linux-kernel - Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

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Date:   Wed, 10 Jan 2018 12:27:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Dan Williams <dan.j.williams@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] x86/enter: Use IBRS on syscall and interrupts

On 10/01/2018 11:04, Peter Zijlstra wrote:
> On Tue, Jan 09, 2018 at 06:26:47PM -0800, Tim Chen wrote:
>> Set IBRS upon kernel entrance via syscall and interrupts. Clear it
>> upon exit.  IBRS protects against unsafe indirect branching predictions
>> in the kernel.
>>
>> The NMI interrupt save/restore of IBRS state was based on Andrea
>> Arcangeli's implementation.
>> Here's an explanation by Dave Hansen on why we save IBRS state for NMI.
>>
>> The normal interrupt code uses the 'error_entry' path which uses the
>> Code Segment (CS) of the instruction that was interrupted to tell
>> whether it interrupted the kernel or userspace and thus has to switch
>> IBRS, or leave it alone.
>>
>> The NMI code is different.  It uses 'paranoid_entry' because it can
>> interrupt the kernel while it is running with a userspace IBRS (and %GS
>> and CR3) value, but has a kernel CS.  If we used the same approach as
>> the normal interrupt code, we might do the following;
>>
>> 	SYSENTER_entry
>> <-------------- NMI HERE
>> 	IBRS=1
>> 		do_something()
>> 	IBRS=0
>> 	SYSRET
>>
>> The NMI code might notice that we are running in the kernel and decide
>> that it is OK to skip the IBRS=1.  This would leave it running
>> unprotected with IBRS=0, which is bad.
>>
>> However, if we unconditionally set IBRS=1, in the NMI, we might get the
>> following case:
>>
>> 	SYSENTER_entry
>> 	IBRS=1
>> 		do_something()
>> 	IBRS=0
>> <-------------- NMI HERE (set IBRS=1)
>> 	SYSRET
>>
>> and we would return to userspace with IBRS=1.  Userspace would run
>> slowly until we entered and exited the kernel again.
>>
>> Instead of those two approaches, we chose a third one where we simply
>> save the IBRS value in a scratch register (%r13) and then restore that
>> value, verbatim.
>>
> 
> What this Changelog fails to address is _WHY_ we need this. What does
> this provide that retpoline does not.

Which, for the record, is just that it works better on Skylake+ CPUs.

Paolo