linux-kernel - Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207

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Date:   Wed, 13 Nov 2019 10:10:59 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207


> On Nov 12, 2019, at 1:21 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> CVE-2018-12207 is a microarchitectural implementation issue
> that could allow an unprivileged local attacker to cause system wide
> denial-of-service condition.
> 
> Privileged software may change the page size (ex. 4KB, 2MB, 1GB) in the
> paging structures, without following such paging structure changes with
> invalidation of the TLB entries corresponding to the changed pages. In
> this case, the attacker could invoke instruction fetch, which will result
> in the processor hitting multiple TLB entries, reporting a machine check
> error exception, and ultimately hanging the system.
> 
> The attached patches mitigate the vulnerability by making huge pages
> non-executable. The processor will not be able to execute an instruction
> residing in a large page (ie. 2MB, 1GB, etc.) without causing a trap into
> the host kernel/hypervisor; KVM will then break the large page into 4KB
> pages and gives executable permission to 4KB pages.

It sounds that this mitigation will trigger the “page fracturing” problem
I once encountered [1], causing frequent full TLB flushes when invlpg
runs. I wonder if VMs would benefit in performance from changing
/sys/kernel/debug/x86/tlb_single_page_flush_ceiling to zero.

On a different note - I am not sure I fully understand the exact scenario.
Any chance of getting a kvm-unit-test for this case?

[1] https://patchwork.kernel.org/patch/9099311/