Commit 23c8a812 authored Feb 23, 2016 by David Howells

KEYS: Fix ASN.1 indefinite length object parsing



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>

parent 685764b1

lib/asn1_decoder.c

+9 −7

Original line number	Diff line number	Diff line
		@@ -74,7 +74,7 @@ next_tag:

		/* 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;
		@@ -96,10 +96,8 @@ next_tag:

		/* 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 */
		@@ -110,14 +108,18 @@ next_tag:
		}

		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;

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