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?
