[PATCH] fbdev: udlfb: validate vendor descriptor items

dlfb_parse_vendor_descriptor() walks key-length-value items inside the
DisplayLink vendor descriptor.

Require each item to contain its key, length and declared value bytes
before reading item-specific fields such as max_area.

Signed-off-by: Pengpeng Hou <[email protected]>
---
 drivers/video/fbdev/udlfb.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c
index fdbb8671a810..e78d6f95c9c5 100644
--- a/drivers/video/fbdev/udlfb.c
+++ b/drivers/video/fbdev/udlfb.c
@@ -1586,19 +1586,29 @@ static int dlfb_parse_vendor_descriptor(struct dlfb_data *dlfb,
 		desc += 5; /* the fixed header we've already parsed */
 
 		while (desc < desc_end) {
+			char *value;
 			u8 length;
 			u16 key;
 
-			key = *desc++;
-			key |= (u16)*desc++ << 8;
+			if (desc_end - desc < sizeof(key) + sizeof(length))
+				goto unrecognized;
+
+			key = get_unaligned_le16(desc);
+			desc += sizeof(key);
 			length = *desc++;
 
+			if (length > desc_end - desc)
+				goto unrecognized;
+
+			value = desc;
 			switch (key) {
 			case 0x0200: { /* max_area */
-				u32 max_area = *desc++;
-				max_area |= (u32)*desc++ << 8;
-				max_area |= (u32)*desc++ << 16;
-				max_area |= (u32)*desc++ << 24;
+				u32 max_area;
+
+				if (length < sizeof(max_area))
+					goto unrecognized;
+
+				max_area = get_unaligned_le32(value);
 				dev_warn(&intf->dev,
 					 "DL chip limited to %d pixel modes\n",
 					 max_area);
-- 
2.43.0