[PATCH v3 6/8] arm64: Introduce direct_map_is_ptemap()

[Vincent Donnefort <vdonnefort@google.com>]

Regions marked with MEMBLOCK_PTEMAP are mapped at page granularity so
their direct mapping can be modified without splitting blocks at
runtime. Introduce direct_map_is_ptemap(), which walks the kernel page
tables during init to verify that every page in a range is backed by a
present non-contiguous PTE.

Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
---
 arch/arm64/include/asm/set_memory.h |  4 ++
 arch/arm64/mm/pageattr.c            | 65 +++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/arch/arm64/include/asm/set_memory.h b/arch/arm64/include/asm/set_memory.h
index 0091ba12200e..b6ec938703d6 100644
--- a/arch/arm64/include/asm/set_memory.h
+++ b/arch/arm64/include/asm/set_memory.h
@@ -3,12 +3,16 @@
 #ifndef _ASM_ARM64_SET_MEMORY_H
 #define _ASM_ARM64_SET_MEMORY_H
 
+#include <linux/init.h>
+
 #include <asm/mem_encrypt.h>
 #include <asm-generic/set_memory.h>
 
 bool can_set_direct_map(void);
 #define can_set_direct_map can_set_direct_map
 
+bool __init direct_map_is_ptemap(struct page *page, unsigned int numpages);
+
 int set_memory_valid(unsigned long addr, int numpages, int enable);
 
 int set_direct_map_invalid_noflush(struct page *page, unsigned int numpages);
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index de7bf95003ea..c7662fd2fcb2 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -102,6 +102,71 @@ bool can_set_direct_map(void)
 		arm64_kfence_can_set_direct_map() || is_realm_world();
 }
 
+static int __init direct_map_is_ptemap_pud_entry(pud_t *pud, unsigned long addr,
+						 unsigned long next,
+						 struct mm_walk *walk)
+{
+	if (pud_leaf(pudp_get(pud)))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int __init direct_map_is_ptemap_pmd_entry(pmd_t *pmd, unsigned long addr,
+						 unsigned long next,
+						 struct mm_walk *walk)
+{
+	if (pmd_leaf(pmdp_get(pmd)))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int __init direct_map_is_ptemap_pte_entry(pte_t *pte, unsigned long addr,
+						 unsigned long next,
+						 struct mm_walk *walk)
+{
+	pte_t val = __ptep_get(pte);
+
+	if (!pte_present(val) || pte_cont(val))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int __init direct_map_is_ptemap_pte_hole(unsigned long addr,
+						unsigned long next, int depth,
+						struct mm_walk *walk)
+{
+	return -EINVAL;
+}
+
+static const struct mm_walk_ops direct_map_is_ptemap_ops __initconst = {
+	.pud_entry	= direct_map_is_ptemap_pud_entry,
+	.pmd_entry	= direct_map_is_ptemap_pmd_entry,
+	.pte_entry	= direct_map_is_ptemap_pte_entry,
+	.pte_hole	= direct_map_is_ptemap_pte_hole,
+};
+
+bool __init direct_map_is_ptemap(struct page *page, unsigned int numpages)
+{
+	unsigned long addr, end;
+
+	if (!pfn_valid(page_to_pfn(page)) || !numpages)
+		return false;
+
+	addr = (unsigned long)page_address(page);
+	end = addr + PAGE_SIZE * numpages;
+
+	/*
+	 * Called during boot before runtime linear-map splitting, so it is
+	 * safe to walk the page-table lockless.
+	 */
+	return !walk_kernel_page_table_range_lockless(addr, end,
+						      &direct_map_is_ptemap_ops,
+						      NULL, NULL);
+}
+
 static int update_range_prot(unsigned long start, unsigned long size,
 			     pgprot_t set_mask, pgprot_t clear_mask)
 {
-- 
2.56.0.542.g3ef89b7020-goog