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SubjectRe: [PATCH RFC 2/8] riscv/kaslr: introduce functions to clear page table
On Tue, Apr 7, 2020 at 1:09 PM Alex Ghiti <alex@ghiti.fr> wrote:
>
> On 3/24/20 3:30 AM, Zong Li wrote:
> > In KASLR, we need to re-create page table after getting a random
> > destination. Introduce clear function to clear old content. Also, the
> > page table entries allow writing value when it's empty, so we have to
> > clear the early page table.
> >
> > This patch is a preparation to support KASLR.
> >
> > Signed-off-by: Zong Li <zong.li@sifive.com>
> > ---
> > arch/riscv/mm/init.c | 54 ++++++++++++++++++++++++++++++++++++++++++++
> > 1 file changed, 54 insertions(+)
> >
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index ace5d74fd939..51e263c04fa2 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -315,6 +315,7 @@ static void __init create_pmd_mapping(pmd_t *pmdp,
> > #define get_pgd_next_virt(__pa) get_pmd_virt(__pa)
> > #define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot) \
> > create_pmd_mapping(__nextp, __va, __pa, __sz, __prot)
> > +#define clear_pgd_next_mapping(__nextp) clear_pmd(__nextp)
> > #define fixmap_pgd_next fixmap_pmd
> > #else
> > #define pgd_next_t pte_t
> > @@ -322,6 +323,7 @@ static void __init create_pmd_mapping(pmd_t *pmdp,
> > #define get_pgd_next_virt(__pa) get_pte_virt(__pa)
> > #define create_pgd_next_mapping(__nextp, __va, __pa, __sz, __prot) \
> > create_pte_mapping(__nextp, __va, __pa, __sz, __prot)
> > +#define clear_pgd_next_mapping(__nextp) clear_pte(__nextp)
> > #define fixmap_pgd_next fixmap_pte
> > #endif
> >
> > @@ -361,6 +363,58 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> > return PMD_SIZE;
> > }
> >
> > +#ifdef CONFIG_RANDOMIZE_BASE
> > +static void __init clear_pte(pte_t *ptep)
> > +{
> > + unsigned int i;
> > +
> > + for (i = 0; i < PTRS_PER_PTE; i++)
> > + if (!pte_none(ptep[i]))
> > + ptep[i] = __pte(0);
> > +}
> > +
> > +static void __init clear_pmd(pmd_t *pmdp)
> > +{
> > + unsigned int i;
> > + pte_t *ptep;
> > + phys_addr_t pte_phys;
> > + uintptr_t kaslr_offset = get_kaslr_offset();
> > +
> > + for (i = 0; i < PTRS_PER_PMD; i++)
> > + if (!pmd_none(pmdp[i])) {
> > + if (pmd_leaf(pmdp[i])) {
> > + pmd_clear(&pmdp[i]);
> > + } else {
> > + pte_phys = PFN_PHYS(_pmd_pfn(pmdp[i]));
> > + ptep = get_pte_virt(pte_phys + kaslr_offset);
> > + clear_pte(ptep);
> > + pmd_clear(&pmdp[i]);
> > + }
> > + }
> > +}
> > +
> > +static void __init clear_pgd(pgd_t *pgdp)
> > +{
> > + unsigned int i;
> > + pgd_next_t *nextp;
> > + phys_addr_t next_phys;
> > + uintptr_t kaslr_offset = get_kaslr_offset();
> > +
> > + for (i = 0; i < PTRS_PER_PGD; i++)
> > + if (pgd_val(pgdp[i]) != 0) {
> > + if (pgd_leaf(pgd_val(pgdp[i]))) {
> > + set_pgd(&pgdp[i], __pgd(0));
> > + } else {
> > + next_phys = PFN_PHYS(_pgd_pfn(pgdp[i]));
> > + nextp = get_pgd_next_virt(next_phys +
> > + kaslr_offset);
> > + clear_pgd_next_mapping(nextp);
> > + set_pgd(&pgdp[i], __pgd(0));
> > + }
> > + }
> > +}
> > +#endif
> > +
> > /*
> > * setup_vm() is called from head.S with MMU-off.
> > *
> >
>
> If this is only for clearing early page tables, a memset is way easier
> as there is only one page per level to clear.
>

Yes, it's a better way. Thanks.

> Alex

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