linux-kernel - Re: [PATCH] x86: fix kaslr and memmap collision

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Date:   Tue, 22 Nov 2016 09:47:54 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Dave Jiang <dave.jiang@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        dan.j.williams@...el.com, x86@...nel.org, david@...morbit.com,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] x86: fix kaslr and memmap collision


* Dave Jiang <dave.jiang@...el.com> wrote:

> CONFIG_RANDOMIZE_BASE relocates the kernel to a random base address.
> However it does not take into account the memmap= parameter passed in from
> the kernel commandline.

memmap= parameters are often used as a list.

> [...] This results in the kernel sometimes being put in the middle of the user 
> memmap. [...]

What does this mean? If memmap= is used to re-define the memory map then the 
kernel getting in the middle of a RAM area is what we want, isn't it? What we 
don't want is for the kernel to get into reserved areas, right?

> [...] Check has been added in the kaslr in order to avoid the region marked by 
> memmap.

What does this mean?

> Signed-off-by: Dave Jiang <dave.jiang@...el.com>
> ---
>  arch/x86/boot/boot.h             |    2 ++
>  arch/x86/boot/compressed/kaslr.c |   45 ++++++++++++++++++++++++++++++++++++++
>  arch/x86/boot/string.c           |   25 +++++++++++++++++++++
>  3 files changed, 72 insertions(+)
> 
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index e5612f3..0d5fe5b 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -332,6 +332,8 @@ int strncmp(const char *cs, const char *ct, size_t count);
>  size_t strnlen(const char *s, size_t maxlen);
>  unsigned int atou(const char *s);
>  unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int base);
> +unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base);
> +long simple_strtol(const char *cp, char **endp, unsigned int base);
>  size_t strlen(const char *s);
>  
>  /* tty.c */
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index a66854d..6fb8f1ec 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -11,6 +11,7 @@
>   */
>  #include "misc.h"
>  #include "error.h"
> +#include "../boot.h"
>  
>  #include <generated/compile.h>
>  #include <linux/module.h>
> @@ -61,6 +62,7 @@ enum mem_avoid_index {
>  	MEM_AVOID_INITRD,
>  	MEM_AVOID_CMDLINE,
>  	MEM_AVOID_BOOTPARAMS,
> +	MEM_AVOID_MEMMAP,
>  	MEM_AVOID_MAX,
>  };
>  
> @@ -77,6 +79,37 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
>  	return true;
>  }
>  
> +#include "../../../../lib/cmdline.c"
> +
> +static int
> +parse_memmap(char *p, unsigned long long *start, unsigned long long *size)
> +{
> +	char *oldp;
> +
> +	if (!p)
> +		return -EINVAL;
> +
> +	/* we don't care about this option here */
> +	if (!strncmp(p, "exactmap", 8))
> +		return -EINVAL;
> +
> +	oldp = p;
> +	*size = memparse(p, &p);
> +	if (p == oldp)
> +		return -EINVAL;
> +
> +	switch (*p) {
> +	case '@':
> +	case '#':
> +	case '$':
> +	case '!':
> +		*start = memparse(p+1, &p);
> +		return 0;
> +	}
> +
> +	return -EINVAL;
> +}
> +
>  /*
>   * In theory, KASLR can put the kernel anywhere in the range of [16M, 64T).
>   * The mem_avoid array is used to store the ranges that need to be avoided
> @@ -158,6 +191,8 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
>  	u64 initrd_start, initrd_size;
>  	u64 cmd_line, cmd_line_size;
>  	char *ptr;
> +	char arg[38];

Where does the magic '38' come from?

> +	unsigned long long memmap_start, memmap_size;
>  
>  	/*
>  	 * Avoid the region that is unsafe to overlap during
> @@ -195,6 +230,16 @@ static void mem_avoid_init(unsigned long input, unsigned long input_size,
>  	add_identity_map(mem_avoid[MEM_AVOID_BOOTPARAMS].start,
>  			 mem_avoid[MEM_AVOID_BOOTPARAMS].size);
>  
> +	/* see if we have any memmap areas */
> +	if (cmdline_find_option("memmap", arg, sizeof(arg)) > 0) {
> +		int rc = parse_memmap(arg, &memmap_start, &memmap_size);
> +
> +		if (!rc) {
> +			mem_avoid[MEM_AVOID_MEMMAP].start = memmap_start;
> +			mem_avoid[MEM_AVOID_MEMMAP].size = memmap_size;
> +		}
> +	}
> +

This only handles a single (first) memmap argument, is that sufficient?

Thanks,

	Ingo