Re: [GIT pull] x86 vdso updates

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

Andrew Lutomirski writes:
 > On Sun, May 29, 2011 at 12:01 PM, Mikael Pettersson <mikpe@it.uu.se> wrote:
 > > 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.
 > 
 > Yes and no.
 > 
 > With the code I just posted (and am fixing up now) that will work.
 > But if we want to replace the entire vsyscall page with three int 0xcc
 > and 4090 int3 instructions, then we can't look at eax because it won't
 > contain anything meaningful.

I can relatively easily also consider the original application rIP
when decoding and translating these instructions.

 > 
 > --Andy
 > 
 > >
 > > 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.)
 > 
 > Calls to 0xffffffffff600000, 0xffffffffff600400, and
 > 0xffffffffff600800 are syscalls, as an (unfortunate) part of the ABI.
 > 
 > >
 > >  > 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.
 > 
 > I'll look.
 > 
 > >
 > > Anyway, if I can map that vsyscall to a plain proper syscall, then I'm OK.
 > 
 > All three vsyscalls can be replaced with real syscalls without side
 > effects.  Would it be possible to teach the instrumentation code to
 > deal with that?

Yes, I just need to know how to identify them and what their equivalents are.
E.g., an int3 at <known address> becomes syscall rAX=<some constant>.

Sounds like this change will be manageable after all.  Thanks.

/Mikael