From: Vivek Goyal <vgoyal@redhat.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
viro@zeniv.linux.org.uk, virtio-fs@redhat.com, dwalsh@redhat.com,
dgilbert@redhat.com, casey.schaufler@intel.com,
linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
tytso@mit.edu, miklos@szeredi.hu, gscrivan@redhat.com,
jack@suse.cz, Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files
Date: Fri, 9 Jul 2021 11:27:37 -0400 [thread overview]
Message-ID: <20210709152737.GA398382@redhat.com> (raw)
In-Reply-To: <20210709091915.2bd4snyfjndexw2b@wittgenstein>
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.
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
> >
>
next prev parent reply other threads:[~2021-07-09 15:27 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 [this message]
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 ` [Virtio-fs] " Greg Kurz
2021-07-13 14:28 ` Casey Schaufler
2021-07-09 16:00 ` [RFC PATCH v2 0/1] Relax restrictions on user.* xattr Daniel Walsh
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