linux-kernel - Re: [PATCH] x86: use WRITE

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Date:   Fri, 7 Sep 2018 16:10:57 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...el.com>, Nadav Amit <namit@...are.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs

On 09/06/2018 07:21 PM, Dave Hansen wrote:
> On 09/02/2018 11:14 AM, 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.
> 
> But, our types are already 64-bit, and we're doing a 64-bit pointer
> write.  Our WRITE_ONCE() implementation boils down to:
> 
> static __always_inline void __write_once_size(...
> {
>         switch (size) {
> 	...
>         case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
> 
> 
> For 64-bit types, which is precisely the same thing.  Right?

Notice the volatile cast. While the CPU write itself is fine, the
*compiler* can decide to do partial updates; volatile forbids it.