[PATCH] x86_64: Unmap NULL during early bootup

We should zap the low mappings, as soon as possible, so that we can catch
kernel bugs more effectively. Previously early boot had NULL mapped
and didn't trap on NULL references.

This patch introduces boot_level4_pgt, which will always have low identity
addresses mapped. Druing boot, all the processors will use this as their
level4 pgt. On BP, we will switch to init_level4_pgt as soon as we enter C
code and zap the low mappings as soon as we are done with the usage of
identity low mapped addresses. On AP's we will zap the low mappings as
soon as we jump to C code.

Signed-off-by: Suresh Siddha <[e-mail]>
Signed-off-by: Ashok Raj <[e-mail]>
Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/i386/kernel/acpi/boot.c

···
 	 * RSDP signature.
 	 */
 	for (offset = 0; offset < length; offset += 16) {
-		if (strncmp((char *)(start + offset), "RSD PTR ", sig_len))
+		if (strncmp((char *)(phys_to_virt(start) + offset), "RSD PTR ", sig_len))
 			continue;
 		return (start + offset);
 	}

arch/x86_64/kernel/head.S

···
 
 #include <linux/linkage.h>
 #include <linux/threads.h>
+#include <linux/init.h>
 #include <asm/desc.h>
 #include <asm/segment.h>
 #include <asm/page.h>
···
 	movl	%eax, %cr4
 
 	/* Setup early boot stage 4 level pagetables */
-	movl	$(init_level4_pgt - __START_KERNEL_map), %eax
+	movl	$(boot_level4_pgt - __START_KERNEL_map), %eax
 	movl	%eax, %cr3
 
 	/* Setup EFER (Extended Feature Enable Register) */
···
 	movq	%rax, %cr4
 
 	/* Setup early boot stage 4 level pagetables. */
-	movq	$(init_level4_pgt - __START_KERNEL_map), %rax
+	movq	$(boot_level4_pgt - __START_KERNEL_map), %rax
 	movq	%rax, %cr3
 
 	/* Check if nx is implemented */
···
 ENTRY(stext)
 ENTRY(_stext)
 
-	/*
-	 * This default setting generates an ident mapping at address 0x100000
-	 * and a mapping for the kernel that precisely maps virtual address
-	 * 0xffffffff80000000 to physical address 0x000000. (always using
-	 * 2Mbyte large pages provided by PAE mode)
-	 */
 .org 0x1000
 ENTRY(init_level4_pgt)
-	.quad	0x0000000000002007 + __PHYSICAL_START	/* -> level3_ident_pgt */
-	.fill	255,8,0
-	.quad	0x000000000000a007 + __PHYSICAL_START
-	.fill	254,8,0
-	/* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
-	.quad	0x0000000000003007 + __PHYSICAL_START	/* -> level3_kernel_pgt */
+	/* This gets initialized in x86_64_start_kernel */
+	.fill	512,8,0
 
 .org 0x2000
 ENTRY(level3_ident_pgt)
···
 	/* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
 	.quad	0x0000000000003007 + __PHYSICAL_START	/* -> level3_kernel_pgt */
 #endif
+
+#ifndef CONFIG_HOTPLUG_CPU
+	__INITDATA
+#endif
+	/*
+	 * This default setting generates an ident mapping at address 0x100000
+	 * and a mapping for the kernel that precisely maps virtual address
+	 * 0xffffffff80000000 to physical address 0x000000. (always using
+	 * 2Mbyte large pages provided by PAE mode)
+	 */
+	.align PAGE_SIZE
+ENTRY(boot_level4_pgt)
+	.quad	0x0000000000002007 + __PHYSICAL_START	/* -> level3_ident_pgt */
+	.fill	255,8,0
+	.quad	0x000000000000a007 + __PHYSICAL_START
+	.fill	254,8,0
+	/* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
+	.quad	0x0000000000003007 + __PHYSICAL_START	/* -> level3_kernel_pgt */
 
 	.data
 

arch/x86_64/kernel/head64.c

···
 #include <asm/bootsetup.h>
 #include <asm/setup.h>
 #include <asm/desc.h>
+#include <asm/pgtable.h>
 
 /* Don't add a printk in there. printk relies on the PDA which is not initialized 
    yet. */
···
 		set_intr_gate(i, early_idt_handler);
 	asm volatile("lidt %0" :: "m" (idt_descr));
 	clear_bss();
+
+	/*
+	 * switch to init_level4_pgt from boot_level4_pgt
+	 */
+	memcpy(init_level4_pgt, boot_level4_pgt, PTRS_PER_PGD*sizeof(pgd_t));
+	asm volatile("movq %0,%%cr3" :: "r" (__pa_symbol(&init_level4_pgt)));
+
 	pda_init(0);
 	copy_bootdata(real_mode_data);
 #ifdef CONFIG_SMP

arch/x86_64/kernel/mpparse.c

···
 		 * Read the physical hardware table.  Anything here will
 		 * override the defaults.
 		 */
-		if (!smp_read_mpc((void *)(unsigned long)mpf->mpf_physptr)) {
+		if (!smp_read_mpc(phys_to_virt(mpf->mpf_physptr))) {
 			smp_found_config = 0;
 			printk(KERN_ERR "BIOS bug, MP table errors detected!...\n");
 			printk(KERN_ERR "... disabling SMP support. (tell your hw vendor)\n");

arch/x86_64/kernel/setup.c

···
 
 	init_memory_mapping(0, (end_pfn_map << PAGE_SHIFT));
 
+	zap_low_mappings(0);
+
 #ifdef CONFIG_ACPI
 	/*
 	 * Initialize the ACPI boot-time table parser (gets the RSDP and SDT).

arch/x86_64/kernel/setup64.c

···
 			panic("cannot allocate irqstack for cpu %d", cpu); 
 	}
 
-	asm volatile("movq %0,%%cr3" :: "r" (__pa_symbol(&init_level4_pgt)));
 
 	pda->irqstackptr += IRQSTACKSIZE-64;
 } 
···
 	/* CPU 0 is initialised in head64.c */
 	if (cpu != 0) {
 		pda_init(cpu);
+		zap_low_mappings(cpu);
 	} else 
 		estacks = boot_exception_stacks; 
 

arch/x86_64/kernel/smpboot.c

···
  */
 void __init smp_cpus_done(unsigned int max_cpus)
 {
-#ifndef CONFIG_HOTPLUG_CPU
-	zap_low_mappings();
-#endif
 	smp_cleanup_boot();
 
 #ifdef CONFIG_X86_IO_APIC

arch/x86_64/mm/init.c

···
 
 extern struct x8664_pda cpu_pda[NR_CPUS];
 
-/* Assumes all CPUs still execute in init_mm */
-void zap_low_mappings(void)
+void __cpuinit zap_low_mappings(int cpu)
 {
-	pgd_t *pgd = pgd_offset_k(0UL);
-	pgd_clear(pgd);
-	flush_tlb_all();
+	if (cpu == 0) {
+		pgd_t *pgd = pgd_offset_k(0UL);
+		pgd_clear(pgd);
+	} else {
+		/*
+		 * For AP's, zap the low identity mappings by changing the cr3
+		 * to init_level4_pgt and doing local flush tlb all
+		 */
+		asm volatile("movq %0,%%cr3" :: "r" (__pa_symbol(&init_level4_pgt)));
+	}
+	__flush_tlb_all();
 }
 
 /* Compute zone sizes for the DMA and DMA32 zones in a node. */
···
 		datasize >> 10,
 		initsize >> 10);
 
+#ifdef CONFIG_SMP
 	/*
-	 * Subtle. SMP is doing its boot stuff late (because it has to
-	 * fork idle threads) - but it also needs low mappings for the
-	 * protected-mode entry to work. We zap these entries only after
-	 * the WP-bit has been tested.
+	 * Sync boot_level4_pgt mappings with the init_level4_pgt
+	 * except for the low identity mappings which are already zapped
+	 * in init_level4_pgt. This sync-up is essential for AP's bringup
 	 */
-#ifndef CONFIG_SMP
-	zap_low_mappings();
+	memcpy(boot_level4_pgt+1, init_level4_pgt+1, (PTRS_PER_PGD-1)*sizeof(pgd_t));
 #endif
 }
 

include/asm-x86_64/pgtable.h

···
 extern pud_t level3_ident_pgt[512];
 extern pmd_t level2_kernel_pgt[512];
 extern pgd_t init_level4_pgt[];
+extern pgd_t boot_level4_pgt[];
 extern unsigned long __supported_pte_mask;
 
 #define swapper_pg_dir init_level4_pgt

include/asm-x86_64/proto.h

···
 extern void start_kernel(void);
 extern void pda_init(int); 
 
+extern void zap_low_mappings(int cpu);
+
 extern void early_idt_handler(void);
 
 extern void mcheck_init(struct cpuinfo_x86 *c);

include/asm-x86_64/smp.h

···
 extern void unlock_ipi_call_lock(void);
 extern int smp_num_siblings;
 extern void smp_send_reschedule(int cpu);
-extern void zap_low_mappings(void);
 void smp_stop_cpu(void);
 extern int smp_call_function_single(int cpuid, void (*func) (void *info),
 				void *info, int retry, int wait);