linux-kernel - Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED

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Date:   Fri, 10 Jun 2022 22:18:23 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR

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

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.