Re: [PATCH] LDT improvements

[Thomas Gleixner]

On Fri, 8 Dec 2017, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> > On Fri, 8 Dec 2017, Ingo Molnar wrote:
> > > * Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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