Commit 13c164b1a186 - regression for LSMs/SELinux?

[Stephen Smalley]

On Tue, Sep 22, 2020 at 3:34 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> On Mon, Sep 21, 2020 at 6:30 PM Al Viro <viro at zeniv.linux.org.uk> wrote:
> > On Mon, Sep 21, 2020 at 06:09:22PM +0200, Christoph Hellwig wrote:
> > > [adding Linus and Al]
> > >
> > > On Mon, Sep 21, 2020 at 04:51:35PM +0200, Ondrej Mosnacek wrote:
> > > > Hi folks,
> > > >
> > > > It seems that after commit 13c164b1a186 ("autofs: switch to
> > > > kernel_write") there is now an extra LSM permission required (for the
> > > > current task to write to the automount pipe) for processes accessing
> > > > some yet-to-to-be mounted directory on which an autofs mount is set
> > > > up. The call chain is:
> > > > [...]
> > > > autofs_wait() ->
> > > > autofs_notify_daemon() ->
> > > > autofs_write() ->
> > > > kernel_write() ->
> > > > rw_verify_area() ->
> > > > security_file_permission()
> > > >
> > > > The bug report that led me to this commit is at [1].
> > > >
> > > > Technically, this is a regression for LSM users, since this is a
> > > > kernel-internal operation and an LSM permission for the current task
> > > > shouldn't be required. Can this patch be reverted? Perhaps
> > > > __kernel_{read|write}() could instead be renamed to kernel_*_nocheck()
> > > > so that the name is more descriptive?
> > >
> > > So we obviously should not break existing user space and need to fix
> > > this ASAP.  The trivial "fix" would be to export __kernel_write again
> > > and switch autofs to use it.  The other option would be a FMODE flag
> > > to bypass security checks, only to be set if the callers ensures
> > > they've been valided (i.e. in autofs_prepare_pipe).
>
> IMHO that sounds like an overkill in this scenario. I don't think it
> makes sense to do the LSM check here (or at least not against the
> current task's creds), because it is not the current task that wants
> to communicate with the daemon, it just wants to to access some
> directory on the system that just happens to be special to the kernel,
> which needs to do some communication on the side to service this
> request. So if we do want to do any LSM check here, there should at
> least be some "bool internal" flag passed to the LSM, signalizing that
> this is an internal read/write operation that wasn't directly
> initiated/requested by the current process. SELinux could then either
> use the kernel secid instead of the current task's secid or skip the
> check completely in such case.
>
> I'd like Stephen to weigh in on this, but it looks he might be on
> vacation right now...

No, just wasn't cc'd previously.  I don't think we want any LSM check
here.  As the long term fix, I would suggest moving the
security_file_permission() call up from rw_verify_area() to the
callers (and not call it from kernel_write() at all).

>
> > >
> > > Any opinions?
> >
> > Reexport for now.  Incidentally, what is LSM doing rejecting writes
> > into a pipe?
>
> With SELinux at least, what is allowed or denied is defined in the
> policy. And the policy usually defaults to everything denied and then
> you add rules to allow what needs (and makes sense) to be allowed.
> Since until kernel 5.8 random processes didn't need to write to pipes
> created by the automount daemon, it has never been explicitly allowed
> and so the automounting now fails. It is in no way obvious that all
> processes should have the permission to talk to the automount daemon
> just to traverse the filesystem...

Yes, controlling pipe writes is just one part of controlling
information flow between processes but the intent here is to control
userspace actions, not kernel-internal operations.