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

[Kees Cook <keescook@chromium.org>]

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>
---
 arch/x86/include/uapi/asm/kvm.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index e48deab8901d..8ec3dfd641b0 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -502,13 +502,13 @@ struct kvm_nested_state {
 	} hdr;
 
 	/*
-	 * Define data region as 0 bytes to preserve backwards-compatability
+	 * Define union of flexible arrays to preserve backwards-compatability
 	 * to old definition of kvm_nested_state in order to avoid changing
 	 * 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