From: Greg Kurz <groug@kaod.org>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Vivek Goyal <vgoyal@redhat.com>,
Christian Brauner <christian.brauner@ubuntu.com>,
<gscrivan@redhat.com>, <tytso@mit.edu>, <miklos@szeredi.hu>,
<selinux@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<virtio-fs@redhat.com>, <casey.schaufler@intel.com>,
<linux-security-module@vger.kernel.org>,
<viro@zeniv.linux.org.uk>, Christoph Hellwig <hch@infradead.org>,
<linux-fsdevel@vger.kernel.org>, <jack@suse.cz>
Subject: Re: [Virtio-fs] [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files
Date: Mon, 12 Jul 2021 14:49:30 +0200 [thread overview]
Message-ID: <20210712144849.121c948c@bahia.lan> (raw)
In-Reply-To: <710d1c6f-d477-384f-0cc1-8914258f1fb1@schaufler-ca.com>
On Fri, 9 Jul 2021 08:34:41 -0700
Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 7/9/2021 8:27 AM, Vivek Goyal wrote:
> > On Fri, Jul 09, 2021 at 11:19:15AM +0200, Christian Brauner wrote:
> >> On Thu, Jul 08, 2021 at 01:57:38PM -0400, Vivek Goyal wrote:
> >>> Currently user.* xattr are not allowed on symlink and special files.
> >>>
> >>> man xattr and recent discussion suggested that primary reason for this
> >>> restriction is how file permissions for symlinks and special files
> >>> are little different from regular files and directories.
> >>>
> >>> For symlinks, they are world readable/writable and if user xattr were
> >>> to be permitted, it will allow unpriviliged users to dump a huge amount
> >>> of user.* xattrs on symlinks without any control.
> >>>
> >>> For special files, permissions typically control capability to read/write
> >>> from devices (and not necessarily from filesystem). So if a user can
> >>> write to device (/dev/null), does not necessarily mean it should be allowed
> >>> to write large number of user.* xattrs on the filesystem device node is
> >>> residing in.
> >>>
> >>> This patch proposes to relax the restrictions a bit and allow file owner
> >>> or priviliged user (CAP_FOWNER), to be able to read/write user.* xattrs
> >>> on symlink and special files.
> >>>
> >>> virtiofs daemon has a need to store user.* xatrrs on all the files
> >>> (including symlinks and special files), and currently that fails. This
> >>> patch should help.
> >>>
> >>> Link: https://lore.kernel.org/linux-fsdevel/20210625191229.1752531-1-vgoyal@redhat.com/
> >>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> >>> ---
> >> Seems reasonable and useful.
> >> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> >>
> >> One question, do all filesystem supporting xattrs deal with setting them
> >> on symlinks/device files correctly?
> > Wrote a simple bash script to do setfattr/getfattr user.foo xattr on
> > symlink and device node on ext4, xfs and btrfs and it works fine.
>
> How about nfs, tmpfs, overlayfs and/or some of the other less conventional
> filesystems?
>
How about virtiofs then ? :-)
> >
> > https://github.com/rhvgoyal/misc/blob/master/generic-programs/user-xattr-special-files.sh
> >
> > I probably can add some more filesystems to test.
> >
> > Thanks
> > Vivek
> >
> >>> fs/xattr.c | 10 ++++++----
> >>> 1 file changed, 6 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/fs/xattr.c b/fs/xattr.c
> >>> index 5c8c5175b385..2f1855c8b620 100644
> >>> --- a/fs/xattr.c
> >>> +++ b/fs/xattr.c
> >>> @@ -120,12 +120,14 @@ xattr_permission(struct user_namespace *mnt_userns, struct inode *inode,
> >>> }
> >>>
> >>> /*
> >>> - * In the user.* namespace, only regular files and directories can have
> >>> - * extended attributes. For sticky directories, only the owner and
> >>> - * privileged users can write attributes.
> >>> + * In the user.* namespace, for symlinks and special files, only
> >>> + * the owner and priviliged users can read/write attributes.
> >>> + * For sticky directories, only the owner and privileged users can
> >>> + * write attributes.
> >>> */
> >>> if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
> >>> - if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
> >>> + if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) &&
> >>> + !inode_owner_or_capable(mnt_userns, inode))
> >>> return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
> >>> if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
> >>> (mask & MAY_WRITE) &&
> >>> --
> >>> 2.25.4
> >>>
>
> _______________________________________________
> Virtio-fs mailing list
> Virtio-fs@redhat.com
> https://listman.redhat.com/mailman/listinfo/virtio-fs
>
next prev parent reply other threads:[~2021-07-12 12:49 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-08 17:57 [RFC PATCH v2 0/1] Relax restrictions on user.* xattr Vivek Goyal
2021-07-08 17:57 ` [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files Vivek Goyal
2021-07-09 9:19 ` Christian Brauner
2021-07-09 15:27 ` Vivek Goyal
2021-07-09 15:34 ` Casey Schaufler
2021-07-09 17:59 ` Vivek Goyal
2021-07-09 20:10 ` Bruce Fields
2021-07-12 14:02 ` Vivek Goyal
2021-07-12 15:41 ` J. Bruce Fields
2021-07-12 17:47 ` Vivek Goyal
2021-07-12 19:31 ` J. Bruce Fields
2021-07-12 21:22 ` Vivek Goyal
2021-07-13 14:17 ` Casey Schaufler
2021-08-30 18:45 ` Vivek Goyal
2021-07-09 20:36 ` Theodore Ts'o
2021-07-12 17:50 ` Vivek Goyal
2021-07-12 12:49 ` Greg Kurz [this message]
2021-07-13 14:28 ` [Virtio-fs] " Casey Schaufler
2021-07-09 16:00 ` [RFC PATCH v2 0/1] Relax restrictions on user.* xattr Daniel Walsh
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