[PATCH] x86/sev-es: Invalidate the GHCB after completing VMGEXIT

[PATCH] x86/sev-es: Invalidate the GHCB after completing VMGEXIT

[Tom Lendacky]

Since the VMGEXIT instruction can be issued from userspace, invalidate
the GHCB after performing VMGEXIT processing in the kernel.

Invalidation is only required after userspace is available, so call
vc_ghcb_invalidate() from sev_es_put_ghcb(). Update vc_ghcb_invalidate()
to additionally clear the GHCB exit code, so that a value of 0 is always
present outside VMGEXIT processing in the kernel.

Since vc_ghcb_invalidate() is part of sev-shared.c, move sev_es_put_ghcb()
down to after where sev-shared.c is included.

Fixes: 0786138c78e79 ("x86/sev-es: Add a Runtime #VC Exception Handler")
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/kernel/sev-shared.c |  1 +
 arch/x86/kernel/sev.c        | 37 ++++++++++++++++++------------------
 2 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 6ec8b3bfd76e..9f90f460a28c 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -63,6 +63,7 @@ static bool sev_es_negotiate_protocol(void)
 
 static __always_inline void vc_ghcb_invalidate(struct ghcb *ghcb)
 {
+	ghcb->save.sw_exit_code = 0;
 	memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
 }
 
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index 9578c82832aa..5ccb0218c885 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -221,24 +221,6 @@ static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
 	return ghcb;
 }
 
-static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
-{
-	struct sev_es_runtime_data *data;
-	struct ghcb *ghcb;
-
-	data = this_cpu_read(runtime_data);
-	ghcb = &data->ghcb_page;
-
-	if (state->ghcb) {
-		/* Restore GHCB from Backup */
-		*ghcb = *state->ghcb;
-		data->backup_ghcb_active = false;
-		state->ghcb = NULL;
-	} else {
-		data->ghcb_active = false;
-	}
-}
-
 /* Needed in vc_early_forward_exception */
 void do_early_exception(struct pt_regs *regs, int trapnr);
 
@@ -461,6 +443,25 @@ static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt
 /* Include code shared with pre-decompression boot stage */
 #include "sev-shared.c"
 
+static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
+{
+	struct sev_es_runtime_data *data;
+	struct ghcb *ghcb;
+
+	data = this_cpu_read(runtime_data);
+	ghcb = &data->ghcb_page;
+
+	if (state->ghcb) {
+		/* Restore GHCB from Backup */
+		*ghcb = *state->ghcb;
+		data->backup_ghcb_active = false;
+		state->ghcb = NULL;
+	} else {
+		vc_ghcb_invalidate(ghcb);
+		data->ghcb_active = false;
+	}
+}
+
 void noinstr __sev_es_nmi_complete(void)
 {
 	struct ghcb_state state;
-- 
2.31.0

Re: [PATCH] x86/sev-es: Invalidate the GHCB after completing VMGEXIT

[Tom Lendacky]

On 5/14/21 2:12 PM, Tom Lendacky wrote:
> Since the VMGEXIT instruction can be issued from userspace, invalidate
> the GHCB after performing VMGEXIT processing in the kernel.
> 
> Invalidation is only required after userspace is available, so call
> vc_ghcb_invalidate() from sev_es_put_ghcb(). Update vc_ghcb_invalidate()
> to additionally clear the GHCB exit code, so that a value of 0 is always
> present outside VMGEXIT processing in the kernel.
> 
> Since vc_ghcb_invalidate() is part of sev-shared.c, move sev_es_put_ghcb()
> down to after where sev-shared.c is included.
> 
> Fixes: 0786138c78e79 ("x86/sev-es: Add a Runtime #VC Exception Handler")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/kernel/sev-shared.c |  1 +
>  arch/x86/kernel/sev.c        | 37 ++++++++++++++++++------------------
>  2 files changed, 20 insertions(+), 18 deletions(-)

I was debating whether to do this as two patches, with the first patch
moving sev_es_put_ghcb() and then the second patch would more clearly show
the changes related to the invalidation.

Let me know if that would be preferred and I'll re-submit this as a
two-patch series.

