Re: [Fastboot] [PATCH 03/03] kexec: Avoid overwriting the currentpgd (V2, x86_64)

[Eric W. Biederman]

Magnus Damm <magnus@xxxxxxxxxxxxx> writes:

> --===============37282618571824511==
>
> kexec: Avoid overwriting the current pgd (V2, x86_64)
>
> This patch upgrades the x86_64-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. The already existing page table is renamed to "page_table_b".

As I mentioned earlier you don't need two page tables, because
it is easy to guarantee that the identity mapping and the virtual mapping
will not intersect on x86_64.  Until we have as many physical address bits
as virtual address bits (which requires page table modifications) it
is nonsense to have 2 page tables here.

> KEXEC_CONTROL_CODE_SIZE is changed into a single page. This updated version of
> the patch also moves the segment handling code into the reloacte_kernel.S.

You are doing two things at once here.  If it is worth moving the segment
handling and idt handling it should be done as a separate patch.


> Signed-off-by: Magnus Damm <magnus@xxxxxxxxxxxxx>
> ---
>
>  The patch has been tested with regular kexec and CONFIG_CRASH_DUMP.
>  Applies on top of 2.6.16 and 2.6.17-rc4.
>
>  arch/x86_64/kernel/machine_kexec.c   |  193 +++++++++++++++++-----------------
>  arch/x86_64/kernel/relocate_kernel.S |   84 +++++++++++++-
>  include/asm-x86_64/kexec.h           |   15 ++
>  3 files changed, 189 insertions(+), 103 deletions(-)
>
> --- 0001/arch/x86_64/kernel/machine_kexec.c
> +++ work/arch/x86_64/kernel/machine_kexec.c 2006-05-19 12:09:39.000000000 +0900
> @@ -2,6 +2,10 @@
>   * machine_kexec.c - handle transition of Linux booting another kernel
>   * Copyright (C) 2002-2005 Eric Biederman  <ebiederm@xxxxxxxxxxxx>
>   *
> + * 2006-05-19 Magnus Damm <damm@xxxxxxxxxxxxx>:
> + * - 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.
>   */
> @@ -96,81 +100,110 @@ out:
>  }
>  
>  
> -static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
> +static int create_page_table_b(struct kimage *image)
>  {
> -	pgd_t *level4p;
> -	level4p = (pgd_t *)__va(start_pgtable);
> - 	return init_level4_page(image, level4p, 0, end_pfn << PAGE_SHIFT);
> -}
> +	struct kimage_arch *arch = &image->arch_data;
>  
> -static void set_idt(void *newidt, u16 limit)
> -{
> -	struct desc_ptr curidt;
> +	arch->page_table_b = kimage_alloc_control_pages(image, 0);
>  
> -	/* x86-64 supports unaliged loads & stores */
> -	curidt.size    = limit;
> -	curidt.address = (unsigned long)newidt;
> +	if (!arch->page_table_b)
> +		return -ENOMEM;
>  
> -	__asm__ __volatile__ (
> -		"lidtq %0\n"
> -		: : "m" (curidt)
> -		);
> -};
> + 	return init_level4_page(image, page_address(arch->page_table_b),
> +				0, end_pfn << PAGE_SHIFT);
> +}
>  
> +typedef NORET_TYPE void (*relocate_new_kernel_t)(unsigned long
> indirection_page,
> +					unsigned long control_code_buffer,
> +					unsigned long start_address,
> +					unsigned long page_table_a,
> + unsigned long page_table_b) ATTRIB_NORET;
> +
> +const extern unsigned char relocate_new_kernel[];
> +const extern unsigned long relocate_new_kernel_size;
>  
> -static void set_gdt(void *newgdt, u16 limit)
> +static int allocate_page_table_a(struct kimage *image)
>  {
> -	struct desc_ptr curgdt;
> +	struct kimage_arch *arch = &image->arch_data;
> +	struct page *page;
> +	int k = sizeof(arch->page_table_a) / sizeof(arch->page_table_a[0]);
>  
> -	/* x86-64 supports unaligned loads & stores */
> -	curgdt.size    = limit;
> -	curgdt.address = (unsigned long)newgdt;
> +	for (; k > 0; k--) {
> +		page = kimage_alloc_control_pages(image, 0);
> +		if (!page)
> +			return -ENOMEM;
>  
> -	__asm__ __volatile__ (
> -		"lgdtq %0\n"
> -		: : "m" (curgdt)
> -		);
> -};
> +		clear_page(page_address(page));
> +		arch->page_table_a[k - 1] = page;
> +	}
>  
> -static void load_segments(void)
> -{
> -	__asm__ __volatile__ (
> -		"\tmovl %0,%%ds\n"
> -		"\tmovl %0,%%es\n"
> -		"\tmovl %0,%%ss\n"
> -		"\tmovl %0,%%fs\n"
> -		"\tmovl %0,%%gs\n"
> -		: : "a" (__KERNEL_DS) : "memory"
