Fix integer-overflow corner cases in substring() functions.

If the substring start index and length overflow when added together,
substring() misbehaved, either throwing a bogus "negative substring
length" error on a case that should succeed, or failing to complain that
a negative length is negative (and instead returning the whole string,
in most cases).  Unsurprisingly, the text, bytea, and bit variants of
the function all had this issue.  Rearrange the logic to ensure that
negative lengths are always rejected, and add an overflow check to
handle the other case.

Also install similar guards into detoast_attr_slice() (nee
heap_tuple_untoast_attr_slice()), since it's far from clear that
no other code paths leading to that function could pass it values
that would overflow.

Patch by myself and Pavel Stehule, per bug #16804 from Rafi Shamim.

Back-patch to v11.  While these bugs are old, the common/int.h
infrastructure for overflow-detecting arithmetic didn't exist before
commit 4d6ad31, and it doesn't seem like these misbehaviors are bad
enough to justify developing a standalone fix for the older branches.

Discussion: https://postgr.es/m/16804-f4eeeb6c11ba71d4@postgresql.org

Author: tglsfdc

src/backend/access/common/detoast.c

@@ -17,6 +17,7 @@
 #include "access/table.h"
 #include "access/tableam.h"
 #include "access/toast_internals.h"
+#include "common/int.h"
 #include "common/pg_lzcompress.h"
 #include "utils/expandeddatum.h"
 #include "utils/rel.h"
@@ -196,7 +197,8 @@ detoast_attr(struct varlena *attr)
  *		Public entry point to get back part of a toasted value
  *		from compression or external storage.
  *
- * Note: When slicelength is negative, return suffix of the value.
+ * sliceoffset is where to start (zero or more)
+ * If slicelength < 0, return everything beyond sliceoffset
  * ----------
  */
 struct varlena *
@@ -206,8 +208,21 @@ detoast_attr_slice(struct varlena *attr,
 	struct varlena *preslice;
 	struct varlena *result;
 	char	   *attrdata;
+	int32		slicelimit;
 	int32		attrsize;
 
+	if (sliceoffset < 0)
+		elog(ERROR, "invalid sliceoffset: %d", sliceoffset);
+
+	/*
+	 * Compute slicelimit = offset + length, or -1 if we must fetch all of the
+	 * value.  In case of integer overflow, we must fetch all.
+	 */
+	if (slicelength < 0)
+		slicelimit = -1;
+	else if (pg_add_s32_overflow(sliceoffset, slicelength, &slicelimit))
+		slicelength = slicelimit = -1;
+
 	if (VARATT_IS_EXTERNAL_ONDISK(attr))
 	{
 		struct varatt_external toast_pointer;
@@ -223,15 +238,15 @@ detoast_attr_slice(struct varlena *attr,
 		 * at least the requested part (when a prefix is requested).
 		 * Otherwise, just fetch all slices.
 		 */
-		if (slicelength > 0 && sliceoffset >= 0)
+		if (slicelimit >= 0)
 		{
 			int32		max_size;
 
 			/*
 			 * Determine maximum amount of compressed data needed for a prefix
 			 * of a given length (after decompression).
 			 */
-			max_size = pglz_maximum_compressed_size(sliceoffset + slicelength,
+			max_size = pglz_maximum_compressed_size(slicelimit,
 													toast_pointer.va_extsize);
 
 			/*
@@ -270,8 +285,8 @@ detoast_attr_slice(struct varlena *attr,
 		struct varlena *tmp = preslice;
 
 		/* Decompress enough to encompass the slice and the offset */
-		if (slicelength > 0 && sliceoffset >= 0)
-			preslice = toast_decompress_datum_slice(tmp, slicelength + sliceoffset);
+		if (slicelimit >= 0)
+			preslice = toast_decompress_datum_slice(tmp, slicelimit);
 		else
 			preslice = toast_decompress_datum(tmp);
 
@@ -297,8 +312,7 @@ detoast_attr_slice(struct varlena *attr,
 		sliceoffset = 0;
 		slicelength = 0;
 	}
-
-	if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
+	else if (slicelength < 0 || slicelimit > attrsize)
 		slicelength = attrsize - sliceoffset;
 
 	result = (struct varlena *) palloc(slicelength + VARHDRSZ);
@@ -410,6 +424,11 @@ toast_fetch_datum_slice(struct varlena *attr, int32 sliceoffset,
 	if (VARATT_EXTERNAL_IS_COMPRESSED(toast_pointer) && slicelength > 0)
 		slicelength = slicelength + sizeof(int32);
 
