Re: [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen

[M. Vefa Bicakci]

On 07/21/2018 07:30 PM, Andy Lutomirski wrote:
> On Sat, Jul 21, 2018 at 4:19 PM, M. Vefa Bicakci <m.v.b@xxxxxxxxxx> wrote:
> > On 07/21/2018 05:45 PM, Andy Lutomirski wrote:
> > > On Sat, Jul 21, 2018 at 12:49 PM, M. Vefa Bicakci <m.v.b@xxxxxxxxxx>
> > > wrote:
> > > > Commit 3ac6d8c787b8 ("x86/entry/64: Clear registers for
> > > > exceptions/interrupts, to reduce speculation attack surface")
> > > > unintendedly
> > > > broke Xen PV virtual machines by clearing the %rbx register at the end of
> > > > xen_failsafe_callback before the latter jumps to error_exit.
> > > > error_exit expects the %rbx register to be a flag indicating whether
> > > > there should be a return to kernel mode.
> > > >
> > > > This commit makes sure that the %rbx register is not cleared by
> > > > the PUSH_AND_CLEAR_REGS macro, when the macro in question is instantiated
> > > > by xen_failsafe_callback, to avoid the issue.
> > >
> > >
> > > Seems like a genuine problem, but:
> > >
> > > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > > > index c7449f377a77..96e8ff34129e 100644
> > > > --- a/arch/x86/entry/entry_64.S
> > > > +++ b/arch/x86/entry/entry_64.S
> > > > @@ -1129,7 +1129,7 @@ ENTRY(xen_failsafe_callback)
> > > >           addq    $0x30, %rsp
> > > >           UNWIND_HINT_IRET_REGS
> > > >           pushq   $-1 /* orig_ax = -1 => not a system call */
> > > > -       PUSH_AND_CLEAR_REGS
> > > > +       PUSH_AND_CLEAR_REGS clear_rbx=0
> > > >           ENCODE_FRAME_POINTER
> > > >           jmp     error_exit
> > >
> > >
> > > The old code first set RBX to zero then, if frame pointers are on,
> > > sets it to some special non-zero value, then crosses its fingers and
> > > hopes for the best.  Your patched code just skips the zeroing part, so
> > > RBX either contains the ENCODE_FRAME_POINTER result or is
> > > uninitialized.
> > >
> > > How about actually initializing rbx to something sensible like, say, 1?
> >
> > Hello Andy,
> >
> > Thank you for the review! Apparently, I have not done my homework fully.
> > I will test your suggestion and report back, most likely in a few hours.
> >
> > I have been testing with the next/linux-next tree's master branch
> > (dated 20180720), and I noticed that ENCODE_FRAME_POINTER changes the
> > frame pointer (i.e., RBP) register, as opposed to the RBX register,
> > which the patch aims to avoid changing before jumping to error_exit.
> > It is possible that I am missing something though -- I am not sure about
> > the connection between the RBP and RBX registers.
>
> Sorry, brain fart on my part.

No problem! :-)

> > The change introduced by commit 3ac6d8c787b8 is in the excerpt below. Would
> > it
> > be valid to state that the original code had the same issue that you
> > referred
> > to (i.e., leaving the RBX register uninitialized)?
>
> Presumably.
>
> I would propose a rather different fix:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=bb3d76b50c3bc78b67d79cf90d328f38a435c793
>
> Any chance you could test that and see if it fixes your problem?

Of course; I will report back with the result in a few hours.