Re: [PATCH 2/2] x86/KASLR: Use old ident map page table if physical randomization failed

[Baoquan He]

On 04/26/17 at 06:39pm, Baoquan He wrote:
> Dave found when kdump kernel will reset to bios immediately if kaslr
	      ^~ this 'when' is redundent, sorry 
> is enabled and physical randomization failed to faind a new position
> for kernel. But nokaslr works in this case.
> 
> The reason is kaslr will install a new page table for ident mapping,
> while it missed to consider building ident mapping for original area
> of kernel if kaslr failed on physical randomization.
> 
> In fact bootloaders including kexec/kdump have built ident mapping
> for original place of kernel. We can only install new ident mapping
> page table when physical kaslr succeeds. Otherwise we just keep the
> old page table unchanged just like nokaslr does.
> 
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Dave Jiang <dave.jiang@xxxxxxxxx>
> Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/kaslr.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index e5eb0c3..7a8b443 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -650,10 +650,16 @@ void choose_random_location(unsigned long input,
>  			add_identity_map(random_addr, output_size);
>  			*output = random_addr;
>  		}
> +
> +		/*
> +		 * This actually loads the identity pagetable on x86_64.
> +		 * And this should only be done only if a new position
> +		 * is found. Otherwise we should keep the old page table
> +		 * to make it be like nokaslr case.
> +		 */
> +		finalize_identity_maps();
>  	}
>  
> -	/* This actually loads the identity pagetable on x86_64. */
> -	finalize_identity_maps();
>  
>  	/* Pick random virtual address starting from LOAD_PHYSICAL_ADDR. */
>  	if (IS_ENABLED(CONFIG_X86_64))
> -- 
> 2.5.5
>