_lock);
> 
> -	if (!found) {
> -		/*
> -		 * If it doesn't exist and it isn't a NOT_PINNED item,
> -		 * it must be negative.
> -		 */
> -		if (dentry->d_name.len > NAME_MAX)
> -			return ERR_PTR(-ENAMETOOLONG);
> -		d_add(dentry, NULL);
> -		return NULL;
> -	}
> -
> -out:
> -	return ERR_PTR(err);
> +	/*
> +	 * If it doesn't exist and it isn't a NOT_PINNED item, it must be
> +	 * negative.
> +	 */
> +	if (dentry->d_name.len > NAME_MAX)
> +		return ERR_PTR(-ENAMETOOLONG);
> +done:
> +	d_add(dentry, inode);
> +	return NULL;
> }
> 
> /*