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SubjectRe: [PATCH 2/5] security,overlayfs: Provide security hook for copy up of xattrs for overlay file
On Wed, Jul 06, 2016 at 01:09:00PM -0400, Vivek Goyal wrote:
> On Tue, Jul 05, 2016 at 02:34:43PM -0700, Casey Schaufler wrote:
> > On 7/5/2016 2:15 PM, Vivek Goyal wrote:
> > > On Tue, Jul 05, 2016 at 01:22:22PM -0700, Casey Schaufler wrote:
> > >> On 7/5/2016 8:50 AM, Vivek Goyal wrote:
> > >>> Provide a security hook which is called when xattrs of a file are being
> > >>> copied up. This hook is called once for each xattr and one can either
> > >>> accept or reject xattr. If 0 is returned, xattr will be copied up, if 1
> > >>> is returned, xattr will not be copied up and if negative error code
> > >>> is returned, copy up will be aborted.
> > >>>
> > >>> In SELinux, label of lower file is not copied up. File already has been
> > >>> set with right label at the time of creation and we don't want to overwrite
> > >>> that label.
> > >>>
> > >>> Signed-off-by: David Howells <dhowells@redhat.com>
> > >>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > >>> ---
> > >>> fs/overlayfs/copy_up.c | 8 ++++++++
> > >>> include/linux/lsm_hooks.h | 13 +++++++++++++
> > >>> include/linux/security.h | 10 ++++++++++
> > >>> security/security.c | 9 +++++++++
> > >>> security/selinux/hooks.c | 14 ++++++++++++++
> > >>> 5 files changed, 54 insertions(+)
> > >>>
> > >>> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> > >>> index 90dc362..2c31938 100644
> > >>> --- a/fs/overlayfs/copy_up.c
> > >>> +++ b/fs/overlayfs/copy_up.c
> > >>> @@ -103,6 +103,14 @@ retry:
> > >>> goto retry;
> > >>> }
> > >>>
> > >>> + error = security_inode_copy_up_xattr(old, new,
> > >>> + name, value, size);
> > >>> + if (error < 0)
> > >>> + break;
> > >>> + if (error == 1) {
> > >>> + error = 0;
> > >>> + continue; /* Discard */
> > >>> + }
> > >>> error = vfs_setxattr(new, name, value, size, 0);
> > >>> if (error)
> > >>> break;
> > >>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > >>> index fcde9b9..2a8ee8c 100644
> > >>> --- a/include/linux/lsm_hooks.h
> > >>> +++ b/include/linux/lsm_hooks.h
> > >>> @@ -412,6 +412,16 @@
> > >>> * @src indicates the union dentry of file that is being copied up.
> > >>> * @old indicates the pointer to old_cred returned to caller.
> > >>> * Returns 0 on success or a negative error code on error.
> > >>> + * @inode_copy_up_xattr:
> > >>> + * Filter the xattrs being copied up when a unioned file is copied
> > >>> + * up from a lower layer to the union/overlay layer.
> > >>> + * @src indicates the file that is being copied up.
> > >>> + * @dst indicates the file that has being created by the copy up.
> > >>> + * @name indicates the name of the xattr.
> > >>> + * @value, @size indicate the payload of the xattr.
> > >>> + * Returns 0 to accept the xattr, 1 to discard the xattr or a negative
> > >>> + * error code to abort the copy up. Note that the caller is responsible
> > >>> + * for reading and writing the xattrs as this hook is merely a filter.
> > >> The return should be -EOPNOTSUPP from security modules that don't
> > >> support the attribute "name". This will make it possible to support
> > >> multiple modules that provide attributes. (patches pending)
> > > Hmm.., Sorry I did not understand this one.
> > >
> > > So all modules will not understand all xattrs. So if they start returning
> > > -EOPNOTSUPP, then as per current implementation, copy up operation will
> > > be aborted.
> >
> > Yes, the infrastructure code will have to change to deal with the
> > tri-state returns. That's also true of several other hooks.
> >
> > > Current implementation relies on that a security module, returns 0 if
> > > every thing is "name" xattr should be copied up or lsm does not care.
> > > Negative error code is returned only if something is wrong. Given every
> > > lsm will not understand/care about all the xattrs, we can't return
> > > error code if lsm does not own/understand the "name". In fact
> > > call_int_hook() will bail out the very first time negative error code
> > > is returned.
> > >
> > > IOW, current implementation will work with multiple modules providing
> > > implementation for same hook as long as module returns 0 for the xattrs
> > > it does not understand.
> >
> > There have to be four states. I own this attribute, and want you
> > to use it. I own this attribute and I want you to ignore it. I don't
> > own this attribute. I own this attribute and something went terribly
> > wrong, such as running out of memory.
>
> Ok, so we have 3 states currently and we should have four.
>
> I own this attribute and want you to use it ---> Return 0
> I own this attribute and want you to ignore it --> Return 1
> I don't own this attribute --> -EOPNOTSUPP
> Something went terribly wrong --> Negative error code.
>
> I can modify call_int_hook() to continue if -EOPNOTSUPP is returned. And
> if none of the LSMs claimed xattr, caller will get -EOPNOTSUPP.
>
> But what is caller supposed to do with it. There might be xattrs which
> are just user data (user.foo) and aborting copying up will not make sense.
> That means caller will continue to copy up anyway and treat -EOPNOTSUPP
> as success.
>
> IOW, What are we going to gain by introducing this extra state when none
> of the LSMs claims to know about the xattr name passed in.

Or you are looking for something where caller does not see -EOPNOTSUPP. It
is useful for call_int_hook_foo() where it will return after first LSM
has claimed the "name".

Vivek

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