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author | Yves Orton <demerphq@gmail.com> | 2013-02-12 10:53:05 +0100 |
---|---|---|
committer | Yves Orton <demerphq@gmail.com> | 2013-02-13 20:25:16 +0100 |
commit | f14269908e5f8b4cab4b55643d7dd9de577e7918 (patch) | |
tree | 04e741c57272b1d12d0216c58c4952c3f944be7b | |
parent | ca8de220718ba91d5a5fdd9779497cd5b0250258 (diff) | |
download | perl-f14269908e5f8b4cab4b55643d7dd9de577e7918.tar.gz |
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 f2a571dae7d70f7e3b59022834d8003ecd2df884)
(which was itself cherry picked (with changes) from commit f1220d61455253b170e81427c9d0357831ca0fac)
-rw-r--r-- | ext/Hash-Util-FieldHash/t/10_hash.t | 18 | ||||
-rw-r--r-- | hv.c | 26 | ||||
-rw-r--r-- | t/op/hash.t | 20 |
3 files changed, 39 insertions, 25 deletions
diff --git a/ext/Hash-Util-FieldHash/t/10_hash.t b/ext/Hash-Util-FieldHash/t/10_hash.t index 29c2f4dafa..c266b6a45b 100644 --- a/ext/Hash-Util-FieldHash/t/10_hash.t +++ b/ext/Hash-Util-FieldHash/t/10_hash.t @@ -46,15 +46,29 @@ 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; @@ -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"; @@ -818,23 +819,8 @@ Perl_hv_common(pTHX_ HV *hv, SV *keysv, const char *key, STRLEN klen, 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 @@ 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; } @@ -2506,8 +2492,8 @@ S_share_hek_flags(pTHX_ const char *str, I32 len, register U32 hash, int flags) xhv->xhv_keys++; /* HvTOTALKEYS(hv)++ */ if (!next) { /* initial entry? */ 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); } } diff --git a/t/op/hash.t b/t/op/hash.t index 9bde518732..45eb782b65 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; |