refint: Fix SQL injection and buffer overruns.

Maliciously crafted key value updates could achieve SQL injection
within check_foreign_key().  To fix, ensure new key values are
properly quoted and escaped in the internally generated SQL
statements.  While at it, avoid potential buffer overruns by
replacing the stack buffers for internally generated SQL statements
with StringInfo.

Reported-by: Nikolay Samokhvalov <nik@postgres.ai>
Author: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Security: CVE-2026-6637
Backpatch-through: 14
(cherry picked from commit 1ebda7da9a43d3ae3564d08612de9cb27fbaf482)

Author: nathan-bossart

contrib/spi/refint.c

@@ -171,21 +171,24 @@ check_primary_key(PG_FUNCTION_ARGS)
 	if (plan->nplans <= 0)
 	{
 		SPIPlanPtr	pplan;
-		char		sql[8192];
+		StringInfoData sql;
+
+		initStringInfo(&sql);
 
 		/*
 		 * Construct query: SELECT 1 FROM _referenced_relation_ WHERE Pkey1 =
 		 * $1 [AND Pkey2 = $2 [...]]
 		 */
-		snprintf(sql, sizeof(sql), "select 1 from %s where ", relname);
-		for (i = 0; i < nkeys; i++)
+		appendStringInfo(&sql, "select 1 from %s where ", relname);
+		for (i = 1; i <= nkeys; i++)
 		{
-			snprintf(sql + strlen(sql), sizeof(sql) - strlen(sql), "%s = $%d %s",
-					 args[i + nkeys + 1], i + 1, (i < nkeys - 1) ? "and " : "");
+			appendStringInfo(&sql, "%s = $%d ", args[i + nkeys], i);
+			if (i < nkeys)
+				appendStringInfoString(&sql, "and ");
 		}
 
 		/* Prepare plan for query */
-		pplan = SPI_prepare(sql, nkeys, argtypes);
+		pplan = SPI_prepare(sql.data, nkeys, argtypes);
 		if (pplan == NULL)
 			/* internal error */
 			elog(ERROR, "check_primary_key: SPI_prepare returned %s", SPI_result_code_string(SPI_result));
@@ -201,6 +204,8 @@ check_primary_key(PG_FUNCTION_ARGS)
 														sizeof(SPIPlanPtr));
 		*(plan->splan) = pplan;
 		plan->nplans = 1;
+
+		pfree(sql.data);
 	}
 
 	/*
@@ -423,14 +428,17 @@ check_foreign_key(PG_FUNCTION_ARGS)
 	if (plan->nplans <= 0)
 	{
 		SPIPlanPtr	pplan;
-		char		sql[8192];
 		char	  **args2 = args;
 
 		plan->splan = (SPIPlanPtr *) MemoryContextAlloc(TopMemoryContext,
 														nrefs * sizeof(SPIPlanPtr));
 
 		for (r = 0; r < nrefs; r++)
 		{
+			StringInfoData sql;
+
+			initStringInfo(&sql);
+
 			relname = args2[0];
 
 			/*---------
@@ -444,8 +452,7 @@ check_foreign_key(PG_FUNCTION_ARGS)
 			 *---------
 			 */
 			if (action == 'r')
-
-				snprintf(sql, sizeof(sql), "select 1 from %s where ", relname);
+				appendStringInfo(&sql, "select 1 from %s where ", relname);
 
 			/*---------
 			 * For 'C'ascade action we construct DELETE query
@@ -472,42 +479,23 @@ check_foreign_key(PG_FUNCTION_ARGS)
 					char	   *nv;
 					int			k;
 
