[PATCH] x86/efi: Don't require non-blocking EFI callbacks

[PATCH] x86/efi: Don't require non-blocking EFI callbacks

[Ross Lagerwall]

If a backend does not implement non-blocking EFI operations, it implies
that the normal operations are non-blocking. Instead of crashing
dereferencing a NULL pointer, fallback to the normal operations since it
is safe to do so.

Fixes: 5a58bc1b1edc ("efi/x86: Use non-blocking SetVariable() for efi_delete_dummy_variable()")
Fixes: ca0e30dcaa53 ("efi: Add nonblocking option to efi_query_variable_store()")
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
---
 arch/x86/platform/efi/quirks.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
index 3b9fd679cea9..4167f5e8f3e8 100644
--- a/arch/x86/platform/efi/quirks.c
+++ b/arch/x86/platform/efi/quirks.c
@@ -106,11 +106,13 @@ early_param("efi_no_storage_paranoia", setup_storage_paranoia);
 */
 void efi_delete_dummy_variable(void)
 {
-	efi.set_variable_nonblocking((efi_char16_t *)efi_dummy_name,
-				     &EFI_DUMMY_GUID,
-				     EFI_VARIABLE_NON_VOLATILE |
-				     EFI_VARIABLE_BOOTSERVICE_ACCESS |
-				     EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
+	efi_set_variable_t *set_variable = efi.set_variable_nonblocking ?:
+					   efi.set_variable;
+
+	set_variable((efi_char16_t *)efi_dummy_name, &EFI_DUMMY_GUID,
+		     EFI_VARIABLE_NON_VOLATILE |
+		     EFI_VARIABLE_BOOTSERVICE_ACCESS |
+		     EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
 }
 
 /*
@@ -127,10 +129,12 @@ query_variable_store_nonblocking(u32 attributes, unsigned long size)
 {
 	efi_status_t status;
 	u64 storage_size, remaining_size, max_size;
+	efi_query_variable_info_t *query_variable_info =
+		efi.query_variable_info_nonblocking ?:
+		efi.query_variable_info;
 
-	status = efi.query_variable_info_nonblocking(attributes, &storage_size,
-						     &remaining_size,
-						     &max_size);
+	status = query_variable_info(attributes, &storage_size,
+				     &remaining_size, &max_size);
 	if (status != EFI_SUCCESS)
 		return status;
 
-- 
2.21.0

Re: [PATCH] x86/efi: Don't require non-blocking EFI callbacks

[Ard Biesheuvel]

On Thu, 26 Sep 2019 at 16:12, Ross Lagerwall <ross.lagerwall@citrix.com> wrote:
>
> If a backend does not implement non-blocking EFI operations, it implies
> that the normal operations are non-blocking.

Is that documented anywhere?

> Instead of crashing
> dereferencing a NULL pointer, fallback to the normal operations since it
> is safe to do so.
>

I agree that crashing is never the right thing to do, but I wonder
whether we shouldn't just bail instead. If the provided default
operation is non-blocking, the platform can populate the function
pointer with a reference to the default implementation.


