X.509: Fix indefinite length element skip error handling

asn1_find_indefinite_length() returns an error indicator of -1, which the
caller asn1_ber_decoder() places in a size_t (which is usually unsigned) and
then checks to see whether it is less than 0 (which it can't be).  This can
lead to the following warning:

	lib/asn1_decoder.c:320 asn1_ber_decoder()
		warn: unsigned 'len' is never less than zero.

Instead, asn1_find_indefinite_length() update the caller's idea of the data
cursor and length separately from returning the error code.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 19 insertions, 9 deletions

diff --git a/lib/asn1_decoder.c b/lib/asn1_decoder.c
index 2e4196ddf06f..de2c8b5a715b 100644
--- a/lib/asn1_decoder.c
+++ b/lib/asn1_decoder.c
@@ -46,12 +46,18 @@ static const unsigned char asn1_op_lengths[ASN1_OP__NR] = {
 
 /*
  * Find the length of an indefinite length object
+ * @data: The data buffer
+ * @datalen: The end of the innermost containing element in the buffer
+ * @_dp: The data parse cursor (updated before returning)
+ * @_len: Where to return the size of the element.
+ * @_errmsg: Where to return a pointer to an error message on error
  */
 static int asn1_find_indefinite_length(const unsigned char *data, size_t datalen,
-                                       const char **_errmsg, size_t *_err_dp)
+                                       size_t *_dp, size_t *_len,
+                                       const char **_errmsg)
 {
         unsigned char tag, tmp;
-        size_t dp = 0, len, n;
+        size_t dp = *_dp, len, n;
         int indef_level = 1;
 
 next_tag:
@@ -67,8 +73,11 @@ next_tag:
                 /* It appears to be an EOC. */
                 if (data[dp++] != 0)
                         goto invalid_eoc;
-                if (--indef_level <= 0)
-                        return dp;
+                if (--indef_level <= 0) {
+                        *_len = dp - *_dp;
+                        *_dp = dp;
+                        return 0;
+                }
                 goto next_tag;
         }
 
@@ -122,7 +131,7 @@ data_overrun_error:
 missing_eoc:
         *_errmsg = "Missing EOC in indefinite len cons";
 error:
-        *_err_dp = dp;
+        *_dp = dp;
         return -1;
 }
 
@@ -315,13 +324,14 @@ next_op:
         skip_data:
                 if (!(flags & FLAG_CONS)) {
                         if (flags & FLAG_INDEFINITE_LENGTH) {
-                                len = asn1_find_indefinite_length(
-                                        data + dp, datalen - dp, &errmsg, &dp);
-                                if (len < 0)
+                                ret = asn1_find_indefinite_length(
+                                        data, datalen, &dp, &len, &errmsg);
+                                if (ret < 0)
                                         goto error;
+                        } else {
+                                dp += len;
                         }
                         pr_debug("- LEAF: %zu\n", len);
-                        dp += len;
                 }
                 pc += asn1_op_lengths[op];
                 goto next_op;