Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 21 Nov 2017 19:44:39 -0800 | Subject | Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace |
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On Tue, Nov 21, 2017 at 2:09 PM, Dave Hansen <dave.hansen@linux.intel.com> wrote: > 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.
Is this code path ever hit in any case other than tearing down an LDT?
I'm tempted to suggest that KAISER just disable the MODIFY_LDT config option for now...
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