[kernel-hardening] [PATCH v6 11/11] arm64: mm: set the contiguous bit for kernel mappings where appropriate

Ard Biesheuvel
2017-03-09T20:52:09Z

This is the third attempt at enabling the use of contiguous hints for
kernel mappings. The most recent attempt 0bfc445dec9d was reverted after
it turned out that updating permission attributes on live contiguous ranges
may result in TLB conflicts. So this time, the contiguous hint is not set
for .rodata or for the linear alias of .text/.rodata, both of which are
mapped read-write initially, and remapped read-only at a later stage.
(Note that the latter region could also be unmapped and remapped again
with updated permission attributes, given that the region, while live, is
only mapped for the convenience of the hibernation code, but that also
means the TLB footprint is negligible anyway, so why bother)

This enables the following contiguous range sizes for the virtual mapping
of the kernel image, and for the linear mapping:

granule size | cont PTE | cont PMD |
-------------+------------+------------+
4 KB | 64 KB | 32 MB |
16 KB | 2 MB | 1 GB* |
64 KB | 2 MB | 16 GB* |

* Only when built for 3 or more levels of translation. This is due to the
fact that a 2 level configuration only consists of PGDs and PTEs, and the
added complexity of dealing with folded PMDs is not justified considering
that 16 GB contiguous ranges are likely to be ignored by the hardware (and
16k/2 levels is a niche configuration)

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/arm64/include/asm/pgtable.h | 10 ++
arch/arm64/mm/mmu.c | 140 ++++++++++++++------
2 files changed, 107 insertions(+), 43 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0eef6064bf3b..c213fdbd056c 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -74,6 +74,16 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
#define pte_user_exec(pte) (!(pte_val(pte) & PTE_UXN))
#define pte_cont(pte) (!!(pte_val(pte) & PTE_CONT))

+#define pte_cont_addr_end(addr, end) \
+({ unsigned long __boundary = ((addr) + CONT_PTE_SIZE) & CONT_PTE_MASK; \
+ (__boundary - 1 < (end) - 1) ? __boundary : (end); \
+})
+
+#define pmd_cont_addr_end(addr, end) \
+({ unsigned long __boundary = ((addr) + CONT_PMD_SIZE) & CONT_PMD_MASK; \
+ (__boundary - 1 < (end) - 1) ? __boundary : (end); \
+})
+
#ifdef CONFIG_ARM64_HW_AFDBM
#define pte_hw_dirty(pte) (pte_write(pte) && !(pte_val(pte) & PTE_RDONLY))
#else
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 85ab82f5a0bc..91502e36e6d9 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -44,6 +44,7 @@
#include <asm/ptdump.h>

#define NO_BLOCK_MAPPINGS BIT(0)
+#define NO_CONT_MAPPINGS BIT(1)

u64 idmap_t0sz = TCR_T0SZ(VA_BITS);

@@ -116,22 +117,11 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
return ((old ^ new) & ~mask) == 0;
}

-static void alloc_init_pte(pmd_t *pmd, unsigned long addr,
- unsigned long end, phys_addr_t phys,
- pgprot_t prot,
- phys_addr_t (*pgtable_alloc)(void))
+static void init_pte(pmd_t *pmd, unsigned long addr, unsigned long end,
+ phys_addr_t phys, pgprot_t prot)
{
pte_t *pte;

- BUG_ON(pmd_sect(*pmd));
- if (pmd_none(*pmd)) {
- phys_addr_t pte_phys;
- BUG_ON(!pgtable_alloc);
- pte_phys = pgtable_alloc();
- __pmd_populate(pmd, pte_phys, PMD_TYPE_TABLE);
- }
- BUG_ON(pmd_bad(*pmd));
-
pte = pte_set_fixmap_offset(pmd, addr);
do {
pte_t old_pte = *pte;
@@ -150,25 +140,45 @@ static void alloc_init_pte(pmd_t *pmd, unsigned long addr,
pte_clear_fixmap();
}

-static void alloc_init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
- phys_addr_t phys, pgprot_t prot,
- phys_addr_t (*pgtable_alloc)(void),
- int flags)
+static void alloc_init_cont_pte(pmd_t *pmd, unsigned long addr,
+ unsigned long end, phys_addr_t phys,
+ pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(void),
+ int flags)
{
- pmd_t *pmd;
unsigned long next;

