linux-kernel - Re: [Patch v2 1/2] kaslr: check if kernel location is changed

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Date:	Fri, 12 Sep 2014 08:04:47 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Baoquan He <bhe@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	ebiederm@...ssion.com, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, keescook@...omium.org, ak@...ux.intel.com,
	kumagai-atsushi@....nes.nec.co.jp
Subject: Re: [Patch v2 1/2] kaslr: check if kernel location is changed

On Fri, Sep 12, 2014 at 02:20:31PM +0800, Baoquan He wrote:
> Function handle_relocations() is used to do the relocations handling
> for i686 and kaslr of x86_64. For 32 bit the relocation handling is
> mandotary to perform. For x86_64 only when kaslr is enabled and a
> random kernel location is chosen successfully the relocation handling
> shound be done. However previous implementation only compared the
> kernel loading address and LOAD_PHYSICAL_ADDR where kernel were
> compiled to run at.

> This would casue system to hang when kernel
> loading address is not equal to LOAD_PHYSICAL_ADDR.

I don't think above is correct. It will not always fails. It will fail
only in few conditions like when delta between load address and compiled
address is bigger than what 32bit signed relocations can handle. 

Also there will be limitations that delta can't be too big otherwise
kernel text virtual addresses will overflow in module address space.

Thanks
Vivek
> 
> So in this patch check if kernel location is changed after
> choose_kernel_location() when x86_64. If and only if in x86_64
> and kernel location is changed, we say a kaslr random kernel
> location is chosen, then the relocation handling need be done.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
>  arch/x86/boot/compressed/misc.c | 26 ++++++++++++++++++++++----
>  1 file changed, 22 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 57ab74d..3bb2a17 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -230,8 +230,9 @@ static void error(char *x)
>  		asm("hlt");
>  }
>  
> -#if CONFIG_X86_NEED_RELOCS
> -static void handle_relocations(void *output, unsigned long output_len)
> +#ifdef CONFIG_X86_NEED_RELOCS
> +static void handle_relocations(void *output_orig, void *output,
> +			       unsigned long output_len)
>  {
>  	int *reloc;
>  	unsigned long delta, map, ptr;
> @@ -239,6 +240,20 @@ static void handle_relocations(void *output, unsigned long output_len)
>  	unsigned long max_addr = min_addr + output_len;
>  
>  	/*
> +	* 32bit always requires relocations to be performed. For x86_64,
> +	* relocations need to be performed only if kaslr has chosen a
> +	* different load address then kernel was originally loaded at.
> +	*
> +	* If we are here, either kaslr is not configured in or kaslr is disabled
> +	* or kaslr has chosen not to change the load location of kernel. Don't
> +	* perform any relocations.
> +	*/
> +#if CONFIG_X86_64
> +	if (output_orig == output)
> +		return;
> +#endif
> +
> +	/*
>  	 * Calculate the delta between where vmlinux was linked to load
>  	 * and where it was actually loaded.
>  	 */
> @@ -299,7 +314,8 @@ static void handle_relocations(void *output, unsigned long output_len)
>  #endif
>  }
>  #else
> -static inline void handle_relocations(void *output, unsigned long output_len)
> +static inline void handle_relocations(void *output_orig, void *output,
> +				      unsigned long output_len)
>  { }
>  #endif
>  
> @@ -360,6 +376,8 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
>  				  unsigned char *output,
>  				  unsigned long output_len)
>  {
> +	unsigned char *output_orig = output;
> +
>  	real_mode = rmode;
>  
>  	sanitize_boot_params(real_mode);
> @@ -402,7 +420,7 @@ asmlinkage __visible void *decompress_kernel(void *rmode, memptr heap,
>  	debug_putstr("\nDecompressing Linux... ");
>  	decompress(input_data, input_len, NULL, NULL, output, NULL, error);
>  	parse_elf(output);
> -	handle_relocations(output, output_len);
> +	handle_relocations(output_orig, output, output_len);
>  	debug_putstr("done.\nBooting the kernel.\n");
>  	return output;
>  }
> -- 
> 1.8.5.3
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