X.509: Fix leap year handling again

There are still a couple of minor issues in the X.509 leap year handling:

 (1) To avoid doing a modulus-by-400 in addition to a modulus-by-100 when
     determining whether the year is a leap year or not, I divided the year
     by 100 after doing the modulus-by-100, thereby letting the compiler do
     one instruction for both, and then did a modulus-by-4.

     Unfortunately, I then passed the now-modified year value to mktime64()
     to construct a time value.

     Since this isn't a fast path and since mktime64() does a bunch of
     divisions, just condense down to "% 400".  It's also easier to read.

 (2) The default month length for any February where the year doesn't
     divide by four exactly is obtained from the month_length[] array where
     the value is 29, not 28.

     This is fixed by altering the table.

Reported-by: Rudolf Polzer <rpolzer@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
cc: stable@vger.kernel.org

1 files changed, 4 insertions, 4 deletions

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 7502029e3385..794cf0eac2c9 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -471,7 +471,7 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
                      unsigned char tag,
                      const unsigned char *value, size_t vlen)
 {
-        static const unsigned char month_lengths[] = { 31, 29, 31, 30, 31, 30,
+        static const unsigned char month_lengths[] = { 31, 28, 31, 30, 31, 30,
                                                        31, 31, 30, 31, 30, 31 };
         const unsigned char *p = value;
         unsigned year, mon, day, hour, min, sec, mon_len;
@@ -517,9 +517,9 @@ int x509_decode_time(time64_t *_t,  size_t hdrlen,
                 if (year % 4 == 0) {
                         mon_len = 29;
                         if (year % 100 == 0) {
-                                year /= 100;
-                                if (year % 4 != 0)
-                                        mon_len = 28;
+                                mon_len = 28;
+                                if (year % 400 == 0)
+                                        mon_len = 29;
                         }
                 }
         }