arm64: consistently use reserved_pg_dir

[ Upstream commit 833be850f1cabd0e3b5337c0fcab20a6e936dd48 ]

Depending on configuration options and specific code paths, we either
use the empty_zero_page or the configuration-dependent reserved_ttbr0
as a reserved value for TTBR{0,1}_EL1.

To simplify this code, let's always allocate and use the same
reserved_pg_dir, replacing reserved_ttbr0. Note that this is allocated
(and hence pre-zeroed), and is also marked as read-only in the kernel
Image mapping.

Keeping this separate from the empty_zero_page potentially helps with
robustness as the empty_zero_page is used in a number of cases where a
failure to map it read-only could allow it to become corrupted.

The (presently unused) swapper_pg_end symbol is also removed, and
comments are added wherever we rely on the offsets between the
pre-allocated pg_dirs to keep these cases easily identifiable.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20201103102229.8542-1-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/arm64/include/asm/asm-uaccess.h

@@ -15,10 +15,10 @@
 	.macro	__uaccess_ttbr0_disable, tmp1
 	mrs	\tmp1, ttbr1_el1			// swapper_pg_dir
 	bic	\tmp1, \tmp1, #TTBR_ASID_MASK
-	sub	\tmp1, \tmp1, #RESERVED_TTBR0_SIZE	// reserved_ttbr0 just before swapper_pg_dir
+	sub	\tmp1, \tmp1, #PAGE_SIZE		// reserved_pg_dir just before swapper_pg_dir
 	msr	ttbr0_el1, \tmp1			// set reserved TTBR0_EL1
 	isb
-	add	\tmp1, \tmp1, #RESERVED_TTBR0_SIZE
+	add	\tmp1, \tmp1, #PAGE_SIZE
 	msr	ttbr1_el1, \tmp1		// set reserved ASID
 	isb
 	.endm

arch/arm64/include/asm/kernel-pgtable.h

@@ -89,12 +89,6 @@
 #define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end))
 #define IDMAP_DIR_SIZE		(IDMAP_PGTABLE_LEVELS * PAGE_SIZE)
 
-#ifdef CONFIG_ARM64_SW_TTBR0_PAN
-#define RESERVED_TTBR0_SIZE	(PAGE_SIZE)
-#else
-#define RESERVED_TTBR0_SIZE	(0)
-#endif
-
 /* Initial memory map size */
 #if ARM64_SWAPPER_USES_SECTION_MAPS
 #define SWAPPER_BLOCK_SHIFT	SECTION_SHIFT

arch/arm64/include/asm/mmu_context.h

@@ -36,11 +36,11 @@ static inline void contextidr_thread_switch(struct task_struct *next)
 }
 
 /*
- * Set TTBR0 to empty_zero_page. No translations will be possible via TTBR0.
+ * Set TTBR0 to reserved_pg_dir. No translations will be possible via TTBR0.
  */
 static inline void cpu_set_reserved_ttbr0(void)
 {
-	unsigned long ttbr = phys_to_ttbr(__pa_symbol(empty_zero_page));
+	unsigned long ttbr = phys_to_ttbr(__pa_symbol(reserved_pg_dir));
 
 	write_sysreg(ttbr, ttbr0_el1);
 	isb();
@@ -192,7 +192,7 @@ static inline void update_saved_ttbr0(struct task_struct *tsk,
 		return;
 
 	if (mm == &init_mm)
-		ttbr = __pa_symbol(empty_zero_page);
+		ttbr = __pa_symbol(reserved_pg_dir);
 	else
 		ttbr = virt_to_phys(mm->pgd) | ASID(mm) << 48;
 

arch/arm64/include/asm/pgtable.h

@@ -530,6 +530,7 @@ extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
 extern pgd_t idmap_pg_dir[PTRS_PER_PGD];
 extern pgd_t idmap_pg_end[];
 extern pgd_t tramp_pg_dir[PTRS_PER_PGD];
+extern pgd_t reserved_pg_dir[PTRS_PER_PGD];
 
