Re: [PATCH] x86, kaslr: randomize module base load address

[Andrew Morton]

On Fri, 21 Feb 2014 12:21:10 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> From: Andy Honig <ahonig@xxxxxxxxxx>
> 
> Randomize the load address of modules in the kernel to make kASLR
> effective for modules.  Modules can only be loaded within a particular
> range of virtual address space.  This patch adds 10 bits of entropy to
> the load address by adding 1-1024 * PAGE_SIZE to the beginning range
> where modules are loaded.
> 
> Example kASLR boot without this change, with a single module loaded:
> ---[ Modules ]---
> 0xffffffffc0000000-0xffffffffc0001000           4K     ro     GLB x  pte
> 0xffffffffc0001000-0xffffffffc0002000           4K     ro     GLB NX pte
> 0xffffffffc0002000-0xffffffffc0004000           8K     RW     GLB NX pte
> 0xffffffffc0004000-0xffffffffc0200000        2032K                   pte
> 0xffffffffc0200000-0xffffffffff000000        1006M                   pmd
> ---[ End Modules ]---
> 
> Example kASLR boot after this change, same module loaded:
> ---[ Modules ]---
> 0xffffffffc0000000-0xffffffffc0200000           2M                   pmd
> 0xffffffffc0200000-0xffffffffc03bf000        1788K                   pte
> 0xffffffffc03bf000-0xffffffffc03c0000           4K     ro     GLB x  pte
> 0xffffffffc03c0000-0xffffffffc03c1000           4K     ro     GLB NX pte
> 0xffffffffc03c1000-0xffffffffc03c3000           8K     RW     GLB NX pte
> 0xffffffffc03c3000-0xffffffffc0400000         244K                   pte
> 0xffffffffc0400000-0xffffffffff000000        1004M                   pmd
> ---[ End Modules ]---
> 
> ...
>
> --- a/arch/x86/kernel/module.c
> +++ b/arch/x86/kernel/module.c
> @@ -28,6 +28,7 @@
>  #include <linux/mm.h>
>  #include <linux/gfp.h>
>  #include <linux/jump_label.h>
> +#include <linux/random.h>
>  
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
> @@ -43,13 +44,49 @@ do {							\
>  } while (0)
>  #endif
>  
> +#ifdef CONFIG_RANDOMIZE_BASE
> +static unsigned long module_load_offset;
> +static int randomize_modules = 1;

It's pretty common for people to later come back and say "hey I want to
set the default in Kconfig".  Perhaps we should do that from day 1.

This implies that parse_nokaslr() will need to be renamed and taught to
handle 0->1 changing.

> +static int __init parse_nokaslr(char *p)
> +{
> +	randomize_modules = 0;
> +	return 0;
> +}
> +early_param("nokaslr", parse_nokaslr);

Documentation/kernel-parameters.txt, please.   This isn't hard :(

> +static unsigned long int get_module_load_offset(void)
> +{
> +	if (randomize_modules) {
> +		mutex_lock(&module_mutex);
> +		/*
> +		 * Calculate the module_load_offset the first time this
> +		 * code is called. Once calculated it stays the same until
> +		 * reboot.
> +		 */
> +		if (module_load_offset == 0)
> +			module_load_offset =
> +				(get_random_int() % 1024 + 1) * PAGE_SIZE;
> +		mutex_unlock(&module_mutex);
> +	}
> +	return module_load_offset;
> +}

This seems unnecessarily complex and inefficient.  We only set
module_load_offset a single time, so why not do it that way? 
Mark it __init, run it during initcalls then throw it away.

> +#else
> +static unsigned long int get_module_load_offset(void)
> +{
> +	return 0;
> +}
> +#endif
> +
>  void *module_alloc(unsigned long size)
>  {
>  	if (PAGE_ALIGN(size) > MODULES_LEN)
>  		return NULL;
> -	return __vmalloc_node_range(size, 1, MODULES_VADDR, MODULES_END,
> -				GFP_KERNEL | __GFP_HIGHMEM, PAGE_KERNEL_EXEC,
> -				NUMA_NO_NODE, __builtin_return_address(0));
> +	return __vmalloc_node_range(size, 1,
> +				    MODULES_VADDR + get_module_load_offset(),
> +				    MODULES_END, GFP_KERNEL | __GFP_HIGHMEM,
> +				    PAGE_KERNEL_EXEC, NUMA_NO_NODE,
> +				    __builtin_return_address(0));
>  }
>  
>  #ifdef CONFIG_X86_32

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