Messages in this thread | | | Subject | Re: [PATCH 1/2] x86/entry/64: Do not clear %rbx under Xen | From | "M. Vefa Bicakci" <> | Date | Sat, 21 Jul 2018 19:37:05 -0400 |
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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@runbox.com> 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@runbox.com> >>> 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.
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