- /*
- * Check for initial section mappings in the pgd/pud and remove them.
- */
- BUG_ON(pud_sect(*pud));
- if (pud_none(*pud)) {
- phys_addr_t pmd_phys;
+ BUG_ON(pmd_sect(*pmd));
+ if (pmd_none(*pmd)) {
+ phys_addr_t pte_phys;
BUG_ON(!pgtable_alloc);
- pmd_phys = pgtable_alloc();
- __pud_populate(pud, pmd_phys, PUD_TYPE_TABLE);
+ pte_phys = pgtable_alloc();
+ __pmd_populate(pmd, pte_phys, PMD_TYPE_TABLE);
}
- BUG_ON(pud_bad(*pud));
+ BUG_ON(pmd_bad(*pmd));
+
+ do {
+ pgprot_t __prot = prot;
+
+ next = pte_cont_addr_end(addr, end);
+
+ /* use a contiguous mapping if the range is suitably aligned */
+ if ((((addr | next | phys) & ~CONT_PTE_MASK) == 0) &&
+ (flags & NO_CONT_MAPPINGS) == 0)
+ __prot = __pgprot(pgprot_val(prot) | PTE_CONT);
+
+ init_pte(pmd, addr, next, phys, __prot);
+
+ phys += next - addr;
+ } while (addr = next, addr != end);
+}
+
+static void init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
+ phys_addr_t phys, pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(void), int flags)
+{
+ unsigned long next;
+ pmd_t *pmd;

pmd = pmd_set_fixmap_offset(pud, addr);
do {
@@ -188,8 +198,8 @@ static void alloc_init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
BUG_ON(!pgattr_change_is_safe(pmd_val(old_pmd),
pmd_val(*pmd)));
} else {
- alloc_init_pte(pmd, addr, next, phys,
- prot, pgtable_alloc);
+ alloc_init_cont_pte(pmd, addr, next, phys, prot,
+ pgtable_alloc, flags);

BUG_ON(pmd_val(old_pmd) != 0 &&
pmd_val(old_pmd) != pmd_val(*pmd));
@@ -200,6 +210,41 @@ static void alloc_init_pmd(pud_t *pud, unsigned long addr, unsigned long end,
pmd_clear_fixmap();
}

+static void alloc_init_cont_pmd(pud_t *pud, unsigned long addr,
+ unsigned long end, phys_addr_t phys,
+ pgprot_t prot,
+ phys_addr_t (*pgtable_alloc)(void), int flags)
+{
+ unsigned long next;
+
+ /*
+ * Check for initial section mappings in the pgd/pud.
+ */
+ BUG_ON(pud_sect(*pud));
+ if (pud_none(*pud)) {
+ phys_addr_t pmd_phys;
+ BUG_ON(!pgtable_alloc);
+ pmd_phys = pgtable_alloc();
+ __pud_populate(pud, pmd_phys, PUD_TYPE_TABLE);
+ }
+ BUG_ON(pud_bad(*pud));
+
+ do {
+ pgprot_t __prot = prot;
+
+ next = pmd_cont_addr_end(addr, end);
+
+ /* use a contiguous mapping if the range is suitably aligned */
+ if ((((addr | next | phys) & ~CONT_PMD_MASK) == 0) &&
+ (flags & NO_CONT_MAPPINGS) == 0)
+ __prot = __pgprot(pgprot_val(prot) | PTE_CONT);
+
+ init_pmd(pud, addr, next, phys, __prot, pgtable_alloc, flags);
+
+ phys += next - addr;
+ } while (addr = next, addr != end);
+}
+
static inline bool use_1G_block(unsigned long addr, unsigned long next,
unsigned long phys)
{
@@ -248,8 +293,8 @@ static void alloc_init_pud(pgd_t *pgd, unsigned long addr, unsigned long end,
BUG_ON(!pgattr_change_is_safe(pud_val(old_pud),
pud_val(*pud)));
} else {
- alloc_init_pmd(pud, addr, next, phys, prot,
- pgtable_alloc, flags);
+ alloc_init_cont_pmd(pud, addr, next, phys, prot,
+ pgtable_alloc, flags);