> -		);
> +	return 0;
>  }
>  
> -typedef NORET_TYPE void (*relocate_new_kernel_t)(unsigned long
> indirection_page,
> -					unsigned long control_code_buffer,
> -					unsigned long start_address,
> -					unsigned long pgtable) ATTRIB_NORET;
> +#define _PAGE_KERNEL_EXEC __PAGE_KERNEL_EXEC
> +#define pa_page(page) __pa_symbol(page_address(page)) /* __pa() miscompiles */
>  
> -const extern unsigned char relocate_new_kernel[];
> -const extern unsigned long relocate_new_kernel_size;
> +static int create_mapping(struct page *root, struct page **pages, 
> +			  unsigned long va, unsigned long pa)
> +{
> +	pgd_t *pgd;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +	pte_t *pte;
> +	int k = 0;
> +
> +	pgd = (pgd_t *)page_address(root) + pgd_index(va);
> +	if (!pgd_present(*pgd))
> +		set_pgd(pgd, __pgd(pa_page(pages[k++]) | _KERNPG_TABLE));
> +
> +	pud = pud_offset(pgd, va);
> +	if (!pud_present(*pud))
> +		set_pud(pud, __pud(pa_page(pages[k++]) | _KERNPG_TABLE));
> +
> +	pmd = pmd_offset(pud, va);
> +	if (!pmd_present(*pmd))
> +		set_pmd(pmd, __pmd(pa_page(pages[k++]) | _KERNPG_TABLE));
> +
> +	pte = (pte_t *)page_address(pmd_page(*pmd)) + pte_index(va);
> +	set_pte(pte, __pte(pa | _PAGE_KERNEL_EXEC));
> +
> +	return k;
> +}
>  
>  int machine_kexec_prepare(struct kimage *image)
>  {
> -	unsigned long start_pgtable, control_code_buffer;
> -	int result;
> +	void *control_page;
> +	unsigned long pa;
> +	int k;
>  
> -	/* Calculate the offsets */
> -	start_pgtable = page_to_pfn(image->control_code_page) << PAGE_SHIFT;
> -	control_code_buffer = start_pgtable + PAGE_SIZE;
> -
> -	/* Setup the identity mapped 64bit page table */
> -	result = init_pgtable(image, start_pgtable);
> -	if (result)
> -		return result;
> -
> -	/* Place the code in the reboot code buffer */
> -	memcpy(__va(control_code_buffer), relocate_new_kernel,
> -						relocate_new_kernel_size);
> +	memset(&image->arch_data, 0, sizeof(image->arch_data));
>  
> -	return 0;
> +	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);
> +
> +	/* create identity mapped page table aka page_table_b */
> +
> +	return create_page_table_b(image);
>  }
>  
>  void machine_kexec_cleanup(struct kimage *image)
> @@ -185,47 +218,17 @@ void machine_kexec_cleanup(struct kimage
>  NORET_TYPE void machine_kexec(struct kimage *image)
>  {
>  	unsigned long page_list;
> -	unsigned long control_code_buffer;
> -	unsigned long start_pgtable;
> +	unsigned long control_code;
> +	unsigned long page_table_a;
> +	unsigned long page_table_b;
>  	relocate_new_kernel_t rnk;
>  
> -	/* Interrupts aren't acceptable while we reboot */
> -	local_irq_disable();
> -
> -	/* Calculate the offsets */
>  	page_list = image->head;
> -	start_pgtable = page_to_pfn(image->control_code_page) << PAGE_SHIFT;
> -	control_code_buffer = start_pgtable + PAGE_SIZE;
> +	control_code = __pa(page_address(image->control_code_page));
> +	page_table_a = __pa(page_address(image->arch_data.page_table_a[0]));
> +	page_table_b = __pa(page_address(image->arch_data.page_table_b));
>  
> -	/* Set the low half of the page table to my identity mapped
> -	 * page table for kexec.  Leave the high half pointing at the
> -	 * kernel pages.   Don't bother to flush the global pages
> -	 * as that will happen when I fully switch to my identity mapped
> -	 * page table anyway.
> -	 */
> -	memcpy(__va(read_cr3()), __va(start_pgtable), PAGE_SIZE/2);
> -	__flush_tlb();
> -
> -
> -	/* 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 unless you set the segment to a different selector.
> -	 *
> -	 * The more common model 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);
>  	/* now call it */
> -	rnk = (relocate_new_kernel_t) control_code_buffer;
> -	(*rnk)(page_list, control_code_buffer, image->start, start_pgtable);
> +	rnk = (relocate_new_kernel_t) relocate_new_kernel;
> + (*rnk)(page_list, control_code, image->start, page_table_a, page_table_b);
>  }
> --- 0001/arch/x86_64/kernel/relocate_kernel.S
> +++ work/arch/x86_64/kernel/relocate_kernel.S 2006-05-19 12:11:13.000000000
