[PATCH 02/03] kexec: Avoid overwriting the current pgd (V2, i386)

[Magnus Damm <magnus@valinux.co.jp>]

kexec: Avoid overwriting the current pgd (V2, i386)

This patch upgrades the i386-specific kexec code to avoid overwriting the
current pgd. Overwriting the current pgd is bad when CONFIG_CRASH_DUMP is used
to start a secondary kernel that dumps the memory of the previous kernel.

The code introduces a new set of page tables called "page_table_a". These
tables are used to provide an executable identity mapping without overwriting
the current pgd. This updated version of the patch fixes a PAE bug and moves
the segment handling code into the reloacte_kernel.S.

Signed-off-by: Magnus Damm <magnus@valinux.co.jp>
---

 The patch has been tested with regular kexec and CONFIG_CRASH_DUMP.
 Both PAE and non-PAE configurations work well.
 Applies on top of 2.6.16 and 2.6.17-rc4.

 arch/i386/kernel/machine_kexec.c   |  230 ++++++++++++++----------------------
 arch/i386/kernel/relocate_kernel.S |   92 ++++++++++++++
 include/asm-i386/kexec.h           |   12 +
 3 files changed, 192 insertions(+), 142 deletions(-)

--- 0001/arch/i386/kernel/machine_kexec.c
+++ work/arch/i386/kernel/machine_kexec.c	2006-05-22 15:57:50.000000000 +0900
@@ -2,6 +2,10 @@
  * machine_kexec.c - handle transition of Linux booting another kernel
  * Copyright (C) 2002-2005 Eric Biederman  <ebiederm@xmission.com>
  *
+ * 2006-05-19 Magnus Damm <damm@opensource.se>:
+ * - rewrote identity map code to avoid overwriting current pgd
+ * - moved segment handling code into relocate_kernel.S
+ *
  * This source code is licensed under the GNU General Public License,
  * Version 2.  See the file COPYING for more details.
  */
@@ -19,123 +23,73 @@
 #include <asm/desc.h>
 #include <asm/system.h>
 
-#define PAGE_ALIGNED __attribute__ ((__aligned__(PAGE_SIZE)))
-
-#define L0_ATTR (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | _PAGE_DIRTY)
-#define L1_ATTR (_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED | _PAGE_DIRTY)
-#define L2_ATTR (_PAGE_PRESENT)
-
-#define LEVEL0_SIZE (1UL << 12UL)
+typedef asmlinkage NORET_TYPE void (*relocate_new_kernel_t)(
+					unsigned long indirection_page,
+					unsigned long reboot_code_buffer,
+					unsigned long start_address,
+					unsigned long page_table_a,
+					unsigned long has_pae) ATTRIB_NORET;
 
-#ifndef CONFIG_X86_PAE
-#define LEVEL1_SIZE (1UL << 22UL)
-static u32 pgtable_level1[1024] PAGE_ALIGNED;
+const extern unsigned char relocate_new_kernel[];
+extern void relocate_new_kernel_end(void);
+const extern unsigned int relocate_new_kernel_size;
 
-static void identity_map_page(unsigned long address)
+static int allocate_page_table_a(struct kimage *image)
 {
-	unsigned long level1_index, level2_index;
-	u32 *pgtable_level2;
-
-	/* Find the current page table */
-	pgtable_level2 = __va(read_cr3());
+	struct kimage_arch *arch = &image->arch_data;
+	struct page *page;
+	int k = sizeof(arch->page_table_a) / sizeof(arch->page_table_a[0]);
+
+	for (; k > 0; k--) {
+		page = kimage_alloc_control_pages(image, 0);
+		if (!page)
+			return -ENOMEM;
+
+		clear_page(page_address(page));
+		arch->page_table_a[k - 1] = page;
+	}
 
-	/* Find the indexes of the physical address to identity map */
-	level1_index = (address % LEVEL1_SIZE)/LEVEL0_SIZE;
-	level2_index = address / LEVEL1_SIZE;
-
-	/* Identity map the page table entry */
-	pgtable_level1[level1_index] = address | L0_ATTR;
-	pgtable_level2[level2_index] = __pa(pgtable_level1) | L1_ATTR;
-
-	/* Flush the tlb so the new mapping takes effect.
-	 * Global tlb entries are not flushed but that is not an issue.
-	 */
-	load_cr3(pgtable_level2);
+	return 0;
 }
 
