Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs

From: Peter Zijlstra <peterz@infradead.org>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Nadav Amit <namit@vmware.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>, X86 ML <x86@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.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
Date: Thu, 6 Sep 2018 22:27:46 +0200	[thread overview]
Message-ID: <20180906202746.GD9358@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <10030BE1-FE29-4C60-9963-4BE932EF09BA@gmail.com>

On Thu, Sep 06, 2018 at 01:12:14PM -0700, Nadav Amit wrote:
> at 12:57 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > 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.
> 
> I don’t know. This would obviously be much more intrusive. I can add a
> get_pte() and write a Coccinelle script to use it instead of reading the
> PTE, but in most cases, I presume, it would be an overkill.
> 
> The reason for that is that the PTEs are supposed to be accessed while
> holding the page-table lock, and the hardware can only change dirty & access
> bits. I think that any code that assumes that these bits do not change while
> holding the lock is already broken in more ways.

There are lockless readers, but I just checked, mm/gup.c already uses
READ_ONCE(), so that should be fine.