Re: [PATCH v4 4/5] x86/head/64: Check SEV encryption before switching to kernel page-table

[Tom Lendacky]

On 10/28/20 11:46 AM, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> When SEV is enabled the kernel requests the C-Bit position again from
> the hypervisor to built its own page-table. Since the hypervisor is an

s/built/build/

> untrusted source the C-bit position needs to be verified before the
> kernel page-table is used.
> 
> Call the sev_verify_cbit() function before writing the CR3.
> 
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>  arch/x86/kernel/head_64.S | 16 ++++++++++++++++
>  arch/x86/mm/mem_encrypt.c |  1 +
>  2 files changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> index 7eb2a1c87969..3c417734790f 100644
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -161,6 +161,21 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>  
>  	/* Setup early boot stage 4-/5-level pagetables. */
>  	addq	phys_base(%rip), %rax
> +
> +	/*
> +	 * For SEV guests: Verify that the C-bit is correct. A malicious
> +	 * hypervisor could lie about the C-bit position to perform a ROP
> +	 * attack on the guest by writing to the unencrypted stack and wait for
> +	 * the next RET instruction.
> +	 * %rsi carries pointer to realmode data and is callee-clobbered. Save
> +	 * and restore it.
> +	 */
> +	pushq	%rsi
> +	movq	%rax, %rdi
> +	call	sev_verify_cbit
> +	popq	%rsi
> +
> +	/* Switch to new page-table */
>  	movq	%rax, %cr3
>  
>  	/* Ensure I am executing from virtual addresses */
> @@ -279,6 +294,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
>  SYM_CODE_END(secondary_startup_64)
>  
>  #include "verify_cpu.S"
> +#include "sev_verify_cbit.S"
>  
>  #ifdef CONFIG_HOTPLUG_CPU
>  /*
> diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
> index efbb3de472df..bc0833713be9 100644
> --- a/arch/x86/mm/mem_encrypt.c
> +++ b/arch/x86/mm/mem_encrypt.c
> @@ -39,6 +39,7 @@
>   */
>  u64 sme_me_mask __section(".data") = 0;
>  u64 sev_status __section(".data") = 0;
> +u64 sev_check_data __section(".data") = 0;
>  EXPORT_SYMBOL(sme_me_mask);
>  DEFINE_STATIC_KEY_FALSE(sev_enable_key);
>  EXPORT_SYMBOL_GPL(sev_enable_key);
>