+	/*
+	 * Adjust length request if needed.  (Note: our sole caller,
+	 * detoast_attr_slice, protects us against sliceoffset + slicelength
+	 * overflowing.)
+	 */
 	if (((sliceoffset + slicelength) > attrsize) || slicelength < 0)
 		slicelength = attrsize - sliceoffset;
 

src/backend/utils/adt/varbit.c

@@ -1059,7 +1059,7 @@ bitsubstring(VarBit *arg, int32 s, int32 l, bool length_not_specified)
 				len,
 				ishift,
 				i;
-	int			e,
+	int32		e,
 				s1,
 				e1;
 	bits8	   *r,
@@ -1072,18 +1072,24 @@ bitsubstring(VarBit *arg, int32 s, int32 l, bool length_not_specified)
 	{
 		e1 = bitlen + 1;
 	}
-	else
+	else if (l < 0)
+	{
+		/* SQL99 says to throw an error for E < S, i.e., negative length */
+		ereport(ERROR,
+				(errcode(ERRCODE_SUBSTRING_ERROR),
+				 errmsg("negative substring length not allowed")));
+		e1 = -1;				/* silence stupider compilers */
+	}
+	else if (pg_add_s32_overflow(s, l, &e))
 	{
-		e = s + l;
-
 		/*
-		 * A negative value for L is the only way for the end position to be
-		 * before the start. SQL99 says to throw an error.
+		 * L could be large enough for S + L to overflow, in which case the
+		 * substring must run to end of string.
 		 */
-		if (e < s)
-			ereport(ERROR,
-					(errcode(ERRCODE_SUBSTRING_ERROR),
-					 errmsg("negative substring length not allowed")));
+		e1 = bitlen + 1;
+	}
+	else
+	{
 		e1 = Min(e, bitlen + 1);
 	}
 	if (s1 > bitlen || e1 <= s1)

src/backend/utils/adt/varlena.c

@@ -868,29 +868,38 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 	int32		S = start;		/* start position */
 	int32		S1;				/* adjusted start position */
 	int32		L1;				/* adjusted substring length */
+	int32		E;				/* end position */
+
+	/*
+	 * SQL99 says S can be zero or negative, but we still must fetch from the
+	 * start of the string.
+	 */
+	S1 = Max(S, 1);
 
 	/* life is easy if the encoding max length is 1 */
 	if (eml == 1)
 	{
-		S1 = Max(S, 1);
-
 		if (length_not_specified)	/* special case - get length to end of
 									 * string */
 			L1 = -1;
-		else
+		else if (length < 0)
+		{
+			/* SQL99 says to throw an error for E < S, i.e., negative length */
+			ereport(ERROR,
+					(errcode(ERRCODE_SUBSTRING_ERROR),
+					 errmsg("negative substring length not allowed")));
+			L1 = -1;			/* silence stupider compilers */
+		}
+		else if (pg_add_s32_overflow(S, length, &E))
 		{
-			/* end position */
-			int			E = S + length;
-
 			/*
-			 * A negative value for L is the only way for the end position to
-			 * be before the start. SQL99 says to throw an error.
+			 * L could be large enough for S + L to overflow, in which case
+			 * the substring must run to end of string.
 			 */
-			if (E < S)
-				ereport(ERROR,
-						(errcode(ERRCODE_SUBSTRING_ERROR),
-						 errmsg("negative substring length not allowed")));
-
+			L1 = -1;
+		}
+		else
+		{
 			/*
 			 * A zero or negative value for the end position can happen if the
 			 * start was negative or one. SQL99 says to return a zero-length
@@ -904,8 +913,8 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 
 		/*
 		 * If the start position is past the end of the string, SQL99 says to
-		 * return a zero-length string -- PG_GETARG_TEXT_P_SLICE() will do
-		 * that for us. Convert to zero-based starting position
+		 * return a zero-length string -- DatumGetTextPSlice() will do that
+		 * for us.  We need only convert S1 to zero-based starting position.
 		 */
 		return DatumGetTextPSlice(str, S1 - 1, L1);
 	}
@@ -926,12 +935,6 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 		char	   *s;
 		text	   *ret;
 
