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

[Baoquan He]

On 09/12/14 at 08:04am, Vivek Goyal wrote:
> 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.

Yes, will repost with changed description.

> 
> 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@xxxxxxxxxx>
> > ---
> >  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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