 extern void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd);
 

arch/arm64/include/asm/uaccess.h

@@ -113,8 +113,8 @@ static inline void __uaccess_ttbr0_disable(void)
 	local_irq_save(flags);
 	ttbr = read_sysreg(ttbr1_el1);
 	ttbr &= ~TTBR_ASID_MASK;
-	/* reserved_ttbr0 placed before swapper_pg_dir */
+	/* reserved_pg_dir placed before swapper_pg_dir */
-	write_sysreg(ttbr - RESERVED_TTBR0_SIZE, ttbr0_el1);
+	write_sysreg(ttbr - PAGE_SIZE, ttbr0_el1);
 	isb();
 	/* Set reserved ASID */
 	write_sysreg(ttbr, ttbr1_el1);

arch/arm64/kernel/entry.S

@@ -770,9 +770,10 @@ SYM_CODE_END(ret_to_user)
  */
 	.pushsection ".entry.tramp.text", "ax"
 
+	// Move from tramp_pg_dir to swapper_pg_dir
 	.macro tramp_map_kernel, tmp
 	mrs	\tmp, ttbr1_el1
-	add	\tmp, \tmp, #(PAGE_SIZE + RESERVED_TTBR0_SIZE)
+	add	\tmp, \tmp, #(2 * PAGE_SIZE)
 	bic	\tmp, \tmp, #USER_ASID_FLAG
 	msr	ttbr1_el1, \tmp
 #ifdef CONFIG_QCOM_FALKOR_ERRATUM_1003
@@ -789,9 +790,10 @@ alternative_else_nop_endif
 #endif /* CONFIG_QCOM_FALKOR_ERRATUM_1003 */
 	.endm
 
+	// Move from swapper_pg_dir to tramp_pg_dir
 	.macro tramp_unmap_kernel, tmp
 	mrs	\tmp, ttbr1_el1
-	sub	\tmp, \tmp, #(PAGE_SIZE + RESERVED_TTBR0_SIZE)
+	sub	\tmp, \tmp, #(2 * PAGE_SIZE)
 	orr	\tmp, \tmp, #USER_ASID_FLAG
 	msr	ttbr1_el1, \tmp
 	/*

arch/arm64/kernel/setup.c

@@ -366,7 +366,7 @@ void __init __no_sanitize_address setup_arch(char **cmdline_p)
 	 * faults in case uaccess_enable() is inadvertently called by the init
 	 * thread.
 	 */
-	init_task.thread_info.ttbr0 = __pa_symbol(empty_zero_page);
+	init_task.thread_info.ttbr0 = __pa_symbol(reserved_pg_dir);
 #endif
 
 	if (boot_args[1] || boot_args[2] || boot_args[3]) {

arch/arm64/kernel/vmlinux.lds.S

@@ -164,13 +164,11 @@ SECTIONS
 	. += PAGE_SIZE;
 #endif
 
-#ifdef CONFIG_ARM64_SW_TTBR0_PAN
+	reserved_pg_dir = .;
-	reserved_ttbr0 = .;
+	. += PAGE_SIZE;
-	. += RESERVED_TTBR0_SIZE;
+
-#endif
 	swapper_pg_dir = .;
 	. += PAGE_SIZE;
-	swapper_pg_end = .;
 
 	. = ALIGN(SEGMENT_ALIGN);
 	__init_begin = .;

arch/arm64/mm/proc.S

@@ -168,7 +168,7 @@ SYM_FUNC_END(cpu_do_resume)
 	.pushsection ".idmap.text", "awx"
 
 .macro	__idmap_cpu_set_reserved_ttbr1, tmp1, tmp2
-	adrp	\tmp1, empty_zero_page
+	adrp	\tmp1, reserved_pg_dir
 	phys_to_ttbr \tmp2, \tmp1
 	offset_ttbr1 \tmp2, \tmp1
 	msr	ttbr1_el1, \tmp2