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Date:   Mon, 15 May 2023 15:58:42 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Zhihao Cheng <chengzhihao1@huawei.com>
Cc:     miklos@szeredi.hu, amir73il@gmail.com,
        linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] ovl: ovl_permission: Fix null pointer dereference
 at realinode in rcu-walk mode
Message-ID: <20230515-sprachen-zeltlager-6925dfbe19c1@brauner>
References: <20230515133629.1974610-1-chengzhihao1@huawei.com>
 <20230515133629.1974610-2-chengzhihao1@huawei.com>
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On Mon, May 15, 2023 at 09:36:28PM +0800, Zhihao Cheng wrote:
> Following process:
>           P1                     P2
>  path_lookupat
>   link_path_walk
>    inode_permission
>     ovl_permission
>       ovl_i_path_real(inode, &realpath)
>         path->dentry = ovl_i_dentry_upper(inode)
>                           drop_cache
> 			   __dentry_kill(ovl_dentry)
> 		            iput(ovl_inode)
> 		             ovl_destroy_inode(ovl_inode)
> 		              dput(oi->__upperdentry)
> 		               dentry_kill(upperdentry)
> 		                dentry_unlink_inode
> 				 upperdentry->d_inode = NULL
>       realinode = d_inode(realpath.dentry) // return NULL
>       inode_permission(realinode)
>        inode->i_sb  // NULL pointer dereference
> , will trigger an null pointer dereference at realinode:
>   [  335.664979] BUG: kernel NULL pointer dereference,
>                  address: 0000000000000002
>   [  335.668032] CPU: 0 PID: 2592 Comm: ls Not tainted 6.3.0
>   [  335.669956] RIP: 0010:inode_permission+0x33/0x2c0
>   [  335.678939] Call Trace:
>   [  335.679165]  <TASK>
>   [  335.679371]  ovl_permission+0xde/0x320
>   [  335.679723]  inode_permission+0x15e/0x2c0
>   [  335.680090]  link_path_walk+0x115/0x550
>   [  335.680771]  path_lookupat.isra.0+0xb2/0x200
>   [  335.681170]  filename_lookup+0xda/0x240
>   [  335.681922]  vfs_statx+0xa6/0x1f0
>   [  335.682233]  vfs_fstatat+0x7b/0xb0
> 
> Fetch a reproducer in [Link].
> 
> Add a new helper ovl_i_path_realinode() to get realpath and real inode
> after non-nullptr checking, use the helper to replace the current realpath
> getting logic.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217405
> Fixes: 4b7791b2e958 ("ovl: handle idmappings in ovl_permission()")
> Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
> Suggested-by: Christian Brauner <brauner@kernel.org>
> ---
>  fs/overlayfs/inode.c | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 541cf3717fc2..cc3ef5a6666a 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -278,6 +278,26 @@ int ovl_getattr(struct mnt_idmap *idmap, const struct path *path,
>  	return err;
>  }
>  
> +static inline int ovl_i_path_realinode(struct inode *inode,
> +				       struct path *realpath,
> +				       struct inode **realinode, int rcu)
> +{
> +	/* Careful in RCU walk mode */
> +	ovl_i_path_real(inode, realpath);
> +	if (!realpath->dentry) {
> +		WARN_ON(!rcu);
> +		return -ECHILD;
> +	}
> +
> +	*realinode = d_inode(realpath->dentry);
> +	if (!*realinode) {
> +		WARN_ON(!rcu);
> +		return -ECHILD;
> +	}
> +
> +	return 0;
> +}

If you want to return the inode wouldn't it possibly make more sense to
return the inode from the function directly? But not fuzzed. Maybe Amir
has a preference. As I said, I'm even fine with the original approach.

static inline struct inode *ovl_i_path_realinode(struct inode *inode,
						 struct path *realpath,
						 int rcu)
{
	struct inode *realinode;

	/* Careful in RCU walk mode */
	ovl_i_path_real(inode, realpath);
	if (!realpath->dentry) {
		WARN_ON(!rcu);
		return ERR_PTR(-ECHILD);
	}

	realinode = d_inode(realpath->dentry);
	if (!realinode) {
		WARN_ON(!rcu);
		return ERR_PTR(-ECHILD);
	}

	return realinode;
}

> +
>  int ovl_permission(struct mnt_idmap *idmap,
>  		   struct inode *inode, int mask)
>  {
> @@ -287,12 +307,10 @@ int ovl_permission(struct mnt_idmap *idmap,
>  	const struct cred *old_cred;
>  	int err;
>  
> -	/* Careful in RCU walk mode */
> -	ovl_i_path_real(inode, &realpath);
> -	if (!realpath.dentry) {
> -		WARN_ON(!(mask & MAY_NOT_BLOCK));
> -		return -ECHILD;
> -	}
> +	err = ovl_i_path_realinode(inode, &realpath, &realinode,
> +				   mask & MAY_NOT_BLOCK);
> +	if (err)
> +		return err;
>  
>  	/*
>  	 * Check overlay inode with the creds of task and underlying inode
> @@ -302,7 +320,6 @@ int ovl_permission(struct mnt_idmap *idmap,
>  	if (err)
>  		return err;
>  
> -	realinode = d_inode(realpath.dentry);
>  	old_cred = ovl_override_creds(inode->i_sb);
>  	if (!upperinode &&
>  	    !special_file(realinode->i_mode) && mask & MAY_WRITE) {
> -- 
> 2.39.2
>