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Date:   Thu, 6 Sep 2018 21:57:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nadav Amit <namit@vmware.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Andi Kleen <ak@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Dave Hansen <dave.hansen@intel.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs
Message-ID: <20180906195731.GE4816@worktop.programming.kicks-ass.net>
References: <20180902181451.80520-1-namit@vmware.com>
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On Sun, Sep 02, 2018 at 11:14:50AM -0700, Nadav Amit wrote:
> When page-table entries are set, the compiler might optimize their
> assignment by using multiple instructions to set the PTE. This might
> turn into a security hazard if the user somehow manages to use the
> interim PTE. L1TF does not make our lives easier, making even an interim
> non-present PTE a security hazard.
> 
> Using WRITE_ONCE() to set PTEs and friends should prevent this potential
> security hazard.
> 
> I skimmed the differences in the binary with and without this patch. The
> differences are (obviously) greater when CONFIG_PARAVIRT=n as more
> code optimizations are possible. For better and worse, the impact on the
> binary with this patch is pretty small. Skimming the code did not cause
> anything to jump out as a security hazard, but it seems that at least
> move_soft_dirty_pte() caused set_pte_at() to use multiple writes.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Also, its corollary would also make sense/be required, use READ_ONCE()
when reading these.