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

[Peter Zijlstra]

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@xxxxxxxxxxxxx>

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