Re: [GIT pull] x86 vdso updates

[Mikael Pettersson <mikpe@it.uu.se>]

Andrew Lutomirski writes:
 > On Sun, May 29, 2011 at 10:39 AM, Mikael Pettersson <mikpe@it.uu.se> wrote:
 > > Ingo Molnar writes:
 > >  >
 > >  > * Andrew Lutomirski <luto@mit.edu> wrote:
 > >  >
 > >  > > On Fri, May 27, 2011 at 7:36 AM, Andrew Lutomirski <luto@mit.edu> wrote:
 > >  > > > 3. Add int 0xcc and use it from vgettimeofday.  It will SIGSEGV if
 > >  > > > called from a user address (so it has no risk of ever becoming ABI)
 > >  > > > and it will do gettimeofday if called from the right address.  (I like
 > > ...
 > >  > > Make it a real syscall but with extra constraints.  It would have the
 > >  > > same calling convention as the syscall instruction, but it would turn
 > >  > > into SIGKILL if the calling address isn't in the VSYSCALL page
 > >
 > > This will make things difficult for user-space dynamic binary instrumentation
 > > applications, since these normally execute generated code at different
 > > addresses than the original code.
 > >
 > > Is there a safe fallback for this particular vsyscall?
 > 
 > All of the vsyscalls have vDSO versions that work like any other code.

Easiest would be if we can simply map int $0xcc with rAX==FOO to syscall or
int 0x80 with rAX==BAR.

We currently don't even know about the vDSO, it's all just user-space code
to us.

 > Alternatively, if the dynamic instrumentation code knew about
 > vsyscalls, it could just not instrument addresses in the vsyscall
 > page.

Not instrumenting code is not an option, unless we can prove that the
code in question has no relevant side-effects or unexpected control-flow.
(Where "side-effects" relate both to the integrity of the instrumentation
engine and the application-specific payload it's attaching to the code.)

 > What existing applications would get broken?

My concern is ThreadSpotter, but any user-space dynamic binary instrumentation
engine that instruments down to the raw kernel interface (syscall/sysenter/int
instructions) would have a problem with syscalls that only work at specific
addresses.

Anyway, if I can map that vsyscall to a plain proper syscall, then I'm OK.

/Mikael