Re: [PATCH v2 3/5] x86/boot/compressed/64: Check SEV encryption in 64-bit boot-path

[Arvind Sankar <nivedita@alum.mit.edu>]

On Tue, Oct 20, 2020 at 02:18:54PM +0200, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
> 
> Check whether the hypervisor reported the correct C-bit when running as
> an SEV guest. Using a wrong C-bit position could be used to leak
> sensitive data from the guest to the hypervisor.
> 
> The check function is in arch/x86/kernel/sev_verify_cbit.S so that it
> can be re-used in the running kernel image.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/boot/compressed/ident_map_64.c |  1 +
>  arch/x86/boot/compressed/mem_encrypt.S  |  4 ++
>  arch/x86/boot/compressed/misc.h         |  2 +
>  arch/x86/kernel/sev_verify_cbit.S       | 91 +++++++++++++++++++++++++
>  4 files changed, 98 insertions(+)
>  create mode 100644 arch/x86/kernel/sev_verify_cbit.S
> 
> diff --git a/arch/x86/boot/compressed/ident_map_64.c b/arch/x86/boot/compressed/ident_map_64.c
> index 063a60edcf99..73abba3312a7 100644
> --- a/arch/x86/boot/compressed/ident_map_64.c
> +++ b/arch/x86/boot/compressed/ident_map_64.c
> @@ -153,6 +153,7 @@ void initialize_identity_maps(void)
>  	 * into cr3.
>  	 */
>  	add_identity_map((unsigned long)_head, (unsigned long)_end);
> +	sev_verify_cbit(top_level_pgt);
>  	write_cr3(top_level_pgt);
>  }
>  
> diff --git a/arch/x86/boot/compressed/mem_encrypt.S b/arch/x86/boot/compressed/mem_encrypt.S
> index 2192b3bd78d8..7409f2343d38 100644
> --- a/arch/x86/boot/compressed/mem_encrypt.S
> +++ b/arch/x86/boot/compressed/mem_encrypt.S
> @@ -68,6 +68,9 @@ SYM_FUNC_START(get_sev_encryption_bit)
>  SYM_FUNC_END(get_sev_encryption_bit)
>  
>  	.code64
> +
> +#include "../../kernel/sev_verify_cbit.S"
> +
>  SYM_FUNC_START(set_sev_encryption_mask)
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>  	push	%rbp
> @@ -105,4 +108,5 @@ SYM_FUNC_END(set_sev_encryption_mask)
>  	.balign	8
>  SYM_DATA(sme_me_mask,		.quad 0)
>  SYM_DATA(sev_status,		.quad 0)
> +SYM_DATA(sev_check_data,	.quad 0)
>  #endif
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 6d31f1b4c4d1..53f4848ad392 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -159,4 +159,6 @@ void boot_page_fault(void);
>  void boot_stage1_vc(void);
>  void boot_stage2_vc(void);
>  
> +void sev_verify_cbit(unsigned long cr3);
> +
>  #endif /* BOOT_COMPRESSED_MISC_H */
> diff --git a/arch/x86/kernel/sev_verify_cbit.S b/arch/x86/kernel/sev_verify_cbit.S
> new file mode 100644
> index 000000000000..3f7153607956
> --- /dev/null
> +++ b/arch/x86/kernel/sev_verify_cbit.S
> @@ -0,0 +1,91 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + *	sev_verify_cbit.S - Code for verification of the C-bit position reported
> + *			    by the Hypervisor when running with SEV enabled.
> + *
> + *	Copyright (c) 2020  Joerg Roedel (jroedel@suse.de)
> + *
> + * Implements sev_verify_cbit() which is called before switching to a new
> + * long-mode page-table at boot.
> + *
> + * It verifies that the C-bit position is correct by writing a random value to
> + * an encrypted memory location while on the current page-table. Then it
> + * switches to the new page-table to verify the memory content is still the
> + * same. After that it switches back to the current page-table and when the
> + * check succeeded it returns. If the check failed the code invalidates the
> + * stack pointer and goes into a hlt loop. The stack-pointer is invalidated to
> + * make sure no interrupt or exception can get the CPU out of the hlt loop.
> + *
> + * New page-table pointer is expected in %rdi (first parameter)
> + *
> + */
> +SYM_FUNC_START(sev_verify_cbit)
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> +	/* First check if a C-bit was detected */
> +	movq	sme_me_mask(%rip), %r10
> +	testq	%r10, %r10
> +	jz	3f
> +
> +	/* sme_me_mask != 0 could mean SME or SEV - Check also for SEV */
> +	movq	sev_status(%rip), %r10
> +	testq	%r10, %r10
> +	jz	3f
> +
> +	/* Save CR4 in %r12 */
> +	pushq	%r12
> +	movq	%cr4, %r12
> +
> +	/* Disable Global Pages */
> +	pushq	%r12
> +	andq	$(~X86_CR4_PGE), %r12
> +	movq	%r12, %cr4
> +	popq	%r12
> +
> +	/*
> +	 * Verified that running under SEV - now get a random value using
> +	 * RDRAND. This instruction is mandatory when running as an SEV guest.
> +	 *
> +	 * Don't bail out of the loop if RDRAND returns errors. It is better to
> +	 * prevent forward progress than to work with a non-random value here.
> +	 */
> +1:	rdrand	%r10
> +	jnc	1b
> +
> +	/* Store value to memory and keep it in %r10 */
> +	movq	%r10, sev_check_data(%rip)
> +
> +	/* Backup current %cr3 value to restore it later */
> +	movq	%cr3, %r11
> +
> +	/* Switch to new %cr3 - This might unmap the stack */
> +	movq	%rdi, %cr3
> +
> +	/*
> +	 * Compare value in %r10 with memory location - If C-Bit is incorrect
> +	 * this would read the encrypted data and make the check fail.
> +	 */
> +	cmpq	%r10, sev_check_data(%rip)
> +
> +	/* Restore old %cr3 */
> +	movq	%r11, %cr3
> +
> +	/* Restore previous CR4 and %r12 */
> +	movq	%r12, %cr4
> +	popq	%r12
> +
> +	/* Check CMPQ result */
> +	je	3f
> +
> +	/*
> +	 * The check failed - Prevent any forward progress to prevent ROP
> +	 * attacks, invalidate the stack and go into a hlt loop.
> +	 */
> +	xorq	%rsp, %rsp
> +	subq	$0x1000, %rsp
> +2:	hlt
> +	jmp 2b
> +3:
> +#endif
> +	ret
> +SYM_FUNC_END(sev_verify_cbit)
> +
> -- 
> 2.28.0
> 

Why use r10-r12 rather than the caller-save registers? Even for the head
code where you need to perserve the cr3 value you can just return it in
rax?