Messages in this thread | | | Subject | Re: [RFC PTI 1/3] x86/pti: Vastly simplify pgd synchronization | From | Dave Hansen <> | Date | Fri, 8 Dec 2017 12:17:34 -0800 |
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From a high level, what finally allowed this to happen? Because kpti_add_user_map() all went away, including the LDT one?
> + if (pgdp_maps_userspace(pgdp)) { > /* > + * The user page tables get the full PGD, > + * accessible from userspace: > */ > + kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd; > + > + /* > + * If this is normal user memory, make it NX in the kernel > + * pagetables so that, if we somehow screw up and return to > + * usermode with the kernel CR3 loaded, we'll get a page > + * fault instead of allowing user code to execute with > + * the wrong CR3. > + * > + * As exceptions, we don't set NX if: > + * - this is EFI or similar, the kernel may execute from it > + * - we don't have NX support > + * - we're clearing the PGD (i.e. pgd.pgd == 0). > + */ > + if ((pgd.pgd & _PAGE_USER) && (__supported_pte_mask & _PAGE_NX)) > + pgd.pgd |= _PAGE_NX;
I scratched my head for a sec to realize why you didn't need an explicit pgd.pgd==0 check. It's part of the _PAGE_USER check, which is a bit confusing. Could we do:
if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) && (__supported_pte_mask & _PAGE_NX))
And change the comment:
* As exceptions, we don't set NX fo: * - PGDs without _PAGE_USER: Assume this is for a weird in-kernel * user like EFI from which we may need to execute. * - PGDs withoout _PAGE_PRESENT: PGD is being cleared, must * not set _PAGE_NX * - we don't have NX support
> } else { > /* > + * Changes to the high (kernel) portion of the kernelmode > + * page tables are not automatically propagated to the > + * usermode tables. > + * > + * Users should keep in mind that, unlike the kernelmode > + * tables, there is no vmalloc_fault equivalent for the > + * usermode tables. Top-level entries added to init_mm's > + * usermode pgd after boot will not be automatically > + * propagated to other mms. > */ > - WARN_ON_ONCE(system_state == SYSTEM_RUNNING); > } > #endif
How does the VSYSCALL page get into the user page tables now?
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