linux-kernel - Re: [RFC v2 00/27] Kernel Address Space Isolation

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Date:   Fri, 12 Jul 2019 06:51:02 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        kvm@...r.kernel.org, x86@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     konrad.wilk@...cle.com, jan.setjeeilers@...cle.com,
        liran.alon@...cle.com, jwadams@...gle.com, graf@...zon.de,
        rppt@...ux.vnet.ibm.com
Subject: Re: [RFC v2 00/27] Kernel Address Space Isolation

On 7/12/19 1:09 AM, Alexandre Chartre wrote:
> On 7/12/19 12:38 AM, Dave Hansen wrote:
>> I don't see the per-cpu areas in here.  But, the ASI macros in
>> entry_64.S (and asi_start_abort()) use per-cpu data.
> 
> We don't map all per-cpu areas, but only the per-cpu variables we need. ASI
> code uses the per-cpu cpu_asi_session variable which is mapped when an ASI
> is created (see patch 15/26):

No fair!  I had per-cpu variables just for PTI at some point and had to
give them up! ;)

> +    /*
> +     * Map the percpu ASI sessions. This is used by interrupt handlers
> +     * to figure out if we have entered isolation and switch back to
> +     * the kernel address space.
> +     */
> +    err = ASI_MAP_CPUVAR(asi, cpu_asi_session);
> +    if (err)
> +        return err;
> 
> 
>> Also, this stuff seems to do naughty stuff (calling C code, touching
>> per-cpu data) before the PTI CR3 writes have been done.  But, I don't
>> see anything excluding PTI and this code from coexisting.
> 
> My understanding is that PTI CR3 writes only happens when switching to/from
> userland. While ASI enter/exit/abort happens while we are already in the
> kernel,
> so asi_start_abort() is not called when coming from userland and so not
> interacting with PTI.

OK, that makes sense.  You only need to call C code when interrupted
from something in the kernel (deeper than the entry code), and those
were already running kernel C code anyway.

If this continues to live in the entry code, I think you have a good
clue where to start commenting.

BTW, the PTI CR3 writes are not *strictly* about the interrupt coming
from user vs. kernel.  It's tricky because there's a window both in the
entry and exit code where you are in the kernel but have a userspace CR3
value.  You end up needing a CR3 write when you have a userspace CR3
value when the interrupt occurred, not only when you interrupt userspace
itself.