Messages in this thread | | | Subject | Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs | From | Dave Hansen <> | Date | Thu, 6 Sep 2018 10:21:38 -0700 |
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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?
So, while I agree that it's nice to document the need for a full 64-bit "atomicity" of the PTE set, I don't see a practical problem here with our implementation.
There's probably a massive number of things that would break if we assumed sane 64-bit writes can be observed piecemeal.
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