KEYS: Fix ASN.1 indefinite length object parsing

commit 23c8a812dc3c621009e4f0e5342aa4e2ede1ceaa upstream.

This fixes CVE-2016-0758.

In the ASN.1 decoder, when the length field of an ASN.1 value is extracted,
it isn't validated against the remaining amount of data before being added
to the cursor.  With a sufficiently large size indicated, the check:

	datalen - dp < 2

may then fail due to integer overflow.

Fix this by checking the length indicated against the amount of remaining
data in both places a definite length is determined.

Whilst we're at it, make the following changes:

 (1) Check the maximum size of extended length does not exceed the capacity
     of the variable it's being stored in (len) rather than the type that
     variable is assumed to be (size_t).

 (2) Compare the EOC tag to the symbolic constant ASN1_EOC rather than the
     integer 0.

 (3) To reduce confusion, move the initialisation of len outside of:

	for (len = 0; n > 0; n--) {

     since it doesn't have anything to do with the loop counter n.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Peter Jones <pjones@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
index 3787d02e2c49ee8d1267999008b342663e56d9fe..b1c885297113f7d42a6f300f396dd77b2543e6e9 100644 (file)
--- a/lib/asn1_decoder.c
+++ b/lib/asn1_decoder.c
@@ -69,7 +69,7 @@ static int asn1_find_indefinite_length(const unsigned char *data, size_t datalen
 
        /* Extract a tag from the data */
        tag = data[dp++];
-       if (tag == 0) {
+       if (tag == ASN1_EOC) {
                /* It appears to be an EOC. */
                if (data[dp++] != 0)
                        goto invalid_eoc;
@@ -91,10 +91,8 @@ static int asn1_find_indefinite_length(const unsigned char *data, size_t datalen
 
        /* Extract the length */
        len = data[dp++];
-       if (len <= 0x7f) {
-               dp += len;
-               goto next_tag;
-       }
+       if (len <= 0x7f)
+               goto check_length;
 
        if (unlikely(len == ASN1_INDEFINITE_LENGTH)) {
                /* Indefinite length */
@@ -105,14 +103,18 @@ static int asn1_find_indefinite_length(const unsigned char *data, size_t datalen
        }
 
        n = len - 0x80;
-       if (unlikely(n > sizeof(size_t) - 1))
+       if (unlikely(n > sizeof(len) - 1))
                goto length_too_long;
        if (unlikely(n > datalen - dp))
                goto data_overrun_error;
-       for (len = 0; n > 0; n--) {
+       len = 0;
+       for (; n > 0; n--) {
                len <<= 8;
                len |= data[dp++];
        }
+check_length:
+       if (len > datalen - dp)
+               goto data_overrun_error;
        dp += len;
        goto next_tag;