Re: [RFC v1 3/4] x86, boot: Implement ASLR for kernel memory sections (x86_64)

[Thomas Garnier]

On Tue, Apr 19, 2016 at 7:27 AM, Joerg Roedel <jroedel@xxxxxxx> wrote:
> Hi Thomas,
>
> On Fri, Apr 15, 2016 at 03:03:12PM -0700, Thomas Garnier wrote:
>> +/*
>> + * Create PGD aligned trampoline table to allow real mode initialization
>> + * of additional CPUs. Consume only 1 additonal low memory page.
>> + */
>> +void __meminit kaslr_trampoline_init(unsigned long page_size_mask)
>> +{
>> +     unsigned long addr, next, end;
>> +     pgd_t *pgd;
>> +     pud_t *pud_page, *tr_pud_page;
>> +     int i;
>> +
>> +     if (!kaslr_enabled()) {
>> +             trampoline_pgd_entry = init_level4_pgt[pgd_index(PAGE_OFFSET)];
>> +             return;
>> +     }
>> +
>> +     tr_pud_page = alloc_low_page();
>> +     set_pgd(&trampoline_pgd_entry, __pgd(_PAGE_TABLE | __pa(tr_pud_page)));
>> +
>> +     addr = 0;
>> +     end = ISA_END_ADDRESS;
>> +     pgd = pgd_offset_k((unsigned long)__va(addr));
>> +     pud_page = (pud_t *) pgd_page_vaddr(*pgd);
>> +
>> +     for (i = pud_index(addr); i < PTRS_PER_PUD; i++, addr = next) {
>> +             pud_t *pud, *tr_pud;
>> +             pmd_t *pmd;
>> +
>> +             tr_pud = tr_pud_page + pud_index(addr);
>> +             pud = pud_page + pud_index((unsigned long)__va(addr));
>> +             next = (addr & PUD_MASK) + PUD_SIZE;
>> +
>> +             if (addr >= end || !pud_val(*pud)) {
>> +                     if (!after_bootmem &&
>> +                         !e820_any_mapped(addr & PUD_MASK, next, E820_RAM) &&
>> +                         !e820_any_mapped(addr & PUD_MASK, next,
>> +                                         E820_RESERVED_KERN))
>> +                             set_pud(tr_pud, __pud(0));
>> +                     continue;
>> +             }
>> +
>> +             if (page_size_mask & (1<<PG_LEVEL_1G)) {
>> +                     set_pte((pte_t *)tr_pud,
>> +                             pfn_pte((__pa(addr) & PUD_MASK) >> PAGE_SHIFT,
>
> Hmm, why do you treat addr as virtual here, before it was a physical
> address, no?
>

Yes, you are right. Good catch.

>> +                                     PAGE_KERNEL_LARGE));
>> +                     continue;
>> +             }
>
> Why do you need to check these two cases above, can't you just copy the
> pud-entries like done below? The direct mapping should already take care
> of unmapped regions and 1gb pages.
>

Yes, that was my original approach though I was not sure it was the
best. It makes sense though so I will update that for the next
iteration.

>> +             pmd = pmd_offset(pud, 0);
>> +             set_pud(tr_pud, __pud(_PAGE_TABLE | __pa(pmd)));
>> +     }
>> +}
>
>
>         Joerg
>