> +0900
> @@ -2,11 +2,18 @@
>   * relocate_kernel.S - put the kernel image in place to boot
>   * Copyright (C) 2002-2005 Eric Biederman  <ebiederm@xxxxxxxxxxxx>
>   *
> + * 2006-05-19 Magnus Damm <damm@xxxxxxxxxxxxx>:
> + * - moved segment handling code from machine_kexec.c
> + *
>   * 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
> @@ -18,21 +25,69 @@ relocate_new_kernel:
>  	/* %rdi page_list
>  	 * %rsi reboot_code_buffer
>  	 * %rdx start address
> -	 * %rcx page_table
> -	 * %r8  arg5
> +	 * %rcx page_table_a
> +	 * %r8  page_table_b
>  	 * %r9  arg6
>  	 */
> -
> +	
>  	/* zero out flags, and disable interrupts */
>  	pushq $0
>  	popfq
>  
> +	/* switch to page_table_a */
> +	movq    %rcx, %cr3
> +
> +	/* setup idt */
> +
> +	movq	%rsi, %rax
> +	addq	$(idt_48 - relocate_new_kernel), %rax
> +	lidtq	(%rax)
> +
> +	/* setup gdt */
> +
> +	movq	%rsi, %rax
> +	addq	$(gdt - relocate_new_kernel), %rax
> +	movq	%rsi, %r9
> +	addq	$((gdt_48 - relocate_new_kernel) + 2), %r9
> +	movq	%rax, (%r9)
> +	
> +	movq	%rsi, %rax
> +	addq	$(gdt_48 - relocate_new_kernel), %rax
> +	lgdtq	(%rax)
> +
> +	/* setup data segment registers */
> +
> +	xorl	%eax,%eax
> +	movl	%eax, %ds
> +	movl	%eax, %es
> +	movl	%eax, %fs
> +	movl	%eax, %gs
> +	movl	%eax, %ss
> +
>  	/* set a new stack at the bottom of our page... */
>  	lea   4096(%rsi), %rsp
>  
> +	/* load new code segment */
> +
> +	movq	%rsp, %rcx
> +	xorq	%rax, %rax
> +	pushq	%rax                                              /* SS */
> +	pushq	%rcx                                              /* ESP */
> +	pushq	%rax                                              /* RFLAGS */
> +
> +	movq	$(gdt_code - gdt), %rax
> +	pushq	%rax                                              /* CS */
> +
> +	movq	%rsi, %rax
> +	addq	$(identity_mapped - relocate_new_kernel), %rax
> +	pushq	%rax                                              /* RIP */
> +
> +	iretq
> +	
> +identity_mapped:
>  	/* store the parameters back on the stack */
>  	pushq	%rdx /* store the start address */
> -
> +	
>  	/* Set cr0 to a known state:
>  	 * 31 1 == Paging enabled
>  	 * 18 0 == Alignment check disabled
> @@ -69,7 +124,7 @@ relocate_new_kernel:
>  	/* Switch to the identity mapped page tables,
>  	 * and flush the TLB.
>  	*/
> -	movq	%rcx, %cr3
> +	movq	%r8, %cr3
>  
>  	/* Do the copies */
>  	movq	%rdi, %rcx 	/* Put the page_list in %rcx */
> @@ -136,6 +191,25 @@ relocate_new_kernel:
>  	xorq	%r15, %r15
>  
>  	ret
> +	.align	16
> +gdt:
> +	.long   0x00000000  /* NULL descriptor */
> +	.long   0x00000000
> +gdt_code:
> +	.long   0x00000000  /* code descriptor */
> +	.long   0x00209800
> +
> +gdt_end:
> +	.align	4
> +	
> +idt_48:
> +	.word	0				# idt limit = 0
> +	.quad	0, 0				# idt base = 0L
> +
> +gdt_48:
> +	.word	gdt_end - gdt - 1		# gdt limit
> +	.quad	0, 0				# gdt base (filled in later)
> +
>  relocate_new_kernel_end:
>  
>  	.globl relocate_new_kernel_size
> --- 0002/include/asm-x86_64/kexec.h
> +++ work/include/asm-x86_64/kexec.h	2006-05-19 12:07:33.000000000 +0900
> @@ -21,15 +21,24 @@
>  /* Maximum address we can use for the control pages */
>  #define KEXEC_CONTROL_MEMORY_LIMIT     (0xFFFFFFFFFFUL)
>  
> -/* Allocate one page for the pdp and the second for the code */
> -#define KEXEC_CONTROL_CODE_SIZE  (4096UL + 4096UL)
> +#define KEXEC_CONTROL_CODE_SIZE  4096
>  
>  /* The native architecture */
>  #define KEXEC_ARCH KEXEC_ARCH_X86_64
>  
>  #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..
> +	 *
> +	 * page_table_b points to the root page of a page table which is used
> +	 * to provide identity mapping of all ram.
> +	 */
> +
> +	struct page *page_table_a[7]; /* 2 * (pte + pud + pmd) + pgd */
> +	struct page *page_table_b;
> +};
>  
>  /*
>   * Saving the registers of the cpu on which panic occured in
>
> --===============37282618571824511==
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> --===============37282618571824511==--
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