Re: [PATCH 4/3] x86/ldt: allow to disable modify_ldt at runtime

[Willy Tarreau]

On Sat, Jul 25, 2015 at 10:42:14AM -0700, Andy Lutomirski wrote:
> On Sat, Jul 25, 2015 at 9:33 AM, Willy Tarreau <w@xxxxxx> wrote:
> > On Sat, Jul 25, 2015 at 09:08:39AM -0700, Andy Lutomirski wrote:
> >> There's one thing that I think is incomplete here.  Currently, espfix
> >> triggers if SS points to the LDT.  It's possible for SS to point to
> >> the LDT even with modify_ldt disabled, and there's a decent amount of
> >> attack surface there.
> >>
> >> Can we improve this?  Two ideas:
> >>
> >> 1. In the asm, patch out or otherwise disable espfix if that sysctl
> >> has never been set.  (Ick.)
> >>
> >> 2. When modify_ldt is runtime-disabled (or compile-time disabled,
> >> perhaps), disallow setting the LDT bit in SS in the handful of places
> >> that would allow it (ptrace and sigreturn off the top of my head).  We
> >> don't need to worry about (regs->ss & 4) being set on kernel entry
> >> because we'll never be in user mode with that bit set if the LDT is
> >> disabled, but that bit could still be set using kernel APIs.  (In
> >> fact, my sigreturn test does exactly that.)
> >>
> >> Hmm.  With synchronous LDT, we could plausibly check at runtime in the
> >> espfix code, too.  We used to use LAR to do this, but hpa removed it
> >> when he realized that it was racy.  It shouldn't be racy any more,
> >> because, with my patches applied, the LDT never changes while
> >> interrupts are off.
> >
> > I understand it's not complete but I'm a bit bothered with conflating
> > this sysctl with other setting methods, because if the purpose of the
> > sysctl is to disable the syscall, it should do that only. I'd rather
> > document that it's less complete than the Kconfig method and continue
> > to recommend using your option whenever possible (eg: all my kernels
> > will use it just as I've already disabled X86_16BIT everywhere).
> >
> 
> Agreed.  We can certainly tighten up the espfix code later.
> 
> > Also one benefit of having both options is that it will mechanically
> > make LDT a much less interesting target for future attacks, since it
> > will significantly reduce the likeliness of success, hence the motivation
> > for writing exploits that only work in conferences.
> >
> 
> Patch looks fine to me.

OK thanks.

Willy

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