Re: [PATCH v5] x86: fix kaslr and memmap collision

[Baoquan He]

On 01/04/17 at 11:29am, Dave Jiang 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 cmdline. This results in the kernel sometimes being put in
> the middle of memmap. Teaching kaslr to not insert the kernel in
> memmap defined regions. We will support up to 4 memmap regions. Any
> additional regions will cause kaslr to disable. The mem_avoid set has
> been augmented to add up to 4 unusable regions of memmaps provided by the
> user to exclude those regions from the set of valid address range to insert
> the uncompressed kernel image. The nn@ss ranges will be skipped by the
> mem_avoid set since it indicates memory useable.
> 
> Signed-off-by: Dave Jiang <dave.jiang@xxxxxxxxx>
> ---
> 
> v2:
> Addressing comments from Ingo.
> - Handle entire list of memmaps
> v3:
> Fix 32bit build issue
> v4:
> Addressing comments from Baoquan
> - Not exclude nn@ss ranges
> v5:
> Addressing additional comments from Baoquan
> - Update commit header and various coding style changes
> 
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index e5612f3..59c2075 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -332,7 +332,10 @@ 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);
> +char *strchr(const char *s, int c);
>  
>  /* tty.c */
>  void puts(const char *);
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index a66854d..036b514 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>
> @@ -56,11 +57,16 @@ struct mem_vector {
>  	unsigned long size;
>  };
>  
> +/* only supporting at most 4 unusable memmap regions with kaslr */
> +#define MAX_MEMMAP_REGIONS	4
> +
>  enum mem_avoid_index {
>  	MEM_AVOID_ZO_RANGE = 0,
>  	MEM_AVOID_INITRD,
>  	MEM_AVOID_CMDLINE,
>  	MEM_AVOID_BOOTPARAMS,
> +	MEM_AVOID_MEMMAP_BEGIN,
> +	MEM_AVOID_MEMMAP_END = MEM_AVOID_MEMMAP_BEGIN + MAX_MEMMAP_REGIONS - 1,
>  	MEM_AVOID_MAX,
>  };
>  
> @@ -77,6 +83,121 @@ static bool mem_overlaps(struct mem_vector *one, struct mem_vector *two)
>  	return true;
>  }
>  
> +/**
> + *	_memparse - parse a string with mem suffixes into a number
> + *	@ptr: Where parse begins
> + *	@retptr: (output) Optional pointer to next char after parse completes
> + *
> + *	Parses a string into a number.  The number stored at @ptr is
> + *	potentially suffixed with K, M, G, T, P, E.
> + */
> +static unsigned long long _memparse(const char *ptr, char **retptr)
> +{
> +	char *endptr;	/* local pointer to end of parsed string */
> +
> +	unsigned long long ret = simple_strtoull(ptr, &endptr, 0);
> +
> +	switch (*endptr) {
> +	case 'E':
> +	case 'e':
> +		ret <<= 10;
> +	case 'P':
> +	case 'p':
> +		ret <<= 10;
> +	case 'T':
> +	case 't':
> +		ret <<= 10;
> +	case 'G':
> +	case 'g':
> +		ret <<= 10;
> +	case 'M':
> +	case 'm':
> +		ret <<= 10;
> +	case 'K':
> +	case 'k':
> +		ret <<= 10;
> +		endptr++;
> +	default:
> +		break;
> +	}
> +
> +	if (retptr)
> +		*retptr = endptr;
> +
> +	return ret;
> +}
> +
> +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 '@':
> +		/* skip this region, usable */
> +		*start = 0;
> +		*size = 0;
> +		return 0;
> +	case '#':
> +	case '$':
> +	case '!':
> +		*start = _memparse(p + 1, &p);
> +		return 0;
> +	}
> +
> +	return -EINVAL;
> +}
> +
> +static int mem_avoid_memmap(void)
> +{
> +	char arg[128];
> +	int rc = 0;
> +
> +	/* see if we have any memmap areas */
> +	if (cmdline_find_option("memmap", arg, sizeof(arg)) > 0) {
> +		int i = 0;
> +		char *str = arg;
> +
> +		while (str && (i < MAX_MEMMAP_REGIONS)) {
> +			unsigned long long start, size;
> +			char *k = strchr(str, ',');
> +
> +			if (k)
> +				*k++ = 0;
> +
> +			rc = parse_memmap(str, &start, &size);
> +			if (rc < 0)
> +				break;
> +			str = k;
> +			/* a usable region that should not be skipped */
> +			if (size == 0)
> +				continue;
> +
> +			mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].start = start;
> +			mem_avoid[MEM_AVOID_MEMMAP_BEGIN + i].size = size;
> +			i++;
> +		}
> +
> +		/* more than 4 memmaps, fail kaslr */
> +		if ((i >= MAX_MEMMAP_REGIONS) && str)
> +			rc = -EINVAL;
> +	}
> +
> +	return rc;
> +}
> +
>  /*
>   * 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
> @@ -438,6 +559,12 @@ void choose_random_location(unsigned long input,
>  		return;
>  	}
>  
> +	/* Mark the memmap regions we need to avoid */
> +	if (mem_avoid_memmap()) {
> +		warn("KASLR disabled: memmap exceeds limit of 4, giving up.");
> +		return;
> +	}

theoretically, mem_avoid_memmap is doing the mem_avoid initialization
job, should be called inside mem_avoid_init(). The reason you put it
here is you want to make it cancel kaslr, both physical and virtual
address randomization, right?

In choose_random_location(), the physical and virtual random are done
separately. You can see that later when find_random_phys_addr failed to
find a suitable random slot, it just prints a warning, virtual
randomization is still be done with calling find_random_virt_addr().
Avoiding memmap reserved region should be physical ram issue, should we
stop the kernel virtual address randomization either?

Kees, what do you think about this?

> +
>  	boot_params->hdr.loadflags |= KASLR_FLAG;
>  
>  	/* Prepare to add new identity pagetables on demand. */
> diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
> index cc3bd58..0464aaa 100644
> --- a/arch/x86/boot/string.c
> +++ b/arch/x86/boot/string.c
> @@ -122,6 +122,31 @@ unsigned long long simple_strtoull(const char *cp, char **endp, unsigned int bas
>  }
>  
>  /**
> + * simple_strtoul - convert a string to an unsigned long
> + * @cp: The start of the string
> + * @endp: A pointer to the end of the parsed string will be placed here
> + * @base: The number base to use
> + */
> +unsigned long simple_strtoul(const char *cp, char **endp, unsigned int base)
> +{
> +	return simple_strtoull(cp, endp, base);
> +}
> +
> +/**
> + * simple_strtol - convert a string to a signed long
> + * @cp: The start of the string
> + * @endp: A pointer to the end of the parsed string will be placed here
> + * @base: The number base to use
> + */
> +long simple_strtol(const char *cp, char **endp, unsigned int base)
> +{
> +	if (*cp == '-')
> +		return -simple_strtoul(cp + 1, endp, base);
> +
> +	return simple_strtoul(cp, endp, base);
> +}
> +
> +/**
>   * strlen - Find the length of a string
>   * @s: The string to be sized
>   */
> @@ -155,3 +180,16 @@ char *strstr(const char *s1, const char *s2)
>  	}
>  	return NULL;
>  }
> +
> +/**
> + * strchr - Find the first occurrence of the character c in the string s.
> + * @s: the string to be searched
> + * @c: the character to search for
> + */
> +char *strchr(const char *s, int c)
> +{
> +	while (*s != (char)c)
> +		if (*s++ == '\0')
> +			return NULL;
> +	return (char *)s;
> +}
>