-		/*
-		 * if S is past the end of the string, the tuple toaster will return a
-		 * zero-length string to us
-		 */
-		S1 = Max(S, 1);
-
 		/*
 		 * We need to start at position zero because there is no way to know
 		 * in advance which byte offset corresponds to the supplied start
@@ -942,19 +945,24 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 		if (length_not_specified)	/* special case - get length to end of
 									 * string */
 			slice_size = L1 = -1;
-		else
+		else if (length < 0)
+		{
+			/* SQL99 says to throw an error for E < S, i.e., negative length */
+			ereport(ERROR,
+					(errcode(ERRCODE_SUBSTRING_ERROR),
+					 errmsg("negative substring length not allowed")));
+			slice_size = L1 = -1;	/* silence stupider compilers */
+		}
+		else if (pg_add_s32_overflow(S, length, &E))
 		{
-			int			E = S + length;
-
 			/*
-			 * A negative value for L is the only way for the end position to
-			 * be before the start. SQL99 says to throw an error.
+			 * L could be large enough for S + L to overflow, in which case
+			 * the substring must run to end of string.
 			 */
-			if (E < S)
-				ereport(ERROR,
-						(errcode(ERRCODE_SUBSTRING_ERROR),
-						 errmsg("negative substring length not allowed")));
-
+			slice_size = L1 = -1;
+		}
+		else
+		{
 			/*
 			 * A zero or negative value for the end position can happen if the
 			 * start was negative or one. SQL99 says to return a zero-length
@@ -972,8 +980,10 @@ text_substring(Datum str, int32 start, int32 length, bool length_not_specified)
 			/*
 			 * Total slice size in bytes can't be any longer than the start
 			 * position plus substring length times the encoding max length.
+			 * If that overflows, we can just use -1.
 			 */
-			slice_size = (S1 + L1) * eml;
+			if (pg_mul_s32_overflow(E, eml, &slice_size))
+				slice_size = -1;
 		}
 
 		/*
@@ -3309,9 +3319,13 @@ bytea_substring(Datum str,
 				int L,
 				bool length_not_specified)
 {
-	int			S1;				/* adjusted start position */
-	int			L1;				/* adjusted substring length */
+	int32		S1;				/* adjusted start position */
+	int32		L1;				/* adjusted substring length */
+	int32		E;				/* end position */
 
+	/*
+	 * The logic here should generally match text_substring().
+	 */
 	S1 = Max(S, 1);
 
 	if (length_not_specified)
@@ -3322,20 +3336,24 @@ bytea_substring(Datum str,
 		 */
 		L1 = -1;
 	}
-	else
+	else if (L < 0)
+	{
+		/* SQL99 says to throw an error for E < S, i.e., negative length */
+		ereport(ERROR,
+				(errcode(ERRCODE_SUBSTRING_ERROR),
+				 errmsg("negative substring length not allowed")));
+		L1 = -1;				/* silence stupider compilers */
+	}
+	else if (pg_add_s32_overflow(S, L, &E))
 	{
-		/* end position */
-		int			E = S + L;
-
 		/*
-		 * A negative value for L is the only way for the end position to be
-		 * before the start. SQL99 says to throw an error.
+		 * L could be large enough for S + L to overflow, in which case the
+		 * substring must run to end of string.
 		 */
-		if (E < S)
-			ereport(ERROR,
-					(errcode(ERRCODE_SUBSTRING_ERROR),
-					 errmsg("negative substring length not allowed")));
-
+		L1 = -1;
+	}
+	else
+	{
 		/*
 		 * A zero or negative value for the end position can happen if the
 		 * start was negative or one. SQL99 says to return a zero-length
@@ -3350,7 +3368,7 @@ bytea_substring(Datum str,
 	/*
 	 * If the start position is past the end of the string, SQL99 says to
 	 * return a zero-length string -- DatumGetByteaPSlice() will do that for
-	 * us. Convert to zero-based starting position
+	 * us.  We need only convert S1 to zero-based starting position.
 	 */
 	return DatumGetByteaPSlice(str, S1 - 1, L1);
 }

src/test/regress/expected/bit.out

@@ -106,6 +106,35 @@ SELECT v,
  01010101010 | 1010    | 01010    | 101010
 (4 rows)
 
+-- test overflow cases
+SELECT SUBSTRING('01010101'::bit(8) FROM 2 FOR 2147483646) AS "1010101";
+ 1010101 
+---------
+ 1010101
+(1 row)
+
+SELECT SUBSTRING('01010101'::bit(8) FROM -10 FOR 2147483646) AS "01010101";
+ 01010101 
+----------
+ 01010101
+(1 row)
+
+SELECT SUBSTRING('01010101'::bit(8) FROM -10 FOR -2147483646) AS "error";
+ERROR:  negative substring length not allowed
+SELECT SUBSTRING('01010101'::varbit FROM 2 FOR 2147483646) AS "1010101";
+ 1010101 
+---------
+ 1010101
+(1 row)
+
+SELECT SUBSTRING('01010101'::varbit FROM -10 FOR 2147483646) AS "01010101";
+ 01010101 
+----------
+ 01010101
+(1 row)
+
+SELECT SUBSTRING('01010101'::varbit FROM -10 FOR -2147483646) AS "error";
+ERROR:  negative substring length not allowed
 --- Bit operations
 DROP TABLE varbit_table;
 CREATE TABLE varbit_table (a BIT VARYING(16), b BIT VARYING(16));