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Date:   Thu, 19 Oct 2023 16:22:57 +0200
From:   Jan Kara <jack@suse.cz>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Jan Kara <jack@suse.cz>, Jeff Layton <jlayton@kernel.org>,
        Chuck Lever <chuck.lever@oracle.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH 1/5] fanotify: limit reporting of event with
 non-decodeable file handles
Message-ID: <20231019142257.yevkf7i2istyzen2@quack3>
References: <20231018100000.2453965-1-amir73il@gmail.com>
 <20231018100000.2453965-2-amir73il@gmail.com>
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On Wed 18-10-23 12:59:56, Amir Goldstein wrote:
> Commit a95aef69a740 ("fanotify: support reporting non-decodeable file
> handles") merged in v6.5-rc1, added the ability to use an fanotify group
> with FAN_REPORT_FID mode to watch filesystems that do not support nfs
> export, but do know how to encode non-decodeable file handles, with the
> newly introduced AT_HANDLE_FID flag.
> 
> At the time that this commit was merged, there were no filesystems
> in-tree with those traits.
> 
> Commit 16aac5ad1fa9 ("ovl: support encoding non-decodable file handles"),
> merged in v6.6-rc1, added this trait to overlayfs, thus allowing fanotify
> watching of overlayfs with FAN_REPORT_FID mode.
> 
> In retrospect, allowing an fanotify filesystem/mount mark on such
> filesystem in FAN_REPORT_FID mode will result in getting events with
> file handles, without the ability to resolve the filesystem objects from
> those file handles (i.e. no open_by_handle_at() support).
> 
> For v6.6, the safer option would be to allow this mode for inode marks
> only, where the caller has the opportunity to use name_to_handle_at() at
> the time of setting the mark. In the future we can revise this decision.
> 
> Fixes: a95aef69a740 ("fanotify: support reporting non-decodeable file handles")
> Signed-off-by: Amir Goldstein <amir73il@gmail.com>

OK, I agree sb/mount marks reporting FIDs are hardly usable without
name_to_handle_at() so better forbid them before someone comes up with some
creative abuse. I've queued the patch into my tree.

								Honza

> ---
>  fs/notify/fanotify/fanotify_user.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
> index f69c451018e3..537c70beaad0 100644
> --- a/fs/notify/fanotify/fanotify_user.c
> +++ b/fs/notify/fanotify/fanotify_user.c
> @@ -1585,16 +1585,25 @@ static int fanotify_test_fsid(struct dentry *dentry, __kernel_fsid_t *fsid)
>  }
>  
>  /* Check if filesystem can encode a unique fid */
> -static int fanotify_test_fid(struct dentry *dentry)
> +static int fanotify_test_fid(struct dentry *dentry, unsigned int flags)
>  {
> +	unsigned int mark_type = flags & FANOTIFY_MARK_TYPE_BITS;
> +	const struct export_operations *nop = dentry->d_sb->s_export_op;
> +
> +	/*
> +	 * We need to make sure that the filesystem supports encoding of
> +	 * file handles so user can use name_to_handle_at() to compare fids
> +	 * reported with events to the file handle of watched objects.
> +	 */
> +	if (!nop)
> +		return -EOPNOTSUPP;
> +
>  	/*
> -	 * We need to make sure that the file system supports at least
> -	 * encoding a file handle so user can use name_to_handle_at() to
> -	 * compare fid returned with event to the file handle of watched
> -	 * objects. However, even the relaxed AT_HANDLE_FID flag requires
> -	 * at least empty export_operations for ecoding unique file ids.
> +	 * For sb/mount mark, we also need to make sure that the filesystem
> +	 * supports decoding file handles, so user has a way to map back the
> +	 * reported fids to filesystem objects.
>  	 */
> -	if (!dentry->d_sb->s_export_op)
> +	if (mark_type != FAN_MARK_INODE && !nop->fh_to_dentry)
>  		return -EOPNOTSUPP;
>  
>  	return 0;
> @@ -1812,7 +1821,7 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
>  		if (ret)
>  			goto path_put_and_out;
>  
> -		ret = fanotify_test_fid(path.dentry);
> +		ret = fanotify_test_fid(path.dentry, flags);
>  		if (ret)
>  			goto path_put_and_out;
>  
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR