Messages in this thread | | | Subject | Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs | From | Vlastimil Babka <> | Date | Fri, 7 Sep 2018 16:10:57 +0200 |
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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.
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