Re: [PATCH v3 3/6] powerpc/fsl_booke/64: implement KASLR for fsl_booke64

From: Scott Wood <oss@buserror.net>
To: Jason Yan <yanaijie@huawei.com>,
	mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org,
	diana.craciun@nxp.com, christophe.leroy@c-s.fr,
	benh@kernel.crashing.org, paulus@samba.org, npiggin@gmail.com,
	keescook@chromium.org, kernel-hardening@lists.openwall.com
Cc: linux-kernel@vger.kernel.org, zhaohongjiang@huawei.com
Subject: Re: [PATCH v3 3/6] powerpc/fsl_booke/64: implement KASLR for fsl_booke64
Date: Wed, 04 Mar 2020 15:44:19 -0600	[thread overview]
Message-ID: <10913e48efea24c1d217bc5a723d6cd827945de7.camel@buserror.net> (raw)
In-Reply-To: <20200206025825.22934-4-yanaijie@huawei.com>

On Thu, 2020-02-06 at 10:58 +0800, Jason Yan wrote:
> The implementation for Freescale BookE64 is similar as BookE32. One
> difference is that Freescale BookE64 set up a TLB mapping of 1G during
> booting. Another difference is that ppc64 needs the kernel to be
> 64K-aligned. So we can randomize the kernel in this 1G mapping and make
> it 64K-aligned. This can save some code to creat another TLB map at
> early boot. The disadvantage is that we only have about 1G/64K = 16384
> slots to put the kernel in.
> 
> To support secondary cpu boot up, a variable __kaslr_offset was added in
> first_256B section. This can help secondary cpu get the kaslr offset
> before the 1:1 mapping has been setup.

What specifically requires __kaslr_offset instead of using kernstart_virt_addr
like 32-bit does?

>  
> diff --git a/arch/powerpc/kernel/head_64.S b/arch/powerpc/kernel/head_64.S
> index ad79fddb974d..744624140fb8 100644
> --- a/arch/powerpc/kernel/head_64.S
> +++ b/arch/powerpc/kernel/head_64.S
> @@ -104,6 +104,13 @@ __secondary_hold_acknowledge:
>  	.8byte	0x0
>  
>  #ifdef CONFIG_RELOCATABLE
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	. = 0x58
> +	.globl	__kaslr_offset
> +__kaslr_offset:
> +DEFINE_FIXED_SYMBOL(__kaslr_offset)
> +	.long	0
> +#endif
>  	/* This flag is set to 1 by a loader if the kernel should run
>  	 * at the loaded address instead of the linked address.  This
>  	 * is used by kexec-tools to keep the the kdump kernel in the

Why does it need to go here at a fixed address?

>  
>  	/* check for a reserved-memory node and record its cell sizes */
>  	regions.reserved_mem = fdt_path_offset(dt_ptr, "/reserved-memory");
> @@ -363,6 +374,17 @@ notrace void __init kaslr_early_init(void *dt_ptr,
> phys_addr_t size)
>  	unsigned long offset;
>  	unsigned long kernel_sz;
>  
> +#ifdef CONFIG_PPC64
> +	unsigned int *__kaslr_offset = (unsigned int *)(KERNELBASE + 0x58);
> +	unsigned int *__run_at_load = (unsigned int *)(KERNELBASE + 0x5c);

Why are you referencing these by magic offset rather than by symbol?

> +	/* Setup flat device-tree pointer */
> +	initial_boot_params = dt_ptr;
> +#endif

Why does 64-bit need this but 32-bit doesn't?

-Scott