linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Tue, 9 Jan 2018 17:11:39 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     Jon Masters <jcm@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andi Kleen <andi@...stfloor.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jeff Law <law@...hat.com>, Nick Clifton <nickc@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Avoid speculative indirect calls in kernel

On 01/09/2018 04:45 PM, Thomas Gleixner wrote:
> On Mon, 8 Jan 2018, Andrea Arcangeli wrote:
>> On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote:
>> Did my best to do the cleanest patch for tip, but I now figured Dave's
>> original comment was spot on: a _PAGE_NX clear then becomes necessary
>> also after pud_alloc not only after p4d_alloc.
>>
>> pmd_alloc would run into the same with x86 32bit non-PAE too.

non-PAE doesn't have an NX bit. :)

But we #define _PAGE_NX down to 0 there so it's harmless.

>> So there are two choices, either going back to one single _PAGE_NX
>> clear from the original Dave's original patch as below, or to add
>> multiple clear after each level which was my objective and is more
>> robust, but it may be overkill in this case. As long as it was one
>> line it looked a clear improvement.
>>
>> Considering the caller in both cases is going to abort I guess we can
>> use the one liner approach as Dave and Jiri did originally.
> 
> Dave ?

I agree with Andrea.  The patch in -tip potentially misses the pgd
clearing if pud_alloc() sets a PGD.  It would also be nice to have that
comment back.

Note that the -tip commit probably works in *practice* because for two
adjacent calls to map_tboot_page() that share a PGD entry, the first
will clear NX, *then* allocate and set the PGD (without NX clear).  The
second call will *not* allocate but will clear the NX bit.

The patch I think we want is attached.

View attachment "pti-tboot-fix.patch" of type "text/x-patch" (1450 bytes)