MFC r275060: Fix b64_pton output buffer overrun test for exact-sized buffer

  b64_pton would sometimes erroneously fail to decode a base64 string into
  a precisely sized buffer. The overflow check was a little too greedy.

1 files changed, 15 insertions, 7 deletions

diff --git a/lib/libc/net/base64.c b/lib/libc/net/base64.c
index 4335030..6737c18 100644
--- a/lib/libc/net/base64.c
+++ b/lib/libc/net/base64.c
@@ -199,6 +199,7 @@ b64_pton(src, target, targsize)
 	size_t targsize;
 {
 	int tarindex, state, ch;
+	u_char nextbyte;
 	char *pos;
 
 	state = 0;
@@ -226,22 +227,28 @@ b64_pton(src, target, targsize)
 			break;
 		case 1:
 			if (target) {
-				if ((size_t)tarindex + 1 >= targsize)
+				if ((size_t)tarindex >= targsize)
 					return (-1);
 				target[tarindex]   |=  (pos - Base64) >> 4;
-				target[tarindex+1]  = ((pos - Base64) & 0x0f)
-							<< 4 ;
+				nextbyte = ((pos - Base64) & 0x0f) << 4;
+				if ((size_t)tarindex + 1 < targsize)
+					target[tarindex + 1] = nextbyte;
+				else if (nextbyte)
+					return (-1);
 			}
 			tarindex++;
 			state = 2;
 			break;
 		case 2:
 			if (target) {
-				if ((size_t)tarindex + 1 >= targsize)
+				if ((size_t)tarindex >= targsize)
 					return (-1);
 				target[tarindex]   |=  (pos - Base64) >> 2;
-				target[tarindex+1]  = ((pos - Base64) & 0x03)
-							<< 6;
+				nextbyte = ((pos - Base64) & 0x03) << 6;
+				if ((size_t)tarindex + 1 < targsize)
+					target[tarindex + 1] = nextbyte;
+				else if (nextbyte)
+					return (-1);
 			}
 			tarindex++;
 			state = 3;
@@ -299,7 +306,8 @@ b64_pton(src, target, targsize)
 			 * zeros.  If we don't check them, they become a
 			 * subliminal channel.
 			 */
-			if (target && target[tarindex] != 0)
+			if (target && (size_t)tarindex < targsize &&
+			    target[tarindex] != 0)
 				return (-1);
 		}
 	} else {