[kernel-hardening] Re: [PATCH v1] kexec/arch/i386: Add support for KASLR memory randomization

[Dave Young <dyoung@redhat.com>]

Hi, Thomas

On 08/17/16 at 09:47am, Thomas Garnier wrote:
> Multiple changes were made on KASLR (right now in linux-next). One of
> them is randomizing the virtual address of the physical mapping, vmalloc
> and vmemmap memory sections. It breaks kdump ability to read physical
> memory.

What is the user visible behavior without this patch? Could you add more
in the patch log?

During my testing seems with or without this patch kdump kernel boot
both fine.

My kernel config options is like below, is it enough to test this patch?
CONFIG_RANDOMIZE_BASE=y
CONFIG_X86_NEED_RELOCS=y
CONFIG_PHYSICAL_ALIGN=0x1000000
CONFIG_RANDOMIZE_MEMORY=y
CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING=0x0

> 
> This change identifies if KASLR memories randomization is used by
> checking if the page_offset_base variable exists. It search for the
> correct PAGE_OFFSET value by looking at the loaded memory section and
> find the lowest aligned on PUD (the randomization level).
> 
> Related commits on linux-next:
>  - 0483e1fa6e09d4948272680f691dccb1edb9677f: Base for randomization
>  - 021182e52fe01c1f7b126f97fd6ba048dc4234fd: Enable for PAGE_OFFSET
> 
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
>  kexec/arch/i386/crashdump-x86.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/kexec/arch/i386/crashdump-x86.c b/kexec/arch/i386/crashdump-x86.c
> index bbc0f35..ab833d4 100644
> --- a/kexec/arch/i386/crashdump-x86.c
> +++ b/kexec/arch/i386/crashdump-x86.c
> @@ -102,11 +102,10 @@ static int get_kernel_paddr(struct kexec_info *UNUSED(info),
>  	return -1;
>  }
>  
> -/* Retrieve kernel _stext symbol virtual address from /proc/kallsyms */
> -static unsigned long long get_kernel_stext_sym(void)
> +/* Retrieve kernel symbol virtual address from /proc/kallsyms */
> +static unsigned long long get_kernel_sym(const char *symbol)

It sounds better to split this to another patch.

>  {
>  	const char *kallsyms = "/proc/kallsyms";
> -	const char *stext = "_stext";
>  	char sym[128];
>  	char line[128];
>  	FILE *fp;
> @@ -122,13 +121,13 @@ static unsigned long long get_kernel_stext_sym(void)
>  	while(fgets(line, sizeof(line), fp) != NULL) {
>  		if (sscanf(line, "%Lx %c %s", &vaddr, &type, sym) != 3)
>  			continue;
> -		if (strcmp(sym, stext) == 0) {
> -			dbgprintf("kernel symbol %s vaddr = %16llx\n", stext, vaddr);
> +		if (strcmp(sym, symbol) == 0) {
> +			dbgprintf("kernel symbol %s vaddr = %16llx\n", symbol, vaddr);
>  			return vaddr;
>  		}
>  	}
>  
> -	fprintf(stderr, "Cannot get kernel %s symbol address\n", stext);
> +	fprintf(stderr, "Cannot get kernel %s symbol address\n", symbol);
>  	return 0;
>  }
>  
> @@ -151,6 +150,8 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
>  	off_t size;
>  	uint32_t elf_flags = 0;
>  	uint64_t stext_sym;
> +	const unsigned long long pud_mask = ~((1 << 30) - 1);
> +	unsigned long long vaddr, lowest_vaddr = 0;
>  
>  	if (elf_info->machine != EM_X86_64)
>  		return 0;
> @@ -180,9 +181,23 @@ static int get_kernel_vaddr_and_size(struct kexec_info *UNUSED(info),
>  
>  	end_phdr = &ehdr.e_phdr[ehdr.e_phnum];
>  
> +	/* Search for the real PAGE_OFFSET when KASLR memory randomization
> +	 * is enabled */
> +	if (get_kernel_sym("page_offset_base") != 0) {
> +		for(phdr = ehdr.e_phdr; phdr != end_phdr; phdr++) {
> +			if (phdr->p_type == PT_LOAD) {
> +				vaddr = phdr->p_vaddr & pud_mask;
> +				if (lowest_vaddr == 0 || lowest_vaddr > vaddr)
> +					lowest_vaddr = vaddr;
> +			}
> +		}
> +		if (lowest_vaddr != 0)
> +			elf_info->page_offset = lowest_vaddr;
> +	}
> +
>  	/* Traverse through the Elf headers and find the region where
>  	 * _stext symbol is located in. That's where kernel is mapped */
> -	stext_sym = get_kernel_stext_sym();
> +	stext_sym = get_kernel_sym("_stext");
>  	for(phdr = ehdr.e_phdr; stext_sym && phdr != end_phdr; phdr++) {
>  		if (phdr->p_type == PT_LOAD) {
>  			unsigned long long saddr = phdr->p_vaddr;
> -- 
> 2.8.0.rc3.226.g39d4020
> 
> 
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Thanks
Dave