Re: [Xen-devel] [RFC 22/22] x86/kaslr: Add option to extend KASLR range from 1GB to 3GB

[Baoquan He <bhe@xxxxxxxxxx>]

On 07/19/17 at 08:10pm, Baoquan He wrote:
> On 07/18/17 at 03:33pm, Thomas Garnier wrote:
> 
> >  quiet_cmd_relocs = RELOCS  $@
> >        cmd_relocs = $(CMD_RELOCS) $< > $@;$(CMD_RELOCS) --abs-relocs $<
> >  $(obj)/vmlinux.relocs: vmlinux FORCE
> > diff --git a/arch/x86/boot/compressed/misc.c 
> > b/arch/x86/boot/compressed/misc.c
> > index a0838ab929f2..0a0c80ab1842 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -170,10 +170,18 @@ void __puthex(unsigned long value)
> >  }
> >  
> >  #if CONFIG_X86_NEED_RELOCS
> > +
> > +/* Large randomization go lower than -2G and use large relocation table */
> > +#ifdef CONFIG_RANDOMIZE_BASE_LARGE
> > +typedef long rel_t;
> > +#else
> > +typedef int rel_t;
> > +#endif
> > +
> >  static void handle_relocations(void *output, unsigned long output_len,
> >                            unsigned long virt_addr)
> >  {
> > -   int *reloc;
> > +   rel_t *reloc;
> >     unsigned long delta, map, ptr;
> >     unsigned long min_addr = (unsigned long)output;
> >     unsigned long max_addr = min_addr + (VO___bss_start - VO__text);
> > diff --git a/arch/x86/include/asm/page_64_types.h 
> > b/arch/x86/include/asm/page_64_types.h
> > index 3f5f08b010d0..6b65f846dd64 100644
> > --- a/arch/x86/include/asm/page_64_types.h
> > +++ b/arch/x86/include/asm/page_64_types.h
> > @@ -48,7 +48,11 @@
> >  #define __PAGE_OFFSET           __PAGE_OFFSET_BASE
> >  #endif /* CONFIG_RANDOMIZE_MEMORY */
> >  
> > +#ifdef CONFIG_RANDOMIZE_BASE_LARGE
> > +#define __START_KERNEL_map _AC(0xffffffff00000000, UL)
> > +#else
> >  #define __START_KERNEL_map _AC(0xffffffff80000000, UL)
> > +#endif /* CONFIG_RANDOMIZE_BASE_LARGE */
> >  
> >  /* See Documentation/x86/x86_64/mm.txt for a description of the memory 
> > map. */
> >  #ifdef CONFIG_X86_5LEVEL
> > @@ -65,9 +69,14 @@
> >   * 512MiB by default, leaving 1.5GiB for modules once the page tables
> >   * are fully set up. If kernel ASLR is configured, it can extend the
> >   * kernel page table mapping, reducing the size of the modules area.
> > + * On PIE, we relocate the binary 2G lower so add this extra space.
> >   */
> >  #if defined(CONFIG_RANDOMIZE_BASE)
> > +#ifdef CONFIG_RANDOMIZE_BASE_LARGE
> > +#define KERNEL_IMAGE_SIZE  (_AC(3, UL) * 1024 * 1024 * 1024)
> > +#else
> >  #define KERNEL_IMAGE_SIZE  (1024 * 1024 * 1024)
> > +#endif
> >  #else
> >  #define KERNEL_IMAGE_SIZE  (512 * 1024 * 1024)
> >  #endif
> > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> > index 4103e90ff128..235c3f7b46c7 100644
> > --- a/arch/x86/kernel/head64.c
> > +++ b/arch/x86/kernel/head64.c
> > @@ -39,6 +39,7 @@ static unsigned int __initdata next_early_pgt;
> >  pmdval_t early_pmd_flags = __PAGE_KERNEL_LARGE & ~(_PAGE_GLOBAL | 
> > _PAGE_NX);
> >  
> >  #define __head     __section(.head.text)
> > +#define pud_count(x)   (((x + (PUD_SIZE - 1)) & ~(PUD_SIZE - 1)) >> 
> > PUD_SHIFT)
> >  
> >  static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
> >  {
> > @@ -54,6 +55,8 @@ unsigned long _text_offset = (unsigned long)(_text - 
> > __START_KERNEL_map);
> >  void __head notrace __startup_64(unsigned long physaddr)
> >  {
> >     unsigned long load_delta, *p;
> > +   unsigned long level3_kernel_start, level3_kernel_count;
> > +   unsigned long level3_fixmap_start;
> >     pgdval_t *pgd;
> >     p4dval_t *p4d;
> >     pudval_t *pud;
> > @@ -74,6 +77,11 @@ void __head notrace __startup_64(unsigned long physaddr)
> >     if (load_delta & ~PMD_PAGE_MASK)
> >             for (;;);
> >  
> > +   /* Look at the randomization spread to adapt page table used */
> > +   level3_kernel_start = pud_index(__START_KERNEL_map);
> > +   level3_kernel_count = pud_count(KERNEL_IMAGE_SIZE);
> > +   level3_fixmap_start = level3_kernel_start + level3_kernel_count;
> > +
> >     /* Fixup the physical addresses in the page table */
> >  
> >     pgd = fixup_pointer(&early_top_pgt, physaddr);
> > @@ -85,8 +93,9 @@ void __head notrace __startup_64(unsigned long physaddr)
> >     }
> >  
> >     pud = fixup_pointer(&level3_kernel_pgt, physaddr);
> > -   pud[510] += load_delta;
> > -   pud[511] += load_delta;
> > +   for (i = 0; i < level3_kernel_count; i++)
> > +           pud[level3_kernel_start + i] += load_delta;
> > +   pud[level3_fixmap_start] += load_delta;
> >  
> >     pmd = fixup_pointer(level2_fixmap_pgt, physaddr);
> >     pmd[506] += load_delta;
> > @@ -137,7 +146,7 @@ void __head notrace __startup_64(unsigned long physaddr)
> >      */
> >  
> >     pmd = fixup_pointer(level2_kernel_pgt, physaddr);
> > -   for (i = 0; i < PTRS_PER_PMD; i++) {
> > +   for (i = 0; i < PTRS_PER_PMD * level3_kernel_count; i++) {
> >             if (pmd[i] & _PAGE_PRESENT)
> >                     pmd[i] += load_delta;
> 
> Wow, this is dangerous. Three pud entries of level3_kernel_pgt all point
> to level2_kernel_pgt, it's out of bound of level2_kernel_pgt and
> overwrite the next data.
> 
> And if only use one page for level2_kernel_pgt, and kernel is randomized
> to cross the pud entry of -4G to -1G, it won't work well.

