Re: [PATCHv2] x86/mm: set x32 syscall bit in SET_PERSONALITY()

[Dmitry Safonov]

On 03/21/2017 08:45 PM, Andy Lutomirski wrote:
> On Tue, Mar 21, 2017 at 10:17 AM, Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> > On Tue, Mar 21, 2017 at 07:37:12PM +0300, Dmitry Safonov wrote:
> > ...
> > > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > > index d6b784a5520d..d3d4d9abcaf8 100644
> > > --- a/arch/x86/kernel/process_64.c
> > > +++ b/arch/x86/kernel/process_64.c
> > > @@ -519,8 +519,14 @@ void set_personality_ia32(bool x32)
> > >               if (current->mm)
> > >                       current->mm->context.ia32_compat = TIF_X32;
> > >               current->personality &= ~READ_IMPLIES_EXEC;
> > > -             /* in_compat_syscall() uses the presence of the x32
> > > -                syscall bit flag to determine compat status */
> > > +             /*
> > > +              * in_compat_syscall() uses the presence of the x32
> > > +              * syscall bit flag to determine compat status.
> > > +              * On the bitness of syscall relies x86 mmap() code,
> > > +              * so set x32 syscall bit right here to make
> > > +              * in_compat_syscall() work during exec().
> > > +              */
> > > +             task_pt_regs(current)->orig_ax |= __X32_SYSCALL_BIT;
> > >               current->thread.status &= ~TS_COMPAT;
> >
> > Hi! I must admit I didn't follow close the overall series (so can't
> > comment much here :) but I have a slightly unrelated question -- is
> > there a way to figure out if task is running in x32 mode say with
> > some ptrace or procfs sign?
>
> You should be able to figure out of a *syscall* is x32 by simply
> looking at bit 30 in the syscall number.  (This is unlike i386, which
> is currently not reflected in ptrace.)

The process could be stopped with PTRACE_SEIZE and I think, it'll not
have x32 syscall bit at that moment.

I guess the question comes from that we're releasing CRIU 3.0 with
32-bit C/R and some other cool stuff, but we don't support x32 yet.
As we don't want release a thing that we aren't properly testing.
So for a while we should error on dumping x32 applications.

I think, the best way for now is to check physicall address of vdso
from /proc/.../pagemap. If it's CONFIG_VDSO=n kernel, I guess we could
also add check for %ds from ptrace's register set. For x32 it's set to
__USER_DS, while for native it's 0 (looking at start_thread() and
compat_start_thread()). The application can simply change it without
any consequence - so it's not very reliable, we could only warn at
catching it, not rely on this.

> Do we actually have an x32 per-task mode at all?  If so, maybe we can
> just remove it on top of Dmitry's series.

--
             Dmitry