[PATCH RFC 2/8] riscv/kaslr: introduce functions to clear page table

[Zong Li]

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@xxxxxxxxxx>
---
 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.
  *
-- 
2.25.1