Re: [PATCH v4 0/6] Micro-optimize vclock_gettime

From: Andrew Lutomirski
Date: Tue May 17 2011 - 07:11:32 EST

On Tue, May 17, 2011 at 4:00 AM, Ingo Molnar <mingo@xxxxxxx> wrote:
> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> > see if I can persuade Uli to take accept a glibc patch to stop calling it
>> > in future static glibc versions.
>> How wide spread is this in reality on 64bit systems ?
>> IOW, what's the damage if we take a trap and emulate it in the most painful
>> way we can come up with ?

I dunno. I'll measure it.

> Well, how does that differ from having the real syscall instruction there? How
> are we going to filter real (old-)glibc calls from exploits?

Because there are only four vsyscalls: vgettimeofday, vtime, vgetcpu,
and venosys. None of them have side-effects, so they only allow an
attacker to write something to user memory somewhere. The
implementation of vgettimeofday needs a syscall instruction internally
for its fallback, which means that an attack could jump there instead
of to the start of the vsyscall implementation.

> If it can be filtered in a meaningful way then we should just do that and
> perhaps offer a (default enabled) .config COMPAT_VDSO_EMU=y switch to turn the
> emulation off.
> That way we keep the ABI and also have a way out for users who *really* need
> this to work in a performant way.

Yeah, that probably makes more sense. It'll make for an uglier
diffstat, though -- there's a lot of ugly duplicate code around to
make vgettimeofday and vgetcpu work.


> Thanks,
>        Ingo
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