* Thomas Gleixner <t...@linutronix.de> wrote:
> On Fri, 8 Dec 2017, Ingo Molnar wrote:
> > * Thomas Gleixner <t...@linutronix.de> wrote:
> >
> > > On Fri, 8 Dec 2017, Ingo Molnar wrote:
> > > > * Andy Lutomirski <l...@amacapital.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.
Yeah, ok. I don't think we should hide it - if it's in the vma space it should
be
listed in the 'maps' file, and with a descriptive name.
Thanks,
Ingo