Thanks,
Tom

> 
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 6ec8b3bfd76e..9f90f460a28c 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -63,6 +63,7 @@ static bool sev_es_negotiate_protocol(void)
>  
>  static __always_inline void vc_ghcb_invalidate(struct ghcb *ghcb)
>  {
> +	ghcb->save.sw_exit_code = 0;
>  	memset(ghcb->save.valid_bitmap, 0, sizeof(ghcb->save.valid_bitmap));
>  }
>  
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 9578c82832aa..5ccb0218c885 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -221,24 +221,6 @@ static __always_inline struct ghcb *sev_es_get_ghcb(struct ghcb_state *state)
>  	return ghcb;
>  }
>  
> -static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
> -{
> -	struct sev_es_runtime_data *data;
> -	struct ghcb *ghcb;
> -
> -	data = this_cpu_read(runtime_data);
> -	ghcb = &data->ghcb_page;
> -
> -	if (state->ghcb) {
> -		/* Restore GHCB from Backup */
> -		*ghcb = *state->ghcb;
> -		data->backup_ghcb_active = false;
> -		state->ghcb = NULL;
> -	} else {
> -		data->ghcb_active = false;
> -	}
> -}
> -
>  /* Needed in vc_early_forward_exception */
>  void do_early_exception(struct pt_regs *regs, int trapnr);
>  
> @@ -461,6 +443,25 @@ static enum es_result vc_slow_virt_to_phys(struct ghcb *ghcb, struct es_em_ctxt
>  /* Include code shared with pre-decompression boot stage */
>  #include "sev-shared.c"
>  
> +static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
> +{
> +	struct sev_es_runtime_data *data;
> +	struct ghcb *ghcb;
> +
> +	data = this_cpu_read(runtime_data);
> +	ghcb = &data->ghcb_page;
> +
> +	if (state->ghcb) {
> +		/* Restore GHCB from Backup */
> +		*ghcb = *state->ghcb;
> +		data->backup_ghcb_active = false;
> +		state->ghcb = NULL;
> +	} else {
> +		vc_ghcb_invalidate(ghcb);
> +		data->ghcb_active = false;
> +	}
> +}
> +
>  void noinstr __sev_es_nmi_complete(void)
>  {
>  	struct ghcb_state state;
> 

Re: [PATCH] x86/sev-es: Invalidate the GHCB after completing VMGEXIT

[Joerg Roedel]

Hi Tom,

On Fri, May 14, 2021 at 02:12:33PM -0500, Tom Lendacky wrote:
>  arch/x86/kernel/sev-shared.c |  1 +
>  arch/x86/kernel/sev.c        | 37 ++++++++++++++++++------------------
>  2 files changed, 20 insertions(+), 18 deletions(-)

Having this change in one patch is okay. No need to split it up.

> +static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
> +{
> +	struct sev_es_runtime_data *data;
> +	struct ghcb *ghcb;
> +
> +	data = this_cpu_read(runtime_data);
> +	ghcb = &data->ghcb_page;
> +
> +	if (state->ghcb) {
> +		/* Restore GHCB from Backup */
> +		*ghcb = *state->ghcb;
> +		data->backup_ghcb_active = false;
> +		state->ghcb = NULL;
> +	} else {
> +		vc_ghcb_invalidate(ghcb);

A comment would be good to explain why the invalidate here is
necessary.

Regards,

	Joerg

Re: [PATCH] x86/sev-es: Invalidate the GHCB after completing VMGEXIT

[Tom Lendacky]

On 5/17/21 7:33 AM, Joerg Roedel wrote:
> Hi Tom,
> 
> On Fri, May 14, 2021 at 02:12:33PM -0500, Tom Lendacky wrote:
>>  arch/x86/kernel/sev-shared.c |  1 +
>>  arch/x86/kernel/sev.c        | 37 ++++++++++++++++++------------------
>>  2 files changed, 20 insertions(+), 18 deletions(-)
> 
> Having this change in one patch is okay. No need to split it up.
> 
>> +static __always_inline void sev_es_put_ghcb(struct ghcb_state *state)
>> +{
>> +	struct sev_es_runtime_data *data;
>> +	struct ghcb *ghcb;
>> +
>> +	data = this_cpu_read(runtime_data);
>> +	ghcb = &data->ghcb_page;
>> +
>> +	if (state->ghcb) {
>> +		/* Restore GHCB from Backup */
>> +		*ghcb = *state->ghcb;
>> +		data->backup_ghcb_active = false;
>> +		state->ghcb = NULL;
>> +	} else {
>> +		vc_ghcb_invalidate(ghcb);
> 
> A comment would be good to explain why the invalidate here is
> necessary.

Ah, good point. I'll add that and send a v2, but I'll wait for further
feedback before sending the next version.

Thanks,
Tom

> 
> Regards,
> 
> 	Joerg
>