linux-kernel - Re: [PATCH] LDT improvements

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Date:   Fri, 8 Dec 2017 10:55:36 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] LDT improvements

On Fri, 8 Dec 2017, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > On Fri, 8 Dec 2017, Ingo Molnar wrote:
> > > * Andy Lutomirski <luto@...capital.net> wrote:
> > > > I don't love mucking with user address space.  I'm also quite nervous about 
> > > > putting it in our near anything that could pass an access_ok check, since we're 
> > > > totally screwed if the bad guys can figure out how to write to it.
> > > 
> > > Hm, robustness of the LDT address wrt. access_ok() is a valid concern.
> > > 
> > > Can we have vmas with high addresses, in the vmalloc space for example?
> > > IIRC the GPU code has precedents in that area.
> > > 
> > > Since this is x86-64, limitation of the vmalloc() space is not an issue.
> > > 
> > > I like Thomas's solution:
> > > 
> > >  - have the LDT in a regular mmap space vma (hence per process ASLR randomized), 
> > >    but with the system bit set.
> > > 
> > >  - That would be an advantage even for non-PTI kernels, because mmap() is probably 
> > >    more randomized than kmalloc().
> > 
> > Randomization is pointless as long as you can get the LDT address in user
> > space, i.e. w/o UMIP.
> 
> But with UMIP unprivileged user-space won't be able to get the linear address of 
> the LDT. Now it's written out in /proc/self/maps.

We can expose it nameless like other VMAs, but then it's 128k sized so it
can be figured out. But when it's RO then it's not really a problem, even
the kernel can't write to it.

> > >  - It would also be a cleaner approach all around, and would avoid the fixmap
> > >    complications and the scheduler muckery.
> > 
> > The error code of such an access is always 0x03. So I added a special
> > handler, which checks whether the address is in the LDT map range and
> > verifies that the access bit in the descriptor is 0. If that's the case it
> > sets it and returns. If not, the thing dies. That works.
> 
> Are SMP races possible? For example two threads both triggering the accessed bit 
> fault, but only one of them succeeding in setting it. The other thread should not 
> die in this case, right?

Right. I'm trying to figure out whether there is a way to reliably detect
that write access bit mode.

Thanks,

	tglx