treat older Release files than we already have as an IMSHit

Valid-Until protects us from long-living downgrade attacks, but not all
repositories have it and an attacker could still use older but still
valid files to downgrade us. While this makes it sounds like a security
improvement now, its a bit theoretical at best as an attacker with
capabilities to pull this off could just as well always keep us days
(but in the valid period) behind and always knows which state we have,
as we tell him with the If-Modified-Since header. This is also why this
is 'silently' ignored and treated as an IMSHit rather than screamed at
the user as this can at best be an annoyance for attackers.

An error here would 'regularily' be encountered by users by out-of-sync
mirrors serving a single run (e.g. load balancer) or in two consecutive
runs on the other hand, so it would just help teaching people ignore it.

That said, most of the code churn is caused by enforcing this additional
requirement. Crisscross from InRelease to Release.gpg is e.g. very
unlikely in practice, but if we would ignore it an attacker could
sidestep it this way.

1 files changed, 16 insertions, 12 deletions

diff --git a/apt-pkg/indexrecords.cc b/apt-pkg/indexrecords.cc
index d65266f64..de2617833 100644
--- a/apt-pkg/indexrecords.cc
+++ b/apt-pkg/indexrecords.cc
@@ -58,6 +58,11 @@ APT_PURE time_t indexRecords::GetValidUntil() const
    return this->ValidUntil;
 }
 
+APT_PURE time_t indexRecords::GetDate() const
+{
+   return this->Date;
+}
+
 APT_PURE indexRecords::checkSum *indexRecords::Lookup(const string MetaKey)
 {
    std::map<std::string, indexRecords::checkSum* >::const_iterator sum = Entries.find(MetaKey);
@@ -133,9 +138,15 @@ bool indexRecords::Load(const string Filename)				/*{{{*/
       return false;
    }
 
-   string Label = Section.FindS("Label");
-   string StrDate = Section.FindS("Date");
-   string StrValidUntil = Section.FindS("Valid-Until");
+   string const StrDate = Section.FindS("Date");
+   if (RFC1123StrToTime(StrDate.c_str(), Date) == false)
+   {
+      strprintf(ErrorText, _("Invalid 'Date' entry in Release file %s"), Filename.c_str());
+      return false;
+   }
+
+   string const Label = Section.FindS("Label");
+   string const StrValidUntil = Section.FindS("Valid-Until");
 
    // if we have a Valid-Until header in the Release file, use it as default
    if (StrValidUntil.empty() == false)
@@ -158,20 +169,13 @@ bool indexRecords::Load(const string Filename)				/*{{{*/
       (MinAge == 0 || ValidUntil == 0)) // No user settings, use the one from the Release file
       return true;
 
-   time_t date;
-   if (RFC1123StrToTime(StrDate.c_str(), date) == false)
-   {
-      strprintf(ErrorText, _("Invalid 'Date' entry in Release file %s"), Filename.c_str());
-      return false;
-   }
-
    if (MinAge != 0 && ValidUntil != 0) {
-      time_t const min_date = date + MinAge;
+      time_t const min_date = Date + MinAge;
       if (ValidUntil < min_date)
 	 ValidUntil = min_date;
    }
    if (MaxAge != 0) {
-      time_t const max_date = date + MaxAge;
+      time_t const max_date = Date + MaxAge;
       if (ValidUntil == 0 || ValidUntil > max_date)
 	 ValidUntil = max_date;
    }