-					snprintf(sql, sizeof(sql), "update %s set ", relname);
+					appendStringInfo(&sql, "update %s set ", relname);
 					for (k = 1; k <= nkeys; k++)
 					{
-						int			is_char_type = 0;
-						char	   *type;
-
 						fn = SPI_fnumber(tupdesc, args_temp[k - 1]);
 						Assert(fn > 0); /* already checked above */
 						nv = SPI_getvalue(newtuple, tupdesc, fn);
-						type = SPI_gettype(tupdesc, fn);
-
-						if (strcmp(type, "text") == 0 ||
-							strcmp(type, "varchar") == 0 ||
-							strcmp(type, "char") == 0 ||
-							strcmp(type, "bpchar") == 0 ||
-							strcmp(type, "date") == 0 ||
-							strcmp(type, "timestamp") == 0)
-							is_char_type = 1;
-#ifdef	DEBUG_QUERY
-						elog(DEBUG4, "check_foreign_key Debug value %s type %s %d",
-							 nv, type, is_char_type);
-#endif
 
-						/*
-						 * is_char_type =1 i set ' ' for define a new value
-						 */
-						snprintf(sql + strlen(sql), sizeof(sql) - strlen(sql),
-								 " %s = %s%s%s %s ",
-								 args2[k], (is_char_type > 0) ? "'" : "",
-								 nv, (is_char_type > 0) ? "'" : "", (k < nkeys) ? ", " : "");
+						appendStringInfo(&sql, " %s = %s ",
+										 args2[k], quote_literal_cstr(nv));
+						if (k < nkeys)
+							appendStringInfoString(&sql, ", ");
 					}
-					strcat(sql, " where ");
+					appendStringInfoString(&sql, " where ");
 				}
 				else
 					/* DELETE */
-					snprintf(sql, sizeof(sql), "delete from %s where ", relname);
+					appendStringInfo(&sql, "delete from %s where ", relname);
 			}
 
 			/*
@@ -518,25 +506,26 @@ check_foreign_key(PG_FUNCTION_ARGS)
 			 */
 			else if (action == 's')
 			{
-				snprintf(sql, sizeof(sql), "update %s set ", relname);
+				appendStringInfo(&sql, "update %s set ", relname);
 				for (i = 1; i <= nkeys; i++)
 				{
-					snprintf(sql + strlen(sql), sizeof(sql) - strlen(sql),
-							 "%s = null%s",
-							 args2[i], (i < nkeys) ? ", " : "");
+					appendStringInfo(&sql, "%s = null", args2[i]);
+					if (i < nkeys)
+						appendStringInfoString(&sql, ", ");
 				}
-				strcat(sql, " where ");
+				appendStringInfoString(&sql, " where ");
 			}
 
 			/* Construct WHERE qual */
 			for (i = 1; i <= nkeys; i++)
 			{
-				snprintf(sql + strlen(sql), sizeof(sql) - strlen(sql), "%s = $%d %s",
-						 args2[i], i, (i < nkeys) ? "and " : "");
+				appendStringInfo(&sql, "%s = $%d ", args2[i], i);
+				if (i < nkeys)
+					appendStringInfoString(&sql, "and ");
 			}
 
 			/* Prepare plan for query */
-			pplan = SPI_prepare(sql, nkeys, argtypes);
+			pplan = SPI_prepare(sql.data, nkeys, argtypes);
 			if (pplan == NULL)
 				/* internal error */
 				elog(ERROR, "check_foreign_key: SPI_prepare returned %s", SPI_result_code_string(SPI_result));
@@ -552,11 +541,14 @@ check_foreign_key(PG_FUNCTION_ARGS)
 			plan->splan[r] = pplan;
 
 			args2 += nkeys + 1; /* to the next relation */
+
+#ifdef DEBUG_QUERY
+			elog(DEBUG4, "check_foreign_key Debug Query is :  %s ", sql.data);
+#endif
+
+			pfree(sql.data);
 		}
 		plan->nplans = nrefs;
-#ifdef	DEBUG_QUERY
-		elog(DEBUG4, "check_foreign_key Debug Query is :  %s ", sql);
-#endif
 	}
 
 	/*