[RFC PATCH v2 3/4] arm64: mm: Create r/o page table region that permits updates

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

Generalize the handling of swapper_pg_dir, and apply the fixmap fallback
for page table updates to all page tables in the .rodata..pgtbl section.

Currently, this holds only swapper_pg_dir, but this will be expanded in
subsequent patches.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/include/asm/pgtable.h |  6 -----
 arch/arm64/kernel/vmlinux.lds.S  |  6 +++++
 arch/arm64/mm/mmu.c              | 26 +++++++++++++++++---
 3 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 799cd9d52c28..de0a07dae83a 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -817,12 +817,6 @@ extern pgd_t reserved_pg_dir[];
 
 void try_set_readonly_ptval(ptval_t *ptvalp, ptval_t ptval);
 
-static inline bool in_swapper_pgdir(void *addr)
-{
-	return ((unsigned long)addr & PAGE_MASK) ==
-	        ((unsigned long)swapper_pg_dir & PAGE_MASK);
-}
-
 static inline void set_pmd(pmd_t *pmdp, pmd_t pmd)
 {
 	__put_kernel_nofault(pmdp, &pmd, pmd_t, fault);
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index af1d72020976..2cda0b7c41d4 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -243,9 +243,15 @@ SECTIONS
 	reserved_pg_dir = .;
 	. += PAGE_SIZE;
 
+	__rodata_pgtbl_start = .;
 	swapper_pg_dir = .;
 	. += PAGE_SIZE;
 
+	.pgtbl.ro_after_init : ALIGN(PAGE_SIZE) {
+		*(.pgtbl.ro_after_init)
+	}
+	__rodata_pgtbl_end = .;
+
 	. = ALIGN(SEGMENT_ALIGN);
 	__init_begin = .;
 	__inittext_begin = .;
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index b97e4bf99ca5..dad482d66f17 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -65,17 +65,27 @@ long __section(".mmuoff.data.write") __early_cpu_boot_status;
 
 static DEFINE_MUTEX(fixmap_lock);
 
+static struct range kimg_ropgtbl_range __ro_after_init;
+
 void noinstr try_set_readonly_ptval(ptval_t *ptvalp, ptval_t ptval)
 {
 	static DEFINE_SPINLOCK(lock);
+	bool is_lm = __is_lm_address(ptvalp);
+	u64 pa = is_lm ? __pa(ptvalp) : __pa_symbol(ptvalp);
 
-	BUG_ON(!in_swapper_pgdir(ptvalp));
+	if (!range_contains(&kimg_ropgtbl_range,
+			    &DEFINE_RANGE(pa, pa + sizeof(ptval_t))))
+		BUG();
 
 	/*
-	 * Don't bother with the fixmap if swapper_pg_dir is still mapped
+	 * Don't bother with the fixmap if .rodata is still mapped
 	 * writable in the kernel mapping.
 	 */
 	if (rodata_is_rw) {
+		/* no fault should have occurred for a kimg address */
+		BUG_ON(!is_lm);
+
+		ptvalp = (ptval_t *)__phys_to_kimg(pa);
 		WRITE_ONCE(*ptvalp, ptval);
 		dsb(ishst);
 		isb();
@@ -83,7 +93,7 @@ void noinstr try_set_readonly_ptval(ptval_t *ptvalp, ptval_t ptval)
 	}
 
 	guard(spinlock)(&lock);
-	ptvalp = (ptval_t *)set_fixmap_offset(FIX_PTVAL, __pa_symbol(ptvalp));
+	ptvalp = (ptval_t *)set_fixmap_offset(FIX_PTVAL, pa);
 	WRITE_ONCE(*ptvalp, ptval);
 	/*
 	 * We need dsb(ishst) here to ensure the page-table-walker sees
@@ -1177,6 +1187,14 @@ static inline void arm64_kfence_map_pool(void) { }
 
 #endif /* CONFIG_KFENCE */
 
+static void __init record_ropgtbl_phys_range(void)
+{
+	extern const char __rodata_pgtbl_start[], __rodata_pgtbl_end[];
+
+	kimg_ropgtbl_range = DEFINE_RANGE(__pa_symbol(__rodata_pgtbl_start),
+					  __pa_symbol(__rodata_pgtbl_end));
+}
+
 static void __init map_mem(void)
 {
 	static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
@@ -1189,6 +1207,8 @@ static void __init map_mem(void)
 	int flags = NO_EXEC_MAPPINGS;
 	u64 i;
 
+	record_ropgtbl_phys_range();
+
 	/*
 	 * Setting hierarchical PXNTable attributes on table entries covering
 	 * the linear region is only possible if it is guaranteed that no table
-- 
2.55.0.887.g758fc8c411-goog