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    SubjectRe: [Virtio-fs] [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files
    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
    > >>>
    >
    > _______________________________________________
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    > Virtio-fs@redhat.com
    > https://listman.redhat.com/mailman/listinfo/virtio-fs
    >

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