Avoid crashes in contrib/intarray gist__int_ops (bug #15518)

1. Integer overflow in internal_size could result in memory corruption
in decompression since a zero-length array would be allocated and then
written to. This leads to crashes or corruption when traversing an
index which has been populated with sufficiently sparse values. Fix by
using int64 for computations and checking for overflow.

2. Integer overflow in g_int_compress could cause pessimal merge
choices, resulting in unnecessarily large ranges (which would in turn
trigger issue 1 above). Fix by using int64 again.

3. Even without overflow, array sizes could become large enough to
cause unexplained memory allocation errors. Fix by capping the sizes
to a safe limit and report actual errors pointing at gist__intbig_ops
as needed.

4. Large inputs to the compression function always consist of large
runs of consecutive integers, and the compression loop was processing
these one at a time in an O(N^2) manner with a lot of overhead. The
expected runtime of this function could easily exceed 6 months for a
single call as a result. Fix by performing a linear-time first pass,
which reduces the worst case to something on the order of seconds.

Backpatch all the way, since this has been wrong forever.

Per bug #15518 from report from irc user "dymk", analysis and patch by
me.

Discussion: https://postgr.es/m/15518-799e426c3b4f8358@postgresql.org

contrib/intarray/_int_gist.c

@@ -12,6 +12,17 @@
 
 #define GETENTRY(vec,pos) ((ArrayType *) DatumGetPointer((vec)->vector[(pos)].key))
 
+/*
+ * Control the maximum sparseness of compressed keys.
+ *
+ * The upper safe bound for this limit is half the maximum allocatable array
+ * size. A lower bound would give more guarantees that pathological data
+ * wouldn't eat excessive CPU and memory, but at the expense of breaking
+ * possibly working (after a fashion) indexes.
+ */
+#define MAXNUMELTS (Min((MaxAllocSize / sizeof(Datum)),((MaxAllocSize - ARR_OVERHEAD_NONULLS(1)) / sizeof(int)))/2)
+/* or: #define MAXNUMELTS 1000000 */
+
 /*
 ** GiST support methods
 */
@@ -141,11 +152,13 @@ g_int_compress(PG_FUNCTION_ARGS)
 	GISTENTRY  *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
 	GISTENTRY  *retval;
 	ArrayType  *r;
-	int			len;
+	int			len,
+				lenr;
 	int		   *dr;
 	int			i,
-				min,
+				j,
 				cand;
+	int64		min;
 
 	if (entry->leafkey)
 	{
@@ -186,23 +199,62 @@ g_int_compress(PG_FUNCTION_ARGS)
 
 		dr = ARRPTR(r);
 
-		for (i = len - 1; i >= 0; i--)
-			dr[2 * i] = dr[2 * i + 1] = dr[i];
+		/*
+		 * "len" at this point is the number of ranges we will construct.
+		 * "lenr" is the number of ranges we must eventually remove by
+		 * merging, we must be careful to remove no more than this number.
+		 */
+		lenr = len - MAXNUMRANGE;
 
-		len *= 2;
+		/*
+		 * Initially assume we can merge consecutive ints into a range. but we
+		 * must count every value removed and stop when lenr runs out
+		 */
+		for (j = i = len - 1; i > 0 && lenr > 0; i--, j--)
+		{
+			int		r_end = dr[i];
+			int		r_start = r_end;
+			while (i > 0 && lenr > 0 && dr[i-1] == r_start - 1)
+				--r_start, --i, --lenr;
+			dr[2*j] = r_start;
+			dr[2*j+1] = r_end;
+		}
+		/* just copy the rest, if any, as trivial ranges */
+		for (; i >= 0; i--, j--)
+			dr[2*j] = dr[2*j + 1] = dr[i];
+
+		if (++j)
+		{
+			/*
+			 * shunt everything down to start at the right place
+			 */
+			memmove((void *) &dr[0], (void *) &dr[2*j], 2*(len - j) * sizeof(int32));
+		}
+		/*
+		 * make "len" be number of array elements, not ranges
+		 */
+		len = 2*(len - j);
 		cand = 1;
 		while (len > MAXNUMRANGE * 2)
 		{
-			min = INT_MAX;
+			min = PG_INT64_MAX;
 			for (i = 2; i < len; i += 2)
-				if (min > (dr[i] - dr[i - 1]))
+				if (min > ((int64)dr[i] - (int64)dr[i - 1]))
 				{
-					min = (dr[i] - dr[i - 1]);
+					min = ((int64)dr[i] - (int64)dr[i - 1]);
 					cand = i;
 				}
 			memmove((void *) &dr[cand - 1], (void *) &dr[cand + 1], (len - cand - 1) * sizeof(int32));
 			len -= 2;
 		}
+		/*
+		 * check sparseness of result
+		 */
+		lenr = internal_size(dr, len);
+		if (lenr < 0 || lenr > MAXNUMELTS)
+			ereport(ERROR,
+					(errmsg("data is too sparse, recreate index using gist__intbig_ops opclass instead")));
+
 		r = resize_intArrayType(r, len);
 		retval = palloc(sizeof(GISTENTRY));
 		gistentryinit(*retval, PointerGetDatum(r),
@@ -260,6 +312,9 @@ g_int_decompress(PG_FUNCTION_ARGS)
 
 	din = ARRPTR(in);
 	lenr = internal_size(din, lenin);
+	if (lenr < 0 || lenr > MAXNUMELTS)
+		ereport(ERROR,
+				(errmsg("compressed array is too big, recreate index using gist__intbig_ops opclass instead")));
 
 	r = new_intArrayType(lenr);
 	dr = ARRPTR(r);

contrib/intarray/_int_tool.c

@@ -3,6 +3,8 @@
  */
 #include "postgres.h"
 
+#include <limits.h>
+
 #include "catalog/pg_type.h"
 
 #include "_int.h"
@@ -225,6 +227,7 @@ new_intArrayType(int num)
 	/* if no elements, return a zero-dimensional array */
 	if (num <= 0)
 	{
+		Assert(num == 0);
 		r = construct_empty_array(INT4OID);
 		return r;
 	}
@@ -252,6 +255,7 @@ resize_intArrayType(ArrayType *a, int num)
 	/* if no elements, return a zero-dimensional array */
 	if (num <= 0)
 	{
+		Assert(num == 0);
 		ARR_NDIM(a) = 0;
 		return a;
 	}
@@ -288,16 +292,18 @@ copy_intArrayType(ArrayType *a)
 int
 internal_size(int *a, int len)
 {
-	int			i,
-				size = 0;
+	int			i;
+	int64		size = 0;
 
 	for (i = 0; i < len; i += 2)
 	{
 		if (!i || a[i] != a[i - 1]) /* do not count repeated range */
-			size += a[i + 1] - a[i] + 1;
+			size += (int64)(a[i + 1]) - (int64)(a[i]) + 1;
 	}
 
-	return size;
+	if (size > (int64)INT_MAX || size < (int64)INT_MIN)
+		return -1;				/* overflow */
+	return (int) size;
 }
 
 /* unique-ify elements of r in-place ... r must be sorted already */