Messages in this thread | | | Subject | Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace | From | Dave Hansen <> | Date | Tue, 21 Nov 2017 14:09:03 -0800 |
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On 11/20/2017 12:12 PM, Thomas Gleixner wrote: >> + */ >> + native_get_shadow_pgd(pgdp)->pgd = pgd.pgd; >> + /* >> + * For the copy of the pgd that the kernel >> + * uses, make it unusable to userspace. This >> + * ensures if we get out to userspace with the >> + * wrong CR3 value, userspace will crash >> + * instead of running. >> + */ >> + pgd.pgd |= _PAGE_NX; >> + } >> + } else if (!pgd.pgd) { >> + /* >> + * We are clearing the PGD and can not check _PAGE_USER >> + * in the zero'd PGD. > > Just the argument cannot be checked because it's clearing the entry. The > pgd entry itself is not yet modified, so it could be checked.
So, I guess we could enforce that only PGDs with _PAGE_USER set can ever be cleared. That has a nice symmetry to it because we set the shadow when we see _PAGE_USER and we would then clear the shadow when we see _PAGE_USER.
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