On Mon, Dec 18, 2017 at 12:42:54PM +0100, Thomas Gleixner wrote:
> This will significantly slow down LDT users, but that shouldn't matter for
> important workloads -- the LDT is only used by DOSEMU(2), Wine, and very
> old libc implementations.
> +/*
> + * If PTI is enabled, this maps the LDT into the kernelmode and
> + * usermode tables for the given mm.
> + *
> + * There is no corresponding unmap function. Even if the LDT is freed, we
> + * leave the PTEs around until the slot is reused or the mm is destroyed.
> + * This is harmless: the LDT is always in ordinary memory, and no one will
> + * access the freed slot.
> + *
> + * If we wanted to unmap freed LDTs, we'd also need to do a flush to make
> + * it useful, and the flush would slow down modify_ldt().
> + */
> +static int
> +map_ldt_struct(struct mm_struct *mm, struct ldt_struct *ldt, int slot)
> +{
> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
> + bool is_vmalloc, had_top_level_entry;
> + unsigned long va;
> + spinlock_t *ptl;
> + pgd_t *pgd;
> + int i;
> +
> + if (!static_cpu_has(X86_FEATURE_PTI))
> + return 0;
> +
> + /*
> + * Any given ldt_struct should have map_ldt_struct() called at most
> + * once.
> + */
> + WARN_ON(ldt->slot != -1);
> +
> + /*
> + * Did we already have the top level entry allocated? We can't
> + * use pgd_none() for this because it doens't do anything on
> + * 4-level page table kernels.
> + */
> + pgd = pgd_offset(mm, LDT_BASE_ADDR);
> + had_top_level_entry = (pgd->pgd != 0);
> +
> + is_vmalloc = is_vmalloc_addr(ldt->entries);
> +
> + for (i = 0; i * PAGE_SIZE < ldt->nr_entries * LDT_ENTRY_SIZE; i++) {
> + unsigned long offset = i << PAGE_SHIFT;
> + const void *src = (char *)ldt->entries + offset;
> + unsigned long pfn;
> + pte_t pte, *ptep;
> +
> + va = (unsigned long)ldt_slot_va(slot) + offset;
> + pfn = is_vmalloc ? vmalloc_to_pfn(src) :
> + page_to_pfn(virt_to_page(src));
> + /*
> + * Treat the PTI LDT range as a *userspace* range.
> + * get_locked_pte() will allocate all needed pagetables
> + * and account for them in this mm.
> + */
> + ptep = get_locked_pte(mm, va, &ptl);
> + if (!ptep)
> + return -ENOMEM;
> + pte = pfn_pte(pfn, __pgprot(__PAGE_KERNEL & ~_PAGE_GLOBAL));
> + set_pte_at(mm, va, ptep, pte);
> + pte_unmap_unlock(ptep, ptl);
> + }
> +
> + if (mm->context.ldt) {
> + /*
> + * We already had an LDT. The top-level entry should already
> + * have been allocated and synchronized with the usermode
> + * tables.
> + */
> + WARN_ON(!had_top_level_entry);
> + if (static_cpu_has(X86_FEATURE_PTI))
> + WARN_ON(!kernel_to_user_pgdp(pgd)->pgd);
> + } else {
> + /*
> + * This is the first time we're mapping an LDT for this process.
> + * Sync the pgd to the usermode tables.
> + */
> + WARN_ON(had_top_level_entry);
> + if (static_cpu_has(X86_FEATURE_PTI)) {
> + WARN_ON(kernel_to_user_pgdp(pgd)->pgd);
> + set_pgd(kernel_to_user_pgdp(pgd), *pgd);
> + }
> + }
> +
> + va = (unsigned long)ldt_slot_va(slot);
> + flush_tlb_mm_range(mm, va, va + LDT_SLOT_STRIDE, 0);
> +
> + ldt->slot = slot;
> +#endif
> + return 0;
> +}
So if I understand this right, we need to remap+tlb-flush every time we
flip LDTs because this still uses the old vmalloc-a-new-ldt thing?
Should we base this on top of the ldt_mapping stuff tglx did? Such that
we keep the 2 arrays of pages immutable and don't need no tlb flushes?
