linux-kernel - Re: [PATCH] x86/entry/64: Fix CR3 restore order in paranoid

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Date:   Wed, 14 Feb 2018 17:16:37 +0000
From:   Andy Lutomirski <luto@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86/entry/64: Fix CR3 restore order in paranoid_exit()

On Wed, Feb 14, 2018 at 7:39 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Dave Hansen <dave.hansen@...el.com> wrote:
>
>> On 02/13/2018 06:27 PM, Josh Poimboeuf wrote:
>> > --- a/arch/x86/entry/entry_64.S
>> > +++ b/arch/x86/entry/entry_64.S
>> > @@ -1167,10 +1167,10 @@ ENTRY(paranoid_exit)
>> >     UNWIND_HINT_REGS
>> >     DISABLE_INTERRUPTS(CLBR_ANY)
>> >     TRACE_IRQS_OFF_DEBUG
>> > +   RESTORE_CR3     scratch_reg=%r15 save_reg=%r14
>> >     testl   %ebx, %ebx                      /* swapgs needed? */
>> >     jnz     .Lparanoid_exit_no_swapgs
>> >     TRACE_IRQS_IRETQ
>> > -   RESTORE_CR3     scratch_reg=%rbx save_reg=%r14
>> >     SWAPGS_UNSAFE_STACK
>> >     jmp     .Lparanoid_exit_restore
>> >  .Lparanoid_exit_no_swapgs:
>>
>> TRACE_IRQS_* call non-entry functions that are not mapped by the user
>> CR3.  How can this possibly work?  What am I missing?
>
> How about something like the patch below? (Totally untested)
>
> Thanks,
>
>         Ingo
> ---
>  arch/x86/entry/entry_64.S | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index cd216c9431e1..8971bd64d515 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1175,6 +1175,7 @@ ENTRY(paranoid_exit)
>         jmp     .Lparanoid_exit_restore
>  .Lparanoid_exit_no_swapgs:
>         TRACE_IRQS_IRETQ_DEBUG
> +       RESTORE_CR3     scratch_reg=%rbx save_reg=%r14
>  .Lparanoid_exit_restore:
>         jmp restore_regs_and_return_to_kernel
>  END(paranoid_exit)

Reviewed-by: Andy Lutomirski <luto@...nel.org>