Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Kirill A. Shutemov]

On Fri, Jun 10, 2022 at 10:18:23PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2022-06-10 at 11:08 -0700, Edgecombe, Richard P wrote:
> > On Fri, 2022-06-10 at 21:06 +0300, Kirill A. Shutemov wrote:
> > > On Fri, Jun 10, 2022 at 04:16:01PM +0000, Edgecombe, Rick P wrote:
> > > > On Fri, 2022-06-10 at 17:35 +0300, Kirill A. Shutemov wrote:
> > > > > +static int prctl_enable_tagged_addr(unsigned long nr_bits)
> > > > > +{
> > > > > +       struct mm_struct *mm = current->mm;
> > > > > +
> > > > > +       /* Already enabled? */
> > > > > +       if (mm->context.lam_cr3_mask)
> > > > > +               return -EBUSY;
> > > > > +
> > > > > +       /* LAM has to be enabled before spawning threads */
> > > > > +       if (get_nr_threads(current) > 1)
> > > > > +               return -EBUSY;
> > > > 
> > > > Does this work for vfork()? I guess the idea is that locking is
> > > > not
> > > > needed below because there is only one thread with the MM, but
> > > > with
> > > > vfork() another task could operate on the MM, call fork(), etc.
> > > > I'm
> > > > not
> > > > sure...
> > > 
> > > I'm not sure I follow. vfork() blocks parent process until child
> > > exit
> > > or
> > > execve(). I don't see how it is a problem.
> > 
> > Oh yea, you're right.
> 
> Actually, I guess vfork() only suspends the calling thread. So what if
> you had:
> 1. Parent spawns a bunch of threads
> 2. vforks()
> 3. Child enables LAM (it only has one thread, so succeeds)
> 4. Child exits()
> 5. Parent has some threads with LAM, and some not

I think it is in "Don't do that" territory. It is very similar to cases
described in "Caveats" section of the vfork(2) man-page.

> It's some weird userspace that doesn't deserve to have things work for
> it, but I wonder if it could open up little races around untagging. As
> an example, KVM might have a super narrow race where it checks for tags
> in memslots using addr != untagged_addr(addr) before checking
> access_ok(addr, ...). See __kvm_set_memory_region(). If mm-
> >context.untag_mask got set in the middle, tagged memslots could be
> added.

Ultimately, a process which calls vfork(2) is in control of what happens
to the new process until execve(2) or exit(2). So, yes it is very creative
way to shoot yourself into leg, but I don't think it worth preventing.

And I'm not sure how the fix would look like.

-- 
 Kirill A. Shutemov