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

[Nadav Amit]

at 12:57 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> 
> Also, its corollary would also make sense/be required, use READ_ONCE()
> when reading these.

I donât 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.