> Fixes: 5a58bc1b1edc ("efi/x86: Use non-blocking SetVariable() for efi_delete_dummy_variable()")
> Fixes: ca0e30dcaa53 ("efi: Add nonblocking option to efi_query_variable_store()")
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
>  arch/x86/platform/efi/quirks.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
> index 3b9fd679cea9..4167f5e8f3e8 100644
> --- a/arch/x86/platform/efi/quirks.c
> +++ b/arch/x86/platform/efi/quirks.c
> @@ -106,11 +106,13 @@ early_param("efi_no_storage_paranoia", setup_storage_paranoia);
>  */
>  void efi_delete_dummy_variable(void)
>  {
> -       efi.set_variable_nonblocking((efi_char16_t *)efi_dummy_name,
> -                                    &EFI_DUMMY_GUID,
> -                                    EFI_VARIABLE_NON_VOLATILE |
> -                                    EFI_VARIABLE_BOOTSERVICE_ACCESS |
> -                                    EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
> +       efi_set_variable_t *set_variable = efi.set_variable_nonblocking ?:
> +                                          efi.set_variable;
> +
> +       set_variable((efi_char16_t *)efi_dummy_name, &EFI_DUMMY_GUID,
> +                    EFI_VARIABLE_NON_VOLATILE |
> +                    EFI_VARIABLE_BOOTSERVICE_ACCESS |
> +                    EFI_VARIABLE_RUNTIME_ACCESS, 0, NULL);
>  }
>
>  /*
> @@ -127,10 +129,12 @@ query_variable_store_nonblocking(u32 attributes, unsigned long size)
>  {
>         efi_status_t status;
>         u64 storage_size, remaining_size, max_size;
> +       efi_query_variable_info_t *query_variable_info =
> +               efi.query_variable_info_nonblocking ?:
> +               efi.query_variable_info;
>
> -       status = efi.query_variable_info_nonblocking(attributes, &storage_size,
> -                                                    &remaining_size,
> -                                                    &max_size);
> +       status = query_variable_info(attributes, &storage_size,
> +                                    &remaining_size, &max_size);
>         if (status != EFI_SUCCESS)
>                 return status;
>
> --
> 2.21.0
>

Re: [PATCH] x86/efi: Don't require non-blocking EFI callbacks

[Ross Lagerwall]

On 9/26/19 4:29 PM, Ard Biesheuvel wrote:
> On Thu, 26 Sep 2019 at 16:12, Ross Lagerwall <ross.lagerwall@citrix.com> wrote:
>>
>> If a backend does not implement non-blocking EFI operations, it implies
>> that the normal operations are non-blocking.
> 
> Is that documented anywhere?

Sort of. From commit 6d80dba1c9fe "efi: Provide a non-blocking 
SetVariable() operation"

"""
Introduce ->set_variable_nonblocking() for this use case. It is an
optional EFI backend operation, and need only be implemented by those
backends that usually acquire locks to serialize access to EFI
variables, as is the case for virt_efi_set_variable() where we now grab
the EFI runtime spinlock.
"""

> 
>> Instead of crashing
>> dereferencing a NULL pointer, fallback to the normal operations since it
>> is safe to do so.
>>
> 
> I agree that crashing is never the right thing to do, but I wonder
> whether we shouldn't just bail instead. If the provided default
> operation is non-blocking, the platform can populate the function
> pointer with a reference to the default implementation.

If you would prefer it that platforms are always required to implement 
the non-blocking functions, then I will just send a simple patch fixing 
up the Xen implementation.

Thanks,
-- 
Ross Lagerwall

Re: [PATCH] x86/efi: Don't require non-blocking EFI callbacks

[Ard Biesheuvel]

On Thu, 26 Sep 2019 at 17:47, Ross Lagerwall <ross.lagerwall@citrix.com> wrote:
>
> On 9/26/19 4:29 PM, Ard Biesheuvel wrote:
> > On Thu, 26 Sep 2019 at 16:12, Ross Lagerwall <ross.lagerwall@citrix.com> wrote:
> >>
> >> If a backend does not implement non-blocking EFI operations, it implies
> >> that the normal operations are non-blocking.
> >
> > Is that documented anywhere?
>
> Sort of. From commit 6d80dba1c9fe "efi: Provide a non-blocking
> SetVariable() operation"
>
> """
> Introduce ->set_variable_nonblocking() for this use case. It is an
> optional EFI backend operation, and need only be implemented by those
> backends that usually acquire locks to serialize access to EFI
> variables, as is the case for virt_efi_set_variable() where we now grab
> the EFI runtime spinlock.
> """
>
> >
> >> Instead of crashing
> >> dereferencing a NULL pointer, fallback to the normal operations since it
> >> is safe to do so.
> >>
> >
> > I agree that crashing is never the right thing to do, but I wonder
> > whether we shouldn't just bail instead. If the provided default
> > operation is non-blocking, the platform can populate the function
> > pointer with a reference to the default implementation.
>
> If you would prefer it that platforms are always required to implement
> the non-blocking functions, then I will just send a simple patch fixing
> up the Xen implementation.
>

Yes, please, that sounds like a safer option to me.