don't create oversize bignums in binary matching

Bignums are artifically restricted in size.  Arithmetic and logical
operations check the sizes of resulting bignums, and turn oversize
results into system_limit exceptions.

However, this check is not performed when bignums are constructed by
binary matching.  The consequence is that such matchings can construct
oversize bignums that satisfy is_integer/1 yet don't work.  Performing
arithmetic such as Term - 0 fails with a system_limit exception.  Worse,
performing a logical operation such as Term band Term results in [].
The latter occurs because the size checking (e.g. in erts_band()) is
a simple ASSERT(is_not_nil(...)) on the result of the bignum operation,
which internally is [] (NIL) in the case of oversize results.  However,
ASSERT is a no-op in release builds, so the error goes unnoticed and []
is returned as the result of the band/2.

This patch addresses this by preventing oversize bignums from entering
the VM via binary matching:

- the internal bytes_to_big() procedure is augmented to return NIL for
  oversize results, just like big_norm()
- callers of bytes_to_big() are augmented to check for NIL returns and
  signal errors in those cases
- erts_bs_get_integer_2() can only fail with badmatch, so that is the
  Erlang-level result of oversize bignums from binary matches
- big_SUITE.erl is extended with a test case that fails without this
  fix (no error signalled) and passes with it (badmatch occurs)

Credit goes to Nico Kruber for the initial bug report.

5 files changed, 21 insertions, 4 deletions

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index cfc6146b0a..41c1b5d2c2 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -4971,7 +4971,8 @@ get_tag_and_value(LoaderState* stp, Uint len_code,
 
     arity = count/sizeof(Eterm);
     *result = new_literal(stp, &hp, arity+1);
-    (void) bytes_to_big(bigbuf, count, neg, hp);
+    if (is_nil(bytes_to_big(bigbuf, count, neg, hp)))
+	goto load_error;
 
     if (bigbuf != default_buf) {
 	erts_free(ERTS_ALC_T_LOADER_TMP, (void *) bigbuf);
diff --git a/erts/emulator/beam/big.c b/erts/emulator/beam/big.c
index de7d370938..d1e46e3063 100644
--- a/erts/emulator/beam/big.c
+++ b/erts/emulator/beam/big.c
@@ -1900,6 +1900,8 @@ Eterm bytes_to_big(byte *xp, dsize_t xsz, int xsgn, Eterm *r)
 	*rwp = d;
 	rwp++;
     }
+    if (rsz > BIG_ARITY_MAX)
+	return NIL;
     if (xsgn) {
       *r = make_neg_bignum_header(rsz);
     }
diff --git a/erts/emulator/beam/erl_bits.c b/erts/emulator/beam/erl_bits.c
index 73765772c8..642f56a15e 100644
--- a/erts/emulator/beam/erl_bits.c
+++ b/erts/emulator/beam/erl_bits.c
@@ -403,7 +403,9 @@ erts_bs_get_integer_2(Process *p, Uint num_bits, unsigned flags, ErlBinMatchBuff
 	words_needed = 1+WSIZE(bytes);
 	hp = HeapOnlyAlloc(p, words_needed);
 	res = bytes_to_big(LSB, bytes, sgn, hp); 
-	if (is_small(res)) {
+	if (is_nil(res)) {
+	    res = THE_NON_VALUE;
+	} else if (is_small(res)) {
 	    p->htop = hp;
 	} else if ((actual = bignum_header_arity(*hp)+1) < words_needed) {
 	    p->htop = hp + actual;
diff --git a/erts/emulator/beam/external.c b/erts/emulator/beam/external.c
index 9b9b4b2a62..45d1f7514e 100644
--- a/erts/emulator/beam/external.c
+++ b/erts/emulator/beam/external.c
@@ -3056,6 +3056,8 @@ dec_term(ErtsDistExternal *edep, Eterm** hpp, byte* ep, ErlOffHeap* off_heap,
 		    big = make_small(0);
 		} else {
 		    big = bytes_to_big(first, n, neg, hp);
+		    if (is_nil(big))
+			goto error;
 		    if (is_big(big)) {
 			hp += big_arity(big) + 1;
 		    }
diff --git a/erts/emulator/test/big_SUITE.erl b/erts/emulator/test/big_SUITE.erl
index 413bd3bcae..3193d56e2a 100644
--- a/erts/emulator/test/big_SUITE.erl
+++ b/erts/emulator/test/big_SUITE.erl
@@ -23,7 +23,7 @@
 	 init_per_group/2,end_per_group/2]).
 -export([t_div/1, eq_28/1, eq_32/1, eq_big/1, eq_math/1, big_literals/1,
 	 borders/1, negative/1, big_float_1/1, big_float_2/1,
-	 shift_limit_1/1, powmod/1, system_limit/1, otp_6692/1]).
+	 shift_limit_1/1, powmod/1, system_limit/1, toobig/1, otp_6692/1]).
 
 %% Internal exports.
 -export([eval/1]).
@@ -40,7 +40,7 @@ suite() -> [{ct_hooks,[ts_install_cth]}].
 all() -> 
     [t_div, eq_28, eq_32, eq_big, eq_math, big_literals,
      borders, negative, {group, big_float}, shift_limit_1,
-     powmod, system_limit, otp_6692].
+     powmod, system_limit, toobig, otp_6692].
 
 groups() -> 
     [{big_float, [], [big_float_1, big_float_2]}].
@@ -370,6 +370,16 @@ maxbig() ->
 
 id(I) -> I.
 
+toobig(Config) when is_list(Config) ->
+    ?line {'EXIT',{{badmatch,_},_}} = (catch toobig()),
+    ok.
+
+toobig() ->
+    A = erlang:term_to_binary(lists:seq(1000000, 2200000)),
+    ASize = erlang:bit_size(A),
+    <<ANr:ASize>> = A, % should fail
+    ANr band ANr.
+
 otp_6692(suite) ->
     [];
 otp_6692(doc) ->