udf: Check component length before reading it

Check that length specified in a component of a symlink fits in the
input buffer we are reading. Also properly ignore component length for
component types that do not use it. Otherwise we read memory after end
of buffer for corrupted udf image.

Reported-by: Carl Henrik Lunde <chlunde@ping.uio.no>
CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/symlink.c

diff --git a/fs/udf/symlink.c b/fs/udf/symlink.c
index 0f1b3a2..ac10ca9 100644
--- a/fs/udf/symlink.c
+++ b/fs/udf/symlink.c
@@ -42,14 +42,17 @@
 	tolen--;
 	while (elen < fromlen) {
 		pc = (struct pathComponent *)(from + elen);
+		elen += sizeof(struct pathComponent);
 		switch (pc->componentType) {
 		case 1:
 			/*
 			 * Symlink points to some place which should be agreed
  			 * upon between originator and receiver of the media. Ignore.
 			 */
-			if (pc->lengthComponentIdent > 0)
+			if (pc->lengthComponentIdent > 0) {
+				elen += pc->lengthComponentIdent;
 				break;
+			}
 			/* Fall through */
 		case 2:
 			if (tolen == 0)
@@ -74,6 +77,9 @@
 			/* that would be . - just ignore */
 			break;
 		case 5:
+			elen += pc->lengthComponentIdent;
+			if (elen > fromlen)
+				return -EIO;
 			comp_len = udf_get_filename(sb, pc->componentIdent,
 						    pc->lengthComponentIdent,
 						    p, tolen);
@@ -85,7 +91,6 @@
 			tolen--;
 			break;
 		}
-		elen += sizeof(struct pathComponent) + pc->lengthComponentIdent;
 	}
 	if (p > to + 1)
 		p[-1] = '\0';