[PATCH v2] KVM: x86: Replace 0-length arrays with flexible arrays

[PATCH v2] KVM: x86: Replace 0-length arrays with flexible arrays

[Kees Cook]

Zero-length arrays are deprecated[1]. Replace struct kvm_nested_state's
"data" union 0-length arrays with flexible arrays. (How are the
sizes of these arrays verified?) Detected with GCC 13, using
-fstrict-flex-arrays=3:

arch/x86/kvm/svm/nested.c: In function 'svm_get_nested_state':
arch/x86/kvm/svm/nested.c:1536:17: error: array subscript 0 is outside array bounds of 'struct kvm_svm_nested_state_data[0]' [-Werror=array-bounds=]
 1536 |                 &user_kvm_nested_state->data.svm[0];
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/uapi/linux/kvm.h:15,
                 from include/linux/kvm_host.h:40,
                 from arch/x86/kvm/svm/nested.c:18:
arch/x86/include/uapi/asm/kvm.h:511:50: note: while referencing 'svm'
  511 |                 struct kvm_svm_nested_state_data svm[0];
      |                                                  ^~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: kvm@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230105190548.never.323-kees@kernel.org
---
v2: add missing stddef.h include
v1: https://lore.kernel.org/all/20230105190548.never.323-kees@kernel.org/
---
 arch/x86/include/uapi/asm/kvm.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index e48deab8901d..bde47f3a8c9d 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -9,6 +9,7 @@
 
 #include <linux/types.h>
 #include <linux/ioctl.h>
+#include <linux/stddef.h>
 
 #define KVM_PIO_PAGE_OFFSET 1
 #define KVM_COALESCED_MMIO_PAGE_OFFSET 2
@@ -507,8 +508,8 @@ struct kvm_nested_state {
 	 * KVM_{GET,PUT}_NESTED_STATE ioctl values.
 	 */
 	union {
-		struct kvm_vmx_nested_state_data vmx[0];
-		struct kvm_svm_nested_state_data svm[0];
+		__DECLARE_FLEX_ARRAY(struct kvm_vmx_nested_state_data, vmx);
+		__DECLARE_FLEX_ARRAY(struct kvm_svm_nested_state_data, svm);
 	} data;
 };
 
-- 
2.34.1

Re: [PATCH v2] KVM: x86: Replace 0-length arrays with flexible arrays

[Sean Christopherson]

On Wed, 18 Jan 2023 11:59:09 -0800, Kees Cook wrote:
> Zero-length arrays are deprecated[1]. Replace struct kvm_nested_state's
> "data" union 0-length arrays with flexible arrays. (How are the
> sizes of these arrays verified?) Detected with GCC 13, using
> -fstrict-flex-arrays=3:
> 
> arch/x86/kvm/svm/nested.c: In function 'svm_get_nested_state':
> arch/x86/kvm/svm/nested.c:1536:17: error: array subscript 0 is outside array bounds of 'struct kvm_svm_nested_state_data[0]' [-Werror=array-bounds=]
>  1536 |                 &user_kvm_nested_state->data.svm[0];
>       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> In file included from include/uapi/linux/kvm.h:15,
>                  from include/linux/kvm_host.h:40,
>                  from arch/x86/kvm/svm/nested.c:18:
> arch/x86/include/uapi/asm/kvm.h:511:50: note: while referencing 'svm'
>   511 |                 struct kvm_svm_nested_state_data svm[0];
>       |                                                  ^~~
> 
> [...]

Applied to kvm-x86 misc, thanks!  Based on the linux-next complaint, I assume
you (temporarily?) applied this to your tree as well.  Holler if I've confused
you :-)

[1/1] KVM: x86: Replace 0-length arrays with flexible arrays
      https://github.com/kvm-x86/linux/commit/2a8c2de0544e

--
https://github.com/kvm-x86/linux/tree/next
https://github.com/kvm-x86/linux/tree/fixes

Re: [PATCH v2] KVM: x86: Replace 0-length arrays with flexible arrays

[Kees Cook]

On Thu, Jan 19, 2023 at 08:48:52PM +0000, Sean Christopherson wrote:
> On Wed, 18 Jan 2023 11:59:09 -0800, Kees Cook wrote:
> > Zero-length arrays are deprecated[1]. Replace struct kvm_nested_state's
> > "data" union 0-length arrays with flexible arrays. (How are the
> > sizes of these arrays verified?) Detected with GCC 13, using
> > -fstrict-flex-arrays=3:
> > 
> > arch/x86/kvm/svm/nested.c: In function 'svm_get_nested_state':
> > arch/x86/kvm/svm/nested.c:1536:17: error: array subscript 0 is outside array bounds of 'struct kvm_svm_nested_state_data[0]' [-Werror=array-bounds=]
> >  1536 |                 &user_kvm_nested_state->data.svm[0];
> >       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > In file included from include/uapi/linux/kvm.h:15,
> >                  from include/linux/kvm_host.h:40,
> >                  from arch/x86/kvm/svm/nested.c:18:
> > arch/x86/include/uapi/asm/kvm.h:511:50: note: while referencing 'svm'
> >   511 |                 struct kvm_svm_nested_state_data svm[0];
> >       |                                                  ^~~
> > 
> > [...]
> 
> Applied to kvm-x86 misc, thanks!  Based on the linux-next complaint, I assume
> you (temporarily?) applied this to your tree as well.  Holler if I've confused
> you :-)

Thanks! I've removed it from my tree.

-- 
Kees Cook