Sorry, I was wrong, the size of level2_kernel_pgt is decided by
KERNEL_IMAGE_SIZE. So it's not a problem, please ignore this comment.

> 
> >     }
> > @@ -268,7 +277,8 @@ asmlinkage __visible void __init 
> > x86_64_start_kernel(char * real_mode_data)
> >      */
> >     BUILD_BUG_ON(MODULES_VADDR < __START_KERNEL_map);
> >     BUILD_BUG_ON(MODULES_VADDR - __START_KERNEL_map < KERNEL_IMAGE_SIZE);
> > -   BUILD_BUG_ON(MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
> > +   BUILD_BUG_ON(!IS_ENABLED(CONFIG_RANDOMIZE_BASE_LARGE) &&
> > +                MODULES_LEN + KERNEL_IMAGE_SIZE > 2*PUD_SIZE);
> >     BUILD_BUG_ON((__START_KERNEL_map & ~PMD_MASK) != 0);
> >     BUILD_BUG_ON((MODULES_VADDR & ~PMD_MASK) != 0);
> >     BUILD_BUG_ON(!(MODULES_VADDR > __START_KERNEL));
> > diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> > index 4d0a7e68bfe8..e8b2d6706eca 100644
> > --- a/arch/x86/kernel/head_64.S
> > +++ b/arch/x86/kernel/head_64.S
> > @@ -39,11 +39,15 @@
> >  
> >  #define p4d_index(x)       (((x) >> P4D_SHIFT) & (PTRS_PER_P4D-1))
> >  #define pud_index(x)       (((x) >> PUD_SHIFT) & (PTRS_PER_PUD-1))
> > +#define pud_count(x)   (((x + (PUD_SIZE - 1)) & ~(PUD_SIZE - 1)) >> 
> > PUD_SHIFT)
> >  
> >  PGD_PAGE_OFFSET = pgd_index(__PAGE_OFFSET_BASE)
> >  PGD_START_KERNEL = pgd_index(__START_KERNEL_map)
> >  L3_START_KERNEL = pud_index(__START_KERNEL_map)
> >  
> > +/* Adapt page table L3 space based on range of randomization */
> > +L3_KERNEL_ENTRY_COUNT = pud_count(KERNEL_IMAGE_SIZE)
> > +
> >     .text
> >     __HEAD
> >     .code64
> > @@ -396,7 +400,12 @@ NEXT_PAGE(level4_kernel_pgt)
> >  NEXT_PAGE(level3_kernel_pgt)
> >     .fill   L3_START_KERNEL,8,0
> >     /* (2^48-(2*1024*1024*1024)-((2^39)*511))/(2^30) = 510 */
> > -   .quad   level2_kernel_pgt - __START_KERNEL_map + _KERNPG_TABLE
> > +   i = 0
> > +   .rept   L3_KERNEL_ENTRY_COUNT
> > +   .quad   level2_kernel_pgt - __START_KERNEL_map + _KERNPG_TABLE \
> > +           + PAGE_SIZE*i
> > +   i = i + 1
> > +   .endr
> >     .quad   level2_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE
> >  
> >  NEXT_PAGE(level2_kernel_pgt)
> > -- 
> > 2.13.2.932.g7449e964c-goog
> > 

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