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Date: Wed, 23 Aug 2017 11:13:02 -0600
From: Tycho Andersen <tycho@docker.com>
Subject: Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one()
Message-ID: <20170823171302.ubnv7qyrexhhpbs7@smitten>
References: <20170809200755.11234-1-tycho@docker.com>
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 <20170812112603.GB16374@remoulade>
 <20170814163536.6njceqc3dip5lrlu@smitten>
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To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel-hardening@lists.openwall.com, Marco Benatto <marco.antonio.780@gmail.com>, Juerg Haefliger <juerg.haefliger@canonical.com>

On Wed, Aug 23, 2017 at 06:04:43PM +0100, Mark Rutland wrote:
> On Wed, Aug 23, 2017 at 10:58:42AM -0600, Tycho Andersen wrote:
> > Hi Mark,
> > 
> > On Mon, Aug 14, 2017 at 05:50:47PM +0100, Mark Rutland wrote:
> > > That said, is there any reason not to use flush_tlb_kernel_range()
> > > directly?
> > 
> > So it turns out that there is a difference between __flush_tlb_one() and
> > flush_tlb_kernel_range() on x86: flush_tlb_kernel_range() flushes all the TLBs
> > via on_each_cpu(), where as __flush_tlb_one() only flushes the local TLB (which
> > I think is enough here).
> 
> That sounds suspicious; I don't think that __flush_tlb_one() is
> sufficient.
> 
> If you only do local TLB maintenance, then the page is left accessible
> to other CPUs via the (stale) kernel mappings. i.e. the page isn't
> exclusively mapped by userspace.

I thought so too, so I tried to test it with something like the patch
below. But it correctly failed for me when using __flush_tlb_one(). I
suppose I'm doing something wrong in the test, but I'm not sure what.

Tycho