From f1220d61455253b170e81427c9d0357831ca0fac Mon Sep 17 00:00:00 2001

From: Yves Orton <[email protected]>

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.

---

 ext/Hash-Util-FieldHash/t/10_hash.t | 18 ++++++++++++++++--

 hv.c                                | 35 ++++++++---------------------------

 t/op/hash.t                         | 20 +++++++++++++++++---

 3 files changed, 41 insertions(+), 32 deletions(-)

diff --git a/ext/Hash-Util-FieldHash/t/10_hash.t b/ext/Hash-Util-FieldHash/t/10_hash.t

index 2cfb4e8..d58f053 100644

--- a/ext/Hash-Util-FieldHash/t/10_hash.t

+++ b/ext/Hash-Util-FieldHash/t/10_hash.t

@@ -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;

diff --git a/hv.c b/hv.c

index 6b66251..a031703 100644

--- a/hv.c

+++ b/hv.c

@@ -35,7 +35,8 @@ holds the key and hash value.

 #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 > (xhv)->xhv_max) /* HvTOTALKEYS(hv) > HvMAX(hv) */

 static const char S_strtab_error[]

     = "Cannot modify shared string table in hv_%s";

@@ -798,29 +799,9 @@ Perl_hv_common(pTHX_ HV *hv, SV *keysv, const char *key, STRLEN klen,

     if (masked_flags & HVhek_ENABLEHVKFLAGS)

 	HvHASKFLAGS_on(hv);

-    {

-	const HE *counter = HeNEXT(entry);

-

-	xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */

-	if (!counter) {				/* initial entry? */

-	} else if (xhv->xhv_keys > xhv->xhv_max) {

-		/* Use only the old HvUSEDKEYS(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);

-	} else if(!HvREHASH(hv)) {

-	    U32 n_links = 1;

-

-	    while ((counter = HeNEXT(counter)))

-		n_links++;

-

-	    if (n_links > HV_MAX_LENGTH_BEFORE_SPLIT) {

-		hsplit(hv);

-	    }

-	}

+    xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */

+    if ( SHOULD_DO_HSPLIT(xhv) ) {

+        hsplit(hv);

     }

     if (return_svp) {

@@ -1197,7 +1178,7 @@ S_hsplit(pTHX_ HV *hv)

     /* 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;

     }

@@ -2782,8 +2763,8 @@ S_share_hek_flags(pTHX_ const char *str, I32 len, register U32 hash, int flags)

 	xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */

 	if (!next) {			/* initial entry? */

-	} else if (xhv->xhv_keys > xhv->xhv_max /* HvUSEDKEYS(hv) > HvMAX(hv) */) {

-		hsplit(PL_strtab);

+	} else if ( SHOULD_DO_HSPLIT(xhv) ) {

+            hsplit(PL_strtab);

 	}

     }

diff --git a/t/op/hash.t b/t/op/hash.t

index ef757a3..97eb81b 100644

--- a/t/op/hash.t

+++ b/t/op/hash.t

@@ -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;

-- 

1.8.1.3