hv_netvsc: Allocate the recv_buf buffers after NVSP_MSG1_TYPE_SEND_RECV_BUF

The recv_buf buffers are allocated in netvsc_device_add().  Later in
netvsc_init_buf() the response to NVSP_MSG1_TYPE_SEND_RECV_BUF allows
the host to set up a recv_section_size that could be bigger than the
(default) value used for that allocation.  The host-controlled value
could be used by a malicious host to bypass the check on the packet's
length in netvsc_receive() and hence to overflow the recv_buf buffer.

Move the allocation of the recv_buf buffers into netvsc_init_but().
Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Fixes: 0ba35fe9 ("hv_netvsc: Copy packets sent by Hyper-V out of the receive buffer")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/hyperv/netvsc.c

@@ -311,7 +311,7 @@ static int netvsc_init_buf(struct hv_device *device,
 	struct nvsp_message *init_packet;
 	unsigned int buf_size;
 	size_t map_words;
-	int ret = 0;
+	int i, ret = 0;
 
 	/* Get receive buffer area. */
 	buf_size = device_info->recv_sections * device_info->recv_section_size;
@@ -405,6 +405,16 @@ static int netvsc_init_buf(struct hv_device *device,
 		goto cleanup;
 	}
 
+	for (i = 0; i < VRSS_CHANNEL_MAX; i++) {
+		struct netvsc_channel *nvchan = &net_device->chan_table[i];
+
+		nvchan->recv_buf = kzalloc(net_device->recv_section_size, GFP_KERNEL);
+		if (nvchan->recv_buf == NULL) {
+			ret = -ENOMEM;
+			goto cleanup;
+		}
+	}
+
 	/* Setup receive completion ring.
 	 * Add 1 to the recv_section_cnt because at least one entry in a
 	 * ring buffer has to be empty.
@@ -1549,12 +1559,6 @@ struct netvsc_device *netvsc_device_add(struct hv_device *device,
 	for (i = 0; i < VRSS_CHANNEL_MAX; i++) {
 		struct netvsc_channel *nvchan = &net_device->chan_table[i];
 
-		nvchan->recv_buf = kzalloc(device_info->recv_section_size, GFP_KERNEL);
-		if (nvchan->recv_buf == NULL) {
-			ret = -ENOMEM;
-			goto cleanup2;
-		}
-
 		nvchan->channel = device->channel;
 		nvchan->net_device = net_device;
 		u64_stats_init(&nvchan->tx_stats.syncp);