-#else
-#define LEVEL1_SIZE (1UL << 21UL)
-#define LEVEL2_SIZE (1UL << 30UL)
-static u64 pgtable_level1[512] PAGE_ALIGNED;
-static u64 pgtable_level2[512] PAGE_ALIGNED;
-
-static void identity_map_page(unsigned long address)
-{
-	unsigned long level1_index, level2_index, level3_index;
-	u64 *pgtable_level3;
+/* workaround for include/asm-i386/pgtable-3level.h */
 
-	/* Find the current page table */
-	pgtable_level3 = __va(read_cr3());
-
-	/* Find the indexes of the physical address to identity map */
-	level1_index = (address % LEVEL1_SIZE)/LEVEL0_SIZE;
-	level2_index = (address % LEVEL2_SIZE)/LEVEL1_SIZE;
-	level3_index = address / LEVEL2_SIZE;
-
-	/* Identity map the page table entry */
-	pgtable_level1[level1_index] = address | L0_ATTR;
-	pgtable_level2[level2_index] = __pa(pgtable_level1) | L1_ATTR;
-	set_64bit(&pgtable_level3[level3_index],
-					       __pa(pgtable_level2) | L2_ATTR);
-
-	/* Flush the tlb so the new mapping takes effect.
-	 * Global tlb entries are not flushed but that is not an issue.
-	 */
-	load_cr3(pgtable_level3);
-}
+#ifdef CONFIG_X86_PAE
+#undef pgd_present
+#define pgd_present(pgd) (pgd_val(pgd) & _PAGE_PRESENT)
+#define _PGD_ATTR _PAGE_PRESENT
+#else
+#define _PGD_ATTR _KERNPG_TABLE
 #endif
 
-static void set_idt(void *newidt, __u16 limit)
-{
-	struct Xgt_desc_struct curidt;
-
-	/* ia32 supports unaliged loads & stores */
-	curidt.size    = limit;
-	curidt.address = (unsigned long)newidt;
-
-	load_idt(&curidt);
-};
+#define pa_page(page) __pa(page_address(page))
 
-
-static void set_gdt(void *newgdt, __u16 limit)
+static int create_mapping(struct page *root, struct page **pages, 
+			  unsigned long va, unsigned long pa)
 {
-	struct Xgt_desc_struct curgdt;
-
-	/* ia32 supports unaligned loads & stores */
-	curgdt.size    = limit;
-	curgdt.address = (unsigned long)newgdt;
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+	int k = 0;
 
-	load_gdt(&curgdt);
-};
+	pgd = (pgd_t *)page_address(root) + pgd_index(va);
+	if (!pgd_present(*pgd))
+		set_pgd(pgd, __pgd(pa_page(pages[k++]) | _PGD_ATTR));
 
-static void load_segments(void)
-{
-#define __STR(X) #X
-#define STR(X) __STR(X)
+	pud = pud_offset(pgd, va);
+	if (!pud_present(*pud))
+		set_pud(pud, __pud(pa_page(pages[k++]) | _KERNPG_TABLE));
 
-	__asm__ __volatile__ (
-		"\tljmp $"STR(__KERNEL_CS)",$1f\n"
-		"\t1:\n"
-		"\tmovl $"STR(__KERNEL_DS)",%%eax\n"
-		"\tmovl %%eax,%%ds\n"
-		"\tmovl %%eax,%%es\n"
-		"\tmovl %%eax,%%fs\n"
-		"\tmovl %%eax,%%gs\n"
-		"\tmovl %%eax,%%ss\n"
-		::: "eax", "memory");
-#undef STR
-#undef __STR
-}
+	pmd = pmd_offset(pud, va);
+	if (!pmd_present(*pmd))
+		set_pmd(pmd, __pmd(pa_page(pages[k++]) | _KERNPG_TABLE));
 
-typedef asmlinkage NORET_TYPE void (*relocate_new_kernel_t)(
-					unsigned long indirection_page,
-					unsigned long reboot_code_buffer,
-					unsigned long start_address,
-					unsigned int has_pae) ATTRIB_NORET;
+	pte = (pte_t *)page_address(pmd_page(*pmd)) + pte_index(va);
+	set_pte(pte, __pte(pa | _PAGE_KERNEL_EXEC));
 
-const extern unsigned char relocate_new_kernel[];
-extern void relocate_new_kernel_end(void);
-const extern unsigned int relocate_new_kernel_size;
+	return k;
+}
 
