Re: [PATCH v7 71/72] x86/efi: Add GHCB mappings when SEV-ES is active

[Borislav Petkov <bp@alien8.de>]

+ Ard so that he can ack the efi bits.

On Mon, Sep 07, 2020 at 03:16:12PM +0200, Joerg Roedel wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Calling down to EFI runtime services can result in the firmware performing
> VMGEXIT calls. The firmware is likely to use the GHCB of the OS (e.g., for
> setting EFI variables), so each GHCB in the system needs to be identity
> mapped in the EFI page tables, as unencrypted, to avoid page faults.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> [ jroedel@suse.de: Moved GHCB mapping loop to sev-es.c ]
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/boot/compressed/sev-es.c |  1 +
>  arch/x86/include/asm/sev-es.h     |  2 ++
>  arch/x86/kernel/sev-es.c          | 30 ++++++++++++++++++++++++++++++
>  arch/x86/platform/efi/efi_64.c    | 10 ++++++++++
>  4 files changed, 43 insertions(+)
> 
> diff --git a/arch/x86/boot/compressed/sev-es.c b/arch/x86/boot/compressed/sev-es.c
> index 45702b866c33..0a9a248ca33d 100644
> --- a/arch/x86/boot/compressed/sev-es.c
> +++ b/arch/x86/boot/compressed/sev-es.c
> @@ -12,6 +12,7 @@
>   */
>  #include "misc.h"
>  
> +#include <asm/pgtable_types.h>
>  #include <asm/sev-es.h>
>  #include <asm/trapnr.h>
>  #include <asm/trap_pf.h>
> diff --git a/arch/x86/include/asm/sev-es.h b/arch/x86/include/asm/sev-es.h
> index e919f09ae33c..cf1d957c7091 100644
> --- a/arch/x86/include/asm/sev-es.h
> +++ b/arch/x86/include/asm/sev-es.h
> @@ -102,11 +102,13 @@ static __always_inline void sev_es_nmi_complete(void)
>  	if (static_branch_unlikely(&sev_es_enable_key))
>  		__sev_es_nmi_complete();
>  }
> +extern int __init sev_es_efi_map_ghcbs(pgd_t *pgd);
>  #else
>  static inline void sev_es_ist_enter(struct pt_regs *regs) { }
>  static inline void sev_es_ist_exit(void) { }
>  static inline int sev_es_setup_ap_jump_table(struct real_mode_header *rmh) { return 0; }
>  static inline void sev_es_nmi_complete(void) { }
> +static inline int sev_es_efi_map_ghcbs(pgd_t *pgd) { return 0; }
>  #endif
>  
>  #endif
> diff --git a/arch/x86/kernel/sev-es.c b/arch/x86/kernel/sev-es.c
> index 9ab3a4dfecd8..4e2b7e4d9b87 100644
> --- a/arch/x86/kernel/sev-es.c
> +++ b/arch/x86/kernel/sev-es.c
> @@ -491,6 +491,36 @@ int sev_es_setup_ap_jump_table(struct real_mode_header *rmh)
>  	return 0;
>  }
>  
> +/*
> + * This is needed by the OVMF UEFI firmware which will use whatever it finds in
> + * the GHCB MSR as its GHCB to talk to the hypervisor. So make sure the per-cpu
> + * runtime GHCBs used by the kernel are also mapped in the EFI page-table.
> + */
> +int __init sev_es_efi_map_ghcbs(pgd_t *pgd)
> +{
> +	struct sev_es_runtime_data *data;
> +	unsigned long address, pflags;
> +	int cpu;
> +	u64 pfn;
> +
> +	if (!sev_es_active())
> +		return 0;
> +
> +	pflags = _PAGE_NX | _PAGE_RW;
> +
> +	for_each_possible_cpu(cpu) {
> +		data = per_cpu(runtime_data, cpu);
> +
> +		address = __pa(&data->ghcb_page);
> +		pfn = address >> PAGE_SHIFT;
> +
> +		if (kernel_map_pages_in_pgd(pgd, pfn, address, 1, pflags))
> +			return 1;
> +	}
> +
> +	return 0;
> +}
> +
>  static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>  {
>  	struct pt_regs *regs = ctxt->regs;
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index 6af4da1149ba..8f5759df7776 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -47,6 +47,7 @@
>  #include <asm/realmode.h>
>  #include <asm/time.h>
>  #include <asm/pgalloc.h>
> +#include <asm/sev-es.h>
>  
>  /*
>   * We allocate runtime services regions top-down, starting from -4G, i.e.
> @@ -229,6 +230,15 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
>  		return 1;
>  	}
>  
> +	/*
> +	 * When SEV-ES is active, the GHCB as set by the kernel will be used
> +	 * by firmware. Create a 1:1 unencrypted mapping for each GHCB.
> +	 */
> +	if (sev_es_efi_map_ghcbs(pgd)) {
> +		pr_err("Failed to create 1:1 mapping for the GHCBs!\n");
> +		return 1;
> +	}
> +
>  	/*
>  	 * When making calls to the firmware everything needs to be 1:1
>  	 * mapped and addressable with 32-bit pointers. Map the kernel
> -- 
> 2.28.0
> 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette