efi_loader: limit output length for VenHw, VenMedia

VenHw and VenMedia device path nodes may carry vendor defined data of
arbitrary length. When converting a device path node to text ensure that we
do not overrun our internal buffer.

In our implementation of
EFI_DEVICE_PATH_TO_TEXT_PROTOCOL.ConvertDevicePathToText() we could first
determine the output length and then allocate buffers but that would nearly
double the code size. Therefore keep the preallocated buffers and truncate
excessive device paths instead.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

lib/efi_loader/efi_device_path_to_text.c

@@ -67,7 +67,8 @@ static char *dp_hardware(char *s, struct efi_device_path *dp)
 
 		s += sprintf(s, "VenHw(%pUl", &vdp->guid);
 		n = (int)vdp->dp.length - sizeof(struct efi_device_path_vendor);
-		if (n > 0) {
+		/* Node must fit into MAX_NODE_LEN) */
+		if (n > 0 && n < MAX_NODE_LEN / 2 - 22) {
 			s += sprintf(s, ",");
 			for (i = 0; i < n; ++i)
 				s += sprintf(s, "%02x", vdp->vendor_data[i]);
@@ -251,7 +252,8 @@ static char *dp_media(char *s, struct efi_device_path *dp)
 
 		s += sprintf(s, "VenMedia(%pUl", &vdp->guid);
 		n = (int)vdp->dp.length - sizeof(struct efi_device_path_vendor);
-		if (n > 0) {
+		/* Node must fit into MAX_NODE_LEN) */
+		if (n > 0 && n < MAX_NODE_LEN / 2 - 24) {
 			s += sprintf(s, ",");
 			for (i = 0; i < n; ++i)
 				s += sprintf(s, "%02x", vdp->vendor_data[i]);