 /*
  * A architecture hook called to validate the
@@ -147,11 +101,38 @@ const extern unsigned int relocate_new_k
  * Do what every setup is needed on image and the
  * reboot code buffer to allow us to avoid allocations
  * later.
- *
- * Currently nothing.
  */
 int machine_kexec_prepare(struct kimage *image)
 {
+	void *control_page;
+	unsigned long pa;
+	int k;
+
+	memset(&image->arch_data, 0, sizeof(image->arch_data));
+
+	k = allocate_page_table_a(image);
+	if (k)
+		return k;
+
+	/* fill in control_page with assembly code */
+
+	control_page = page_address(image->control_code_page);
+	memcpy(control_page, relocate_new_kernel, relocate_new_kernel_size);
+
+	/* map the control_page at the virtual address of relocate_kernel.S */
+
+	pa = __pa(control_page);
+
+	k = create_mapping(image->arch_data.page_table_a[0], 
+			   &image->arch_data.page_table_a[1],
+			   (unsigned long)relocate_new_kernel, pa);
+
+	/* identity map the control_page */
+
+	create_mapping(image->arch_data.page_table_a[0], 
+		       &image->arch_data.page_table_a[k + 1],
+		       pa, pa);
+
 	return 0;
 }
 
@@ -170,45 +151,16 @@ void machine_kexec_cleanup(struct kimage
 NORET_TYPE void machine_kexec(struct kimage *image)
 {
 	unsigned long page_list;
-	unsigned long reboot_code_buffer;
-
+	unsigned long control_code;
+	unsigned long page_table_a;
 	relocate_new_kernel_t rnk;
 
-	/* Interrupts aren't acceptable while we reboot */
-	local_irq_disable();
-
-	/* Compute some offsets */
-	reboot_code_buffer = page_to_pfn(image->control_code_page)
-								<< PAGE_SHIFT;
 	page_list = image->head;
-
-	/* Set up an identity mapping for the reboot_code_buffer */
-	identity_map_page(reboot_code_buffer);
-
-	/* copy it out */
-	memcpy((void *)reboot_code_buffer, relocate_new_kernel,
-						relocate_new_kernel_size);
-
-	/* The segment registers are funny things, they are
-	 * automatically loaded from a table, in memory wherever you
-	 * set them to a specific selector, but this table is never
-	 * accessed again you set the segment to a different selector.
-	 *
-	 * The more common model is are caches where the behide
-	 * the scenes work is done, but is also dropped at arbitrary
-	 * times.
-	 *
-	 * I take advantage of this here by force loading the
-	 * segments, before I zap the gdt with an invalid value.
-	 */
-	load_segments();
-	/* The gdt & idt are now invalid.
-	 * If you want to load them you must set up your own idt & gdt.
-	 */
-	set_gdt(phys_to_virt(0),0);
-	set_idt(phys_to_virt(0),0);
+	control_code = __pa(page_address(image->control_code_page));
+	page_table_a = __pa(page_address(image->arch_data.page_table_a[0]));
 
 	/* now call it */
-	rnk = (relocate_new_kernel_t) reboot_code_buffer;
-	(*rnk)(page_list, reboot_code_buffer, image->start, cpu_has_pae);
+	rnk = (relocate_new_kernel_t) relocate_new_kernel;
+	(*rnk)(page_list, control_code, image->start, 
+	       page_table_a, (unsigned long)cpu_has_pae);
 }
--- 0001/arch/i386/kernel/relocate_kernel.S
+++ work/arch/i386/kernel/relocate_kernel.S	2006-05-22 12:55:37.000000000 +0900
@@ -2,12 +2,20 @@
  * relocate_kernel.S - put the kernel image in place to boot
  * Copyright (C) 2002-2004 Eric Biederman  <ebiederm@xmission.com>
  *
+ * 2006-05-19 Magnus Damm <damm@opensource.se>:
+ * - moved segment handling code from machine_kexec.c
+ * - gdt tables stolen from arch/i386/boot/setup.S
+ *
  * This source code is licensed under the GNU General Public License,
  * Version 2.  See the file COPYING for more details.
  */
 
