[RFC PATCH 04/12] arm64: mm: remap PGD pages r/o in the linear region after allocation
Ard Biesheuvel
ardb at kernel.org
Wed Jan 26 09:30:03 PST 2022
As the first step in restricting write access to all page tables via the
linear mapping, remap the page at the root PGD level of a user space
page table hierarchy read-only after allocation, so that it can only be
manipulated using the dedicated fixmap based API.
Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
arch/arm64/mm/mmu.c | 7 ++++--
arch/arm64/mm/pgd.c | 25 ++++++++++++++------
2 files changed, 23 insertions(+), 9 deletions(-)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index acfae9b41cc8..a52c3162beae 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -394,8 +394,11 @@ static phys_addr_t __pgd_pgtable_alloc(int shift)
void *ptr = (void *)__get_free_page(GFP_PGTABLE_KERNEL);
BUG_ON(!ptr);
- /* Ensure the zeroed page is visible to the page table walker */
- dsb(ishst);
+ if (page_tables_are_ro())
+ set_pgtable_ro(ptr);
+ else
+ /* Ensure the zeroed page is visible to the page table walker */
+ dsb(ishst);
return __pa(ptr);
}
diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
index 4a64089e5771..637d6eceeada 100644
--- a/arch/arm64/mm/pgd.c
+++ b/arch/arm64/mm/pgd.c
@@ -9,8 +9,10 @@
#include <linux/mm.h>
#include <linux/gfp.h>
#include <linux/highmem.h>
+#include <linux/set_memory.h>
#include <linux/slab.h>
+#include <asm/mmu_context.h>
#include <asm/pgalloc.h>
#include <asm/page.h>
#include <asm/tlbflush.h>
@@ -20,24 +22,33 @@ static struct kmem_cache *pgd_cache __ro_after_init;
pgd_t *pgd_alloc(struct mm_struct *mm)
{
gfp_t gfp = GFP_PGTABLE_USER;
+ pgd_t *pgd;
- if (PGD_SIZE == PAGE_SIZE)
- return (pgd_t *)__get_free_page(gfp);
- else
+ if (PGD_SIZE < PAGE_SIZE && !page_tables_are_ro())
return kmem_cache_alloc(pgd_cache, gfp);
+
+ pgd = (pgd_t *)__get_free_page(gfp);
+ if (!pgd)
+ return NULL;
+ if (page_tables_are_ro())
+ set_pgtable_ro(pgd);
+ return pgd;
}
void pgd_free(struct mm_struct *mm, pgd_t *pgd)
{
- if (PGD_SIZE == PAGE_SIZE)
- free_page((unsigned long)pgd);
- else
+ if (PGD_SIZE < PAGE_SIZE && !page_tables_are_ro()) {
kmem_cache_free(pgd_cache, pgd);
+ } else {
+ if (page_tables_are_ro())
+ set_pgtable_rw(pgd);
+ free_page((unsigned long)pgd);
+ }
}
void __init pgtable_cache_init(void)
{
- if (PGD_SIZE == PAGE_SIZE)
+ if (PGD_SIZE == PAGE_SIZE || page_tables_are_ro())
return;
#ifdef CONFIG_ARM64_PA_BITS_52
--
2.30.2
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