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File perl-rehash.diff of Package perl

From 57629630785036482da04228e9bf767b3dac66b6 Mon Sep 17 00:00:00 2001
From: Yves Orton <demerphq@gmail.com>
Date: Tue, 12 Feb 2013 10:53:05 +0100
Subject: [PATCH] Prevent premature hsplit() calls, and only trigger REHASH
 after hsplit()

Triggering a hsplit due to long chain length allows an attacker
to create a carefully chosen set of keys which can cause the hash
to use 2 * (2**32) * sizeof(void *) bytes ram. AKA a DOS via memory
exhaustion. Doing so also takes non trivial time.

Eliminating this check, and only inspecting chain length after a
normal hsplit() (triggered when keys>buckets) prevents the attack
entirely, and makes such attacks relatively benign.

(cherry picked from commit f1220d61455253b170e81427c9d0357831ca0fac)

--- ./ext/Hash/Util/FieldHash/t/10_hash.t.orig	2010-12-30 02:07:16.000000000 +0000
+++ ./ext/Hash/Util/FieldHash/t/10_hash.t	2013-02-26 15:54:36.000000000 +0000
@@ -38,15 +38,29 @@ use constant START     => "a";
 
 # some initial hash data
 fieldhash my %h2;
-%h2 = map {$_ => 1} 'a'..'cc';
+my $counter= "a";
+$h2{$counter++}++ while $counter ne 'cd';
 
 ok (!Internals::HvREHASH(%h2), 
     "starting with pre-populated non-pathological hash (rehash flag if off)");
 
 my @keys = get_keys(\%h2);
+my $buckets= buckets(\%h2);
 $h2{$_}++ for @keys;
+$h2{$counter++}++ while buckets(\%h2) == $buckets; # force a split
 ok (Internals::HvREHASH(%h2), 
-    scalar(@keys) . " colliding into the same bucket keys are triggering rehash");
+    scalar(@keys) . " colliding into the same bucket keys are triggering rehash after split");
+
+# returns the number of buckets in a hash
+sub buckets {
+    my $hr = shift;
+    my $keys_buckets= scalar(%$hr);
+    if ($keys_buckets=~m!/([0-9]+)\z!) {
+        return 0+$1;
+    } else {
+        return 8;
+    }
+}
 
 sub get_keys {
     my $hr = shift;
--- ./t/op/hash.t.orig	2007-12-18 10:47:08.000000000 +0000
+++ ./t/op/hash.t	2013-02-26 15:35:41.000000000 +0000
@@ -39,22 +39,36 @@ use constant THRESHOLD => 14;
 use constant START     => "a";
 
 # some initial hash data
-my %h2 = map {$_ => 1} 'a'..'cc';
+my %h2;
+my $counter= "a";
+$h2{$counter++}++ while $counter ne 'cd';
 
 ok (!Internals::HvREHASH(%h2), 
     "starting with pre-populated non-pathological hash (rehash flag if off)");
 
 my @keys = get_keys(\%h2);
+my $buckets= buckets(\%h2);
 $h2{$_}++ for @keys;
+$h2{$counter++}++ while buckets(\%h2) == $buckets; # force a split
 ok (Internals::HvREHASH(%h2), 
-    scalar(@keys) . " colliding into the same bucket keys are triggering rehash");
+    scalar(@keys) . " colliding into the same bucket keys are triggering rehash after split");
+
+# returns the number of buckets in a hash
+sub buckets {
+    my $hr = shift;
+    my $keys_buckets= scalar(%$hr);
+    if ($keys_buckets=~m!/([0-9]+)\z!) {
+        return 0+$1;
+    } else {
+        return 8;
+    }
+}
 
 sub get_keys {
     my $hr = shift;
 
     # the minimum of bits required to mount the attack on a hash
     my $min_bits = log(THRESHOLD)/log(2);
-
     # if the hash has already been populated with a significant amount
     # of entries the number of mask bits can be higher
     my $keys = scalar keys %$hr;
--- hv.c.orig	2013-03-15 10:02:44.969542617 +0100
+++ hv.c	2013-03-15 10:37:23.853123998 +0100
@@ -35,7 +35,8 @@
 #define PERL_HASH_INTERNAL_ACCESS
 #include "perl.h"
 
-#define HV_MAX_LENGTH_BEFORE_SPLIT 14
+#define HV_MAX_LENGTH_BEFORE_REHASH 14
+#define SHOULD_DO_HSPLIT(xhv) ((xhv)->xhv_keys > (IV)xhv->xhv_max) /* HvTOTALKEYS(hv) > HvMAX(hv) */
 
 static const char S_strtab_error[]
     = "Cannot modify shared string table in hv_%s";
@@ -818,23 +819,8 @@
 	xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */
 	if (!counter) {				/* initial entry? */
 	    xhv->xhv_fill++; /* HvFILL(hv)++ */
-	} else if (xhv->xhv_keys > (IV)xhv->xhv_max) {
+	} else if ( SHOULD_DO_HSPLIT(xhv) ) {
 	    hsplit(hv);
-	} else if(!HvREHASH(hv)) {
-	    U32 n_links = 1;
-
-	    while ((counter = HeNEXT(counter)))
-		n_links++;
-
-	    if (n_links > HV_MAX_LENGTH_BEFORE_SPLIT) {
-		/* Use only the old HvKEYS(hv) > HvMAX(hv) condition to limit
-		   bucket splits on a rehashed hash, as we're not going to
-		   split it again, and if someone is lucky (evil) enough to
-		   get all the keys in one list they could exhaust our memory
-		   as we repeatedly double the number of buckets on every
-		   entry. Linear search feels a less worse thing to do.  */
-		hsplit(hv);
-	    }
 	}
     }
 
@@ -1180,7 +1166,7 @@
 
 
     /* Pick your policy for "hashing isn't working" here:  */
-    if (longest_chain <= HV_MAX_LENGTH_BEFORE_SPLIT /* split worked?  */
+    if (longest_chain <= HV_MAX_LENGTH_BEFORE_REHASH /* split worked?  */
 	|| HvREHASH(hv)) {
 	return;
     }
@@ -2550,9 +2536,8 @@
 
 	xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */
 	if (!next) {			/* initial entry? */
-	    xhv->xhv_fill++; /* HvFILL(hv)++ */
-	} else if (xhv->xhv_keys > (IV)xhv->xhv_max /* HvKEYS(hv) > HvMAX(hv) */) {
-		hsplit(PL_strtab);
+	} else if ( SHOULD_DO_HSPLIT(xhv) ) {
+	    hsplit(PL_strtab);
 	}
     }

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