BUG_ON(pud_val(old_pud) != 0 &&
pud_val(old_pud) != pud_val(*pud));
@@ -313,7 +358,8 @@ static void __init create_mapping_noalloc(phys_addr_t phys, unsigned long virt,
&phys, virt);
return;
}
- __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, 0);
+ __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL,
+ NO_CONT_MAPPINGS);
}

void __init create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
@@ -325,7 +371,7 @@ void __init create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
BUG_ON(mm == &init_mm);

if (page_mappings_only)
- flags = NO_BLOCK_MAPPINGS;
+ flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

__create_pgd_mapping(mm->pgd, phys, virt, size, prot,
pgd_pgtable_alloc, flags);
@@ -340,7 +386,8 @@ static void update_mapping_prot(phys_addr_t phys, unsigned long virt,
return;
}

- __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL, 0);
+ __create_pgd_mapping(init_mm.pgd, phys, virt, size, prot, NULL,
+ NO_CONT_MAPPINGS);

/* flush the TLBs after updating live kernel mappings */
flush_tlb_kernel_range(virt, virt + size);
@@ -353,7 +400,7 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
int flags = 0;

if (debug_pagealloc_enabled())
- flags = NO_BLOCK_MAPPINGS;
+ flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;

/*
* Take care not to create a writable alias for the
@@ -390,10 +437,12 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
* alternative patching has completed). This makes the contents
* of the region accessible to subsystems such as hibernate,
* but protects it from inadvertent modification or execution.
+ * Note that contiguous mappings cannot be remapped in this way,
+ * so we should avoid them here.
*/
__create_pgd_mapping(pgd, kernel_start, __phys_to_virt(kernel_start),
kernel_end - kernel_start, PAGE_KERNEL,
- early_pgtable_alloc, 0);
+ early_pgtable_alloc, NO_CONT_MAPPINGS);
}

void __init mark_linear_text_alias_ro(void)
@@ -440,7 +489,8 @@ void mark_rodata_ro(void)
}

static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,
- pgprot_t prot, struct vm_struct *vma)
+ pgprot_t prot, struct vm_struct *vma,
+ int flags)
{
phys_addr_t pa_start = __pa_symbol(va_start);
unsigned long size = va_end - va_start;
@@ -449,7 +499,7 @@ static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,
BUG_ON(!PAGE_ALIGNED(size));

__create_pgd_mapping(pgd, pa_start, (unsigned long)va_start, size, prot,
- early_pgtable_alloc, 0);
+ early_pgtable_alloc, flags);

vma->addr = va_start;
vma->phys_addr = pa_start;
@@ -481,14 +531,18 @@ static void __init map_kernel(pgd_t *pgd)
*/
pgprot_t text_prot = rodata_enabled ? PAGE_KERNEL_ROX : PAGE_KERNEL_EXEC;

- map_kernel_segment(pgd, _text, _etext, text_prot, &vmlinux_text);
+ /*
+ * Only rodata will be remapped with different permissions later on,
+ * all other segments are allowed to use contiguous mappings.
+ */
+ map_kernel_segment(pgd, _text, _etext, text_prot, &vmlinux_text, 0);
map_kernel_segment(pgd, __start_rodata, __inittext_begin, PAGE_KERNEL,
- &vmlinux_rodata);
+ &vmlinux_rodata, NO_CONT_MAPPINGS);
map_kernel_segment(pgd, __inittext_begin, __inittext_end, text_prot,
- &vmlinux_inittext);
+ &vmlinux_inittext, 0);
map_kernel_segment(pgd, __initdata_begin, __initdata_end, PAGE_KERNEL,
- &vmlinux_initdata);
- map_kernel_segment(pgd, _data, _end, PAGE_KERNEL, &vmlinux_data);
+ &vmlinux_initdata, 0);
+ map_kernel_segment(pgd, _data, _end, PAGE_KERNEL, &vmlinux_data, 0);

if (!pgd_val(*pgd_offset_raw(pgd, FIXADDR_START))) {
/*
--
2.7.4