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

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
>>>          pushq   $-1 /* orig_ax = -1 => not a system call */
>>> +       PUSH_AND_CLEAR_REGS clear_rbx=0
>>>          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.

> 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)?


I would propose a rather different fix:


Any chance you could test that and see if it fixes your problem?

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