Re: [PATCH v3 1/4] ovl: use insert_inode_locked4() to hash a newly created inode

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, May 15, 2018 at 01:26:09PM +0300, Amir Goldstein wrote:
> Currently, there is a small window where ovl_obtain_alias() can
> race with ovl_instantiate() and create two different overlay inodes
> with the same underlying real non-dir non-hardlink inode.
> 
> The race requires an adversary to guess the file handle of the
> yet to be created upper inode and decode the guessed file handle
> after ovl_creat_real(), but before ovl_instantiate().
> 
> This patch fixes the race, by using insert_inode_locked4() to add
> a newly created inode to icache.
> 
> If the newly created inode apears to already exist in icache (hashed
> by the same real upper inode), we export this error to user instead
> of silently not hashing the new inode.

So we might return an error to user saying operation failed, but still
create file on upper. Does that sound little odd?

I am wondering why can't we call ovl_get_inode() in object creation
path. That should take care of race between creation path and file
handle decode and only one of the paths will get to instantiate and
initialize ovl_inode and other path will wait.

Thanks
Vivek


> 
> This race does not affect overlay directory inodes, because those
> are decoded via ovl_lookup_real() and not with ovl_obtain_alias(),
> so avoid using the new helper d_instantiate_new() to reduce backport
> dependencies.
> 
> Backporting only makes sense for v4.16 where NFS export was introduced.
> 
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: <stable@vger.kernel.org> #v4.16
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> ---
>  fs/overlayfs/dir.c       | 24 ++++++++++++++++++------
>  fs/overlayfs/inode.c     | 18 ++++++++++++++++++
>  fs/overlayfs/overlayfs.h |  1 +
>  3 files changed, 37 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 47dc980e8b33..62e6733b755c 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -183,14 +183,24 @@ static int ovl_set_opaque(struct dentry *dentry, struct dentry *upperdentry)
>  }
>  
>  /* Common operations required to be done after creation of file on upper */
> -static void ovl_instantiate(struct dentry *dentry, struct inode *inode,
> -			    struct dentry *newdentry, bool hardlink)
> +static int ovl_instantiate(struct dentry *dentry, struct inode *inode,
> +			   struct dentry *newdentry, bool hardlink)
>  {
>  	ovl_dir_modified(dentry->d_parent, false);
> -	ovl_copyattr(d_inode(newdentry), inode);
>  	ovl_dentry_set_upper_alias(dentry);
>  	if (!hardlink) {
> -		ovl_inode_update(inode, newdentry);
> +		int err;
> +
> +		ovl_inode_init(inode, newdentry, NULL);
> +		/*
> +		 * XXX: if we ever use ovl_obtain_alias() to decode directory
> +		 * file handles, need to use ovl_insert_inode_locked() and
> +		 * d_instantiate_new() here to prevent ovl_obtain_alias()
> +		 * from sneaking in before d_instantiate().
> +		 */
> +		err = ovl_insert_inode(inode, d_inode(newdentry));
> +		if (err)
> +			return err;
>  	} else {
>  		WARN_ON(ovl_inode_real(inode) != d_inode(newdentry));
>  		dput(newdentry);
> @@ -200,6 +210,8 @@ static void ovl_instantiate(struct dentry *dentry, struct inode *inode,
>  	/* Force lookup of new upper hardlink to find its lower */
>  	if (hardlink)
>  		d_drop(dentry);
> +
> +	return 0;
>  }
>  
>  static bool ovl_type_merge(struct dentry *dentry)
> @@ -238,7 +250,7 @@ static int ovl_create_upper(struct dentry *dentry, struct inode *inode,
>  		ovl_set_opaque(dentry, newdentry);
>  	}
>  
> -	ovl_instantiate(dentry, inode, newdentry, !!hardlink);
> +	err = ovl_instantiate(dentry, inode, newdentry, !!hardlink);
>  	newdentry = NULL;
>  out_dput:
>  	dput(newdentry);
> @@ -439,7 +451,7 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
>  		if (err)
>  			goto out_cleanup;
>  	}
> -	ovl_instantiate(dentry, inode, newdentry, !!hardlink);
> +	err = ovl_instantiate(dentry, inode, newdentry, !!hardlink);
>  	newdentry = NULL;
>  out_dput2:
>  	dput(upper);
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 7abcf96e94fc..060c534998d1 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -741,6 +741,24 @@ static bool ovl_verify_inode(struct inode *inode, struct dentry *lowerdentry,
>  	return true;
>  }
>  
> +static int ovl_insert_inode_locked(struct inode *inode, struct inode *realinode)
> +{
> +	return insert_inode_locked4(inode, (unsigned long) realinode,
> +				    ovl_inode_test, realinode);
> +}
> +
> +int ovl_insert_inode(struct inode *inode, struct inode *realinode)
> +{
> +	int err;
> +
> +	err = ovl_insert_inode_locked(inode, realinode);
> +	if (err)
> +		return err;
> +
> +	unlock_new_inode(inode);
> +	return 0;
> +}
> +
>  struct inode *ovl_lookup_inode(struct super_block *sb, struct dentry *real,
>  			       bool is_upper)
>  {
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index caaa47cea2aa..642b25702092 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -343,6 +343,7 @@ int ovl_update_time(struct inode *inode, struct timespec *ts, int flags);
>  bool ovl_is_private_xattr(const char *name);
>  
>  struct inode *ovl_new_inode(struct super_block *sb, umode_t mode, dev_t rdev);
> +int ovl_insert_inode(struct inode *inode, struct inode *realinode);
>  struct inode *ovl_lookup_inode(struct super_block *sb, struct dentry *real,
>  			       bool is_upper);
>  struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
> -- 
> 2.7.4
>