[PATCH v2 2/3] arm64: mm: use XN table mapping attributes for the linear region

[Ard Biesheuvel]

The way the arm64 kernel virtual address space is constructed guarantees
that swapper PGD entries are never shared between the linear region on
the one hand, and the vmalloc region on the other, which is where all
kernel text, module text and BPF text mappings reside.

This means that mappings in the linear region (which never require
executable permissions) never share any table entries at any level with
mappings that do require executable permissions, and so we can set the
table-level PXN attributes for all table entries that are created while
setting up mappings in the linear region. Since swapper's PGD level page
table is mapped r/o itself, this adds another layer of robustness to the
way the kernel manages its own page tables. While at it, set the UXN
attribute as well for all kernel mappings created at boot.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
Acked-by: Mark Rutland <mark.rutland at arm.com>
---
 arch/arm64/include/asm/pgtable-hwdef.h |  6 +++++
 arch/arm64/mm/mmu.c                    | 27 +++++++++++++++-----
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-hwdef.h b/arch/arm64/include/asm/pgtable-hwdef.h
index e64e77a345b2..b82575a33f8b 100644
--- a/arch/arm64/include/asm/pgtable-hwdef.h
+++ b/arch/arm64/include/asm/pgtable-hwdef.h
@@ -101,6 +101,8 @@
 #define P4D_TYPE_MASK		(_AT(p4dval_t, 3) << 0)
 #define P4D_TYPE_SECT		(_AT(p4dval_t, 1) << 0)
 #define P4D_SECT_RDONLY		(_AT(p4dval_t, 1) << 7)		/* AP[2] */
+#define P4D_TABLE_PXN		(_AT(p4dval_t, 1) << 59)
+#define P4D_TABLE_UXN		(_AT(p4dval_t, 1) << 60)
 
 /*
  * Level 1 descriptor (PUD).
@@ -110,6 +112,8 @@
 #define PUD_TYPE_MASK		(_AT(pudval_t, 3) << 0)
 #define PUD_TYPE_SECT		(_AT(pudval_t, 1) << 0)
 #define PUD_SECT_RDONLY		(_AT(pudval_t, 1) << 7)		/* AP[2] */
+#define PUD_TABLE_PXN		(_AT(pudval_t, 1) << 59)
+#define PUD_TABLE_UXN		(_AT(pudval_t, 1) << 60)
 
 /*
  * Level 2 descriptor (PMD).
@@ -131,6 +135,8 @@
 #define PMD_SECT_CONT		(_AT(pmdval_t, 1) << 52)
 #define PMD_SECT_PXN		(_AT(pmdval_t, 1) << 53)
 #define PMD_SECT_UXN		(_AT(pmdval_t, 1) << 54)
+#define PMD_TABLE_PXN		(_AT(pmdval_t, 1) << 59)
+#define PMD_TABLE_UXN		(_AT(pmdval_t, 1) << 60)
 
 /*
  * AttrIndx[2:0] encoding (mapping attributes defined in the MAIR* registers).
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 029091474042..9de59fce0450 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -39,6 +39,7 @@
 
 #define NO_BLOCK_MAPPINGS	BIT(0)
 #define NO_CONT_MAPPINGS	BIT(1)
+#define NO_EXEC_MAPPINGS	BIT(2)
 
 u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
 u64 idmap_ptrs_per_pgd = PTRS_PER_PGD;
@@ -185,10 +186,14 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr,
 
 	BUG_ON(pmd_sect(pmd));
 	if (pmd_none(pmd)) {
+		pmdval_t pmdval = PMD_TYPE_TABLE | PMD_TABLE_UXN;
 		phys_addr_t pte_phys;
+
+		if (flags & NO_EXEC_MAPPINGS)
+			pmdval |= PMD_TABLE_PXN;
 		BUG_ON(!pgtable_alloc);
 		pte_phys = pgtable_alloc(PAGE_SHIFT);
-		__pmd_populate(pmdp, pte_phys, PMD_TYPE_TABLE);
+		__pmd_populate(pmdp, pte_phys, pmdval);
 		pmd = READ_ONCE(*pmdp);
 	}
 	BUG_ON(pmd_bad(pmd));
@@ -259,10 +264,14 @@ static void alloc_init_cont_pmd(pud_t *pudp, unsigned long addr,
 	 */
 	BUG_ON(pud_sect(pud));
 	if (pud_none(pud)) {
+		pudval_t pudval = PUD_TYPE_TABLE | PUD_TABLE_UXN;
 		phys_addr_t pmd_phys;
+
+		if (flags & NO_EXEC_MAPPINGS)
+			pudval |= PUD_TABLE_PXN;
 		BUG_ON(!pgtable_alloc);
 		pmd_phys = pgtable_alloc(PMD_SHIFT);
-		__pud_populate(pudp, pmd_phys, PUD_TYPE_TABLE);
+		__pud_populate(pudp, pmd_phys, pudval);
 		pud = READ_ONCE(*pudp);
 	}
 	BUG_ON(pud_bad(pud));
@@ -306,10 +315,14 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
 	p4d_t p4d = READ_ONCE(*p4dp);
 
 	if (p4d_none(p4d)) {
+		p4dval_t p4dval = P4D_TYPE_TABLE | P4D_TABLE_UXN;
 		phys_addr_t pud_phys;
+
+		if (flags & NO_EXEC_MAPPINGS)
+			p4dval |= P4D_TABLE_PXN;
 		BUG_ON(!pgtable_alloc);
 		pud_phys = pgtable_alloc(PUD_SHIFT);
-		__p4d_populate(p4dp, pud_phys, P4D_TYPE_TABLE);
+		__p4d_populate(p4dp, pud_phys, p4dval);
 		p4d = READ_ONCE(*p4dp);
 	}
 	BUG_ON(p4d_bad(p4d));
@@ -489,11 +502,11 @@ static void __init map_mem(pgd_t *pgdp)
 	phys_addr_t kernel_start = __pa_symbol(_stext);
 	phys_addr_t kernel_end = __pa_symbol(__init_begin);
 	phys_addr_t start, end;
-	int flags = 0;
+	int flags = NO_EXEC_MAPPINGS;
 	u64 i;
 
 	if (rodata_full || crash_mem_map || debug_pagealloc_enabled())
-		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
+		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
 	/*
 	 * Take care not to create a writable alias for the
@@ -1462,7 +1475,7 @@ struct range arch_get_mappable_range(void)
 int arch_add_memory(int nid, u64 start, u64 size,
 		    struct mhp_params *params)
 {
-	int ret, flags = 0;
+	int ret, flags = NO_EXEC_MAPPINGS;
 
 	VM_BUG_ON(!mhp_range_allowed(start, size, true));
 
@@ -1472,7 +1485,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
 	 */
 	if (rodata_full || debug_pagealloc_enabled() ||
 	    IS_ENABLED(CONFIG_KFENCE))
-		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
+		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
 	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
 			     size, params->pgprot, __pgd_pgtable_alloc,
-- 
2.30.1