Re: [PATCH] x86/mm: Create an SME workarea in the kernel for early encryption

[Baoquan He <bhe@redhat.com>]

On 06/12/19 at 01:32pm, Lendacky, Thomas wrote:
> The SME workarea used during early encryption of the kernel during boot
> is situated on a 2MB boundary after the end of the kernel text, data,
> etc. sections (_end).  This works well during initial boot of a compressed
> kernel because of the relocation used for decompression of the kernel.
> But when performing a kexec boot, there's a chance that the SME workarea
> may not be mapped by the kexec pagetables or that some of the other data
> used by kexec could exist in this range.
> 
> Create a section for SME in the vmlinux.lds.S.  Position it after "_end"
> so that the memory will be reclaimed during boot and, since it is all
> zeroes, it compresses well.  Since this new section will be part of the
> kernel, kexec will account for it in pagetable mappings and placement of
> data after the kernel.
> 
> Here's an example of a kernel size without and with the SME section:
> 	without:
> 		vmlinux:	36,501,616
> 		bzImage:	 6,497,344
> 
> 		100000000-47f37ffff : System RAM
> 		  1e4000000-1e47677d4 : Kernel code	(0x7677d4)
> 		  1e47677d5-1e4e2e0bf : Kernel data	(0x6c68ea)
> 		  1e5074000-1e5372fff : Kernel bss	(0x2fefff)
> 
> 	with:
> 		vmlinux:	44,419,408
> 		bzImage:	 6,503,136
> 
> 		880000000-c7ff7ffff : System RAM
> 		  8cf000000-8cf7677d4 : Kernel code	(0x7677d4)
> 		  8cf7677d5-8cfe2e0bf : Kernel data	(0x6c68ea)
> 		  8d0074000-8d0372fff : Kernel bss	(0x2fefff)
> 
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Lianbo Jiang <lijiang@redhat.com>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kernel/vmlinux.lds.S      | 16 ++++++++++++++++
>  arch/x86/mm/mem_encrypt_identity.c | 22 ++++++++++++++++++++--
>  2 files changed, 36 insertions(+), 2 deletions(-)

This patch looks good to me. 

It fixes an issue which breaks kexec/kdump kernel booting. And it can be
reproduced always when 'nokaslr' is added into kexec/kdump kernel
cmdline. Usually we suggest users to add 'nokaslr' to kdump kernel
cmdline, since KASLR functionality doesn't make any sense for kdump
kernel, and it can simplify to avoid unnecessary checking if kdump
kerenl malfunctions. So it's an important fix for sme supporting on
kexec/kdump.

Thanks for quick response and making this patch, Tom.

Reviewed-by: Baoquan He <bhe@redhat.com>

Thanks
Baoquan

> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index 0850b5149345..8c4377983e54 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -379,6 +379,22 @@ SECTIONS
>  	. = ALIGN(PAGE_SIZE);		/* keep VO_INIT_SIZE page aligned */
>  	_end = .;
>  
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	/*
> +	 * SME workarea section: Lives outside of the kernel proper
> +	 * (_text - _end) for performing in-place encryption. Resides
> +	 * on a 2MB boundary to simplify the pagetable setup used for
> +	 * the encryption.
> +	 */
> +	. = ALIGN(HPAGE_SIZE);
> +	.sme : AT(ADDR(.sme) - LOAD_OFFSET) {
> +		__sme_begin = .;
> +		*(.sme)
> +		. = ALIGN(HPAGE_SIZE);
> +		__sme_end = .;
> +	}
> +#endif
> +
>  	STABS_DEBUG
>  	DWARF_DEBUG
>  
> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
> index 4aa9b1480866..c55c2ec8fb12 100644
> --- a/arch/x86/mm/mem_encrypt_identity.c
> +++ b/arch/x86/mm/mem_encrypt_identity.c
> @@ -73,6 +73,19 @@ struct sme_populate_pgd_data {
>  	unsigned long vaddr_end;
>  };
>  
> +/*
> + * This work area lives in the .sme section, which lives outside of
> + * the kernel proper. It is sized to hold the intermediate copy buffer
> + * and more than enough pagetable pages.
> + *
> + * By using this section, the kernel can be encrypted in place and we
> + * avoid any possibility of boot parameters or initramfs images being
> + * placed such that the in-place encryption logic overwrites them.  This
> + * section is 2MB aligned to allow for simple pagetable setup using only
> + * PMD entries (see vmlinux.lds.S).
> + */
> +static char sme_workarea[2 * PMD_PAGE_SIZE] __section(.sme);
> +
>  static char sme_cmdline_arg[] __initdata = "mem_encrypt";
>  static char sme_cmdline_on[]  __initdata = "on";
>  static char sme_cmdline_off[] __initdata = "off";
> @@ -314,8 +327,13 @@ void __init sme_encrypt_kernel(struct boot_params *bp)
>  	}
>  #endif
>  
> -	/* Set the encryption workarea to be immediately after the kernel */
> -	workarea_start = kernel_end;
> +	/*
> +	 * We're running identity mapped, so we must obtain the address to the
> +	 * SME encryption workarea using rip-relative addressing.
> +	 */
> +	asm ("lea sme_workarea(%%rip), %0"
> +	     : "=r" (workarea_start)
> +	     : "p" (sme_workarea));
>  
>  	/*
>  	 * Calculate required number of workarea bytes needed:
> -- 
> 2.17.1
>