From 6e79fe5714a72b1ef86dc890ff60746cdd19f854 Mon Sep 17 00:00:00 2001 From: Yves Orton 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; -- 1.8.1.4