virtchnl: fix fake 1-elem arrays in structs allocated as nents + 1 - 1

The two most problematic virtchnl structures are virtchnl_rss_key and
virtchnl_rss_lut. Their "flex" arrays have the type of u8, thus, when
allocating / checking, the actual size is calculated as `sizeof +
nents - 1 byte`. But their sizeof() is not 1 byte larger than the size
of such structure with proper flex array, it's two bytes larger due to
the padding. That said, their size is always 1 byte larger unless
there are no tail elements -- then it's +2 bytes.
Add virtchnl_struct_size() macro which will handle this case (and later
other cases as well). Make its calling conv the same as we call
struct_size() to allow it to be drop-in, even though it's unlikely to
become possible to switch to generic API. The macro will calculate a
proper size of a structure with a flex array at the end, so that it
becomes transparent for the compilers, but add the difference from the
old values, so that the real size of sorta-ABI-messages doesn't change.
Use it on the allocation side in IAVF and the receiving side (defined
as static inline in virtchnl.h) for the mentioned two structures.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/iavf/iavf_virtchnl.c

@@ -1085,8 +1085,7 @@ void iavf_set_rss_key(struct iavf_adapter *adapter)
 			adapter->current_op);
 		return;
 	}
-	len = sizeof(struct virtchnl_rss_key) +
-	      (adapter->rss_key_size * sizeof(u8)) - 1;
+	len = virtchnl_struct_size(vrk, key, adapter->rss_key_size);
 	vrk = kzalloc(len, GFP_KERNEL);
 	if (!vrk)
 		return;
@@ -1117,8 +1116,7 @@ void iavf_set_rss_lut(struct iavf_adapter *adapter)
 			adapter->current_op);
 		return;
 	}
-	len = sizeof(struct virtchnl_rss_lut) +
-	      (adapter->rss_lut_size * sizeof(u8)) - 1;
+	len = virtchnl_struct_size(vrl, lut, adapter->rss_lut_size);
 	vrl = kzalloc(len, GFP_KERNEL);
 	if (!vrl)
 		return;

include/linux/avf/virtchnl.h

@@ -866,18 +866,20 @@ VIRTCHNL_CHECK_STRUCT_LEN(4, virtchnl_promisc_info);
 struct virtchnl_rss_key {
 	u16 vsi_id;
 	u16 key_len;
-	u8 key[1];         /* RSS hash key, packed bytes */
+	u8 key[];          /* RSS hash key, packed bytes */
 };
 
-VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_key);
+VIRTCHNL_CHECK_STRUCT_LEN(4, virtchnl_rss_key);
+#define virtchnl_rss_key_LEGACY_SIZEOF	6
 
 struct virtchnl_rss_lut {
 	u16 vsi_id;
 	u16 lut_entries;
-	u8 lut[1];        /* RSS lookup table */
+	u8 lut[];         /* RSS lookup table */
 };
 
-VIRTCHNL_CHECK_STRUCT_LEN(6, virtchnl_rss_lut);
+VIRTCHNL_CHECK_STRUCT_LEN(4, virtchnl_rss_lut);
+#define virtchnl_rss_lut_LEGACY_SIZEOF	6
 
 /* VIRTCHNL_OP_GET_RSS_HENA_CAPS
  * VIRTCHNL_OP_SET_RSS_HENA
@@ -1367,6 +1369,17 @@ struct virtchnl_fdir_del {
 
 VIRTCHNL_CHECK_STRUCT_LEN(12, virtchnl_fdir_del);
 
+#define __vss_byone(p, member, count, old)				      \
+	(struct_size(p, member, count) + (old - 1 - struct_size(p, member, 0)))
+
+#define __vss(type, func, p, member, count)		\
+	struct type: func(p, member, count, type##_LEGACY_SIZEOF)
+
+#define virtchnl_struct_size(p, m, c)					      \
+	_Generic(*p,							      \
+		 __vss(virtchnl_rss_key, __vss_byone, p, m, c),		      \
+		 __vss(virtchnl_rss_lut, __vss_byone, p, m, c))
+
 /**
  * virtchnl_vc_validate_vf_msg
  * @ver: Virtchnl version info
@@ -1479,19 +1492,21 @@ virtchnl_vc_validate_vf_msg(struct virtchnl_version_info *ver, u32 v_opcode,
 		}
 		break;
 	case VIRTCHNL_OP_CONFIG_RSS_KEY:
-		valid_len = sizeof(struct virtchnl_rss_key);
+		valid_len = virtchnl_rss_key_LEGACY_SIZEOF;
 		if (msglen >= valid_len) {
 			struct virtchnl_rss_key *vrk =
 				(struct virtchnl_rss_key *)msg;
-			valid_len += vrk->key_len - 1;
+			valid_len = virtchnl_struct_size(vrk, key,
+							 vrk->key_len);
 		}
 		break;
 	case VIRTCHNL_OP_CONFIG_RSS_LUT:
-		valid_len = sizeof(struct virtchnl_rss_lut);
+		valid_len = virtchnl_rss_lut_LEGACY_SIZEOF;
 		if (msglen >= valid_len) {
 			struct virtchnl_rss_lut *vrl =
 				(struct virtchnl_rss_lut *)msg;
-			valid_len += vrl->lut_entries - 1;
+			valid_len = virtchnl_struct_size(vrl, lut,
+							 vrl->lut_entries);
 		}
 		break;
 	case VIRTCHNL_OP_GET_RSS_HENA_CAPS: