linux-kernel - Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb

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Date:   Wed, 23 Aug 2017 18:04:43 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Tycho Andersen <tycho@...ker.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>
Subject: Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one()

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.

Thanks,
Mark.