 #include <linux/linkage.h>
+#include <asm/page.h>
 
+.text
+.align (1 << PAGE_SHIFT)
+	
 	/*
 	 * Must be relocatable PIC code callable as a C function, that once
 	 * it starts can not use the previous processes stack.
@@ -18,18 +26,68 @@ relocate_new_kernel:
 	movl  4(%esp), %ebx /* page_list */
 	movl  8(%esp), %ebp /* reboot_code_buffer */
 	movl  12(%esp), %edx /* start address */
-	movl  16(%esp), %ecx /* cpu_has_pae */
+	movl  16(%esp), %edi /* page_table_a */
+	movl  20(%esp), %ecx /* cpu_has_pae */
 
 	/* zero out flags, and disable interrupts */
 	pushl $0
 	popfl
 
+	/* switch to page_table_a */
+	movl	%edi, %eax
+	movl	%eax, %cr3
+
+	/* setup idt */
+
+	movl	%ebp, %eax
+	addl	$(idt_48 - relocate_new_kernel), %eax
+	lidtl	(%eax)
+
+	/* setup gdt */
+
+	movl	%ebp, %eax
+	addl	$(gdt - relocate_new_kernel), %eax
+	movl	%ebp, %esi
+	addl	$((gdt_48 - relocate_new_kernel) + 2), %esi
+	movl	%eax, (%esi)
+	
+	movl	%ebp, %eax
+	addl	$(gdt_48 - relocate_new_kernel), %eax
+	lgdtl	(%eax)
+
+	/* setup data segment registers */
+	
+	mov	$(gdt_ds - gdt), %eax
+	mov	%eax, %ds
+	mov	%eax, %es
+	mov	%eax, %fs
+	mov	%eax, %gs
+	mov	%eax, %ss
+
 	/* set a new stack at the bottom of our page... */
 	lea   4096(%ebp), %esp
 
+	/* load new code segment */
+
+	movl	%ebp, %esi
+	xorl	%eax, %eax
+	pushl	%eax
+	pushl	%esi
+	pushl	%eax
+	
+	movl	$(gdt_cs - gdt), %eax
+	pushl	%eax
+	
+	movl	%ebp, %eax
+	addl	$(identity_mapped - relocate_new_kernel),%eax
+	pushl	%eax
+	iretl
+
+identity_mapped:	
+
 	/* store the parameters back on the stack */
 	pushl   %edx /* store the start address */
-
+	
 	/* Set cr0 to a known state:
 	 * 31 0 == Paging disabled
 	 * 18 0 == Alignment check disabled
@@ -113,6 +171,36 @@ relocate_new_kernel:
 	xorl    %edi, %edi
 	xorl    %ebp, %ebp
 	ret
+
+	.align	16
+gdt:
+	.fill	1,8,0
+
+gdt_cs:	
+	.word	0xFFFF				# 4Gb - (0x100000*0x1000 = 4Gb)
+	.word	0				# base address = 0
+	.word	0x9A00				# code read/exec
+	.word	0x00CF				# granularity = 4096, 386
+						#  (+5th nibble of limit)
+gdt_ds:
+	.word	0xFFFF				# 4Gb - (0x100000*0x1000 = 4Gb)
+	.word	0				# base address = 0
+	.word	0x9200				# data read/write
+	.word	0x00CF				# granularity = 4096, 386
+						#  (+5th nibble of limit)
+gdt_end:
+	.align	4
+	
+	.word	0				# alignment byte
+idt_48:
+	.word	0				# idt limit = 0
+	.word	0, 0				# idt base = 0L
+
+	.word	0				# alignment byte
+gdt_48:
+	.word	gdt_end - gdt - 1		# gdt limit
+	.word	0, 0				# gdt base (filled in later)
+	
 relocate_new_kernel_end:
 
 	.globl relocate_new_kernel_size
--- 0002/include/asm-i386/kexec.h
+++ work/include/asm-i386/kexec.h	2006-05-22 12:55:38.000000000 +0900
@@ -29,7 +29,17 @@
 
 #define MAX_NOTE_BYTES 1024
 
-struct kimage_arch {};
+struct kimage_arch {
+       /* page_table_a[] holds enough pages to create a new page table
+        * that maps the control page twice..
+        */
+
+#if defined(CONFIG_X86_PAE)
+       struct page *page_table_a[5]; /* (2 * pte) + (2 * pmd) + pgd */
+#else
+       struct page *page_table_a[3]; /* (2 * pte) + pgd */
+#endif
+};
 
 /* CPU does not save ss and esp on stack if execution is already
  * running in kernel mode at the time of NMI occurrence. This code
-
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