On 10/28/20 11:46 AM, Joerg Roedel wrote:
> From: Joerg Roedel <[email protected]>
>
> 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 <[email protected]>
Reviewed-by: Tom Lendacky <[email protected]>
> ---
> 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);
>