Re: [PATCH 4/4] configfs: fix a race in configfs_lookup()

[Joel Becker]

Acked-by: Joel Becker <jlbec@xxxxxxxxxxxx>

On Wed, Aug 25, 2021 at 08:49:06AM +0200, Christoph Hellwig wrote:
> From: Sishuai Gong <sishuai@xxxxxxxxxx>
> 
> 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 by grabbing configfs_dirent_lock in configfs_lookup()
> while iterating ->s_children.
> 
> Signed-off-by: Sishuai Gong <sishuai@xxxxxxxxxx>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  fs/configfs/dir.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
> index fc20bd8a6337..1466b5d01cbb 100644
> --- a/fs/configfs/dir.c
> +++ b/fs/configfs/dir.c
> @@ -439,13 +439,13 @@ static struct dentry * configfs_lookup(struct inode *dir,
>  	if (!configfs_dirent_is_ready(parent_sd))
>  		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) &&
>  		    !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;
>  
> -			spin_lock(&configfs_dirent_lock);
>  			dentry->d_fsdata = configfs_get(sd);
>  			sd->s_dentry = dentry;
>  			spin_unlock(&configfs_dirent_lock);
> @@ -462,10 +462,11 @@ static struct dentry * configfs_lookup(struct inode *dir,
>  				inode->i_size = PAGE_SIZE;
>  				inode->i_fop = &configfs_file_operations;
>  			}
> -			break;
> +			goto done;
>  		}
>  	}
> -
> +	spin_unlock(&configfs_dirent_lock);
> +done:
>  	d_add(dentry, inode);
>  	return NULL;
>  }
> -- 
> 2.30.2
> 

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