Sanity-check consensus param values

We need to make sure that the worst thing that a weird consensus param
can do to us is to break our Tor (and only if the other Tors are
reliably broken in the same way) so that the majority of directory
authorities can't pull any attacks that are worse than the DoS that
they can trigger by simply shutting down.

One of these worse things was the cbtnummodes parameter, which could
lead to heap corruption on some systems if the value was sufficiently
large.

This commit fixes this particular issue and also introduces sanity
checking for all consensus parameters.

1 files changed, 24 insertions, 2 deletions

diff --git a/path-spec.txt b/path-spec.txt
index 2e4207b..4e1bdd0 100644
--- a/path-spec.txt
+++ b/path-spec.txt
@@ -421,12 +421,16 @@ of their choices.
 
       cbtdisabled
         Default: 0
-        Effect: If non-zero, all CircuitBuildTime learning code should be
+        Min: 0
+        Max: 1
+        Effect: If 1, all CircuitBuildTime learning code should be
                 disabled and history should be discarded. For use in
                 emergency situations only.
 
       cbtnummodes
         Default: 3
+        Min: 1
+        Max: 20
         Effect: This value governs how many modes to use in the weighted
         average calculation of Pareto paramter Xm. A value of 3 introduces
         some bias (2-5% of CDF) under ideal conditions, but allows for better
@@ -435,43 +439,61 @@ of their choices.
 
       cbtrecentcount
         Default: 20
+        Min: 3
+        Max: 1000
         Effect: This is the number of circuit build times to keep track of
                 for the following option.
 
       cbtmaxtimeouts
         Default: 18
+        Min: 3
+        Max: 10000
         Effect: When this many timeouts happen in the last 'cbtrecentcount'
                 circuit attempts, the client should discard all of its
                 history and begin learning a fresh timeout value.
 
       cbtmincircs
         Default: 100
+        Min: 1
+        Max: 10000
         Effect: This is the minimum number of circuits to build before
                 computing a timeout.
 
       cbtquantile
         Default: 80
+        Min: 10
+        Max: 99
         Effect: This is the position on the quantile curve to use to set the
-                timeout value. It is a percent (0-99).
+                timeout value. It is a percent (10-99).
 
       cbtclosequantile
         Default: 95
+        Min: Value of cbtquantile parameter
+        Max: 99
         Effect: This is the position on the quantile curve to use to set the
                 timeout value to use to actually close circuits. It is a percent
                 (0-99).
 
       cbttestfreq
         Default: 60
+        Min: 1
+        Max: 2147483647 (INT32_MAX)
         Effect: Describes how often in seconds to build a test circuit to
                 gather timeout values. Only applies if less than 'cbtmincircs'
                 have been recorded.
 
       cbtmintimeout
         Default: 2000
+        Min: 500
+        Max: 2147483647 (INT32_MAX)
         Effect: This is the minimum allowed timeout value in milliseconds.
+                The minimum is to prevent rounding to 0 (we only check once
+                per second).
 
       cbtinitialtimeout
         Default: 60000
+        Min: Value of cbtmintimeout
+        Max: 2147483647 (INT32_MAX)
         Effect: This is the timeout value to use before computing a timeout,
                 in milliseconds.