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

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Message-ID: <2428d66f-3c31-fa73-0d6a-c16fafa99455@canonical.com>
Date:   Wed, 30 Aug 2017 07:31:25 +0200
From:   Juerg Haefliger <juerg.haefliger@...onical.com>
To:     Mark Rutland <mark.rutland@....com>,
        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>
Subject: Re: [kernel-hardening] [PATCH v5 04/10] arm64: Add __flush_tlb_one()



On 08/23/2017 07:04 PM, 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.

We flush all CPUs to get rid of stale entries when a new page is
allocated to userspace that was previously allocated to the kernel.
Is that the scenario you were thinking of?

...Juerg


> Thanks,
> Mark.
> 



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