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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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.

--Andy

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