Ignore attempts to \gset into specially treated variables.

If an interactive psql session used \gset when querying a compromised
server, the attacker could execute arbitrary code as the operating
system account running psql.  Using a prefix not found among specially
treated variables, e.g. every lowercase string, precluded the attack.
Fix by issuing a warning and setting no variable for the column in
question.  Users wanting the old behavior can use a prefix and then a
meta-command like "\set HISTSIZE :prefix_HISTSIZE".  Back-patch to 9.5
(all supported versions).

Reviewed by Robert Haas.  Reported by Nick Cleaton.

Security: CVE-2020-25696

Author: nmisch

src/bin/psql/common.c

@@ -782,6 +782,13 @@ StoreQueryTuple(const PGresult *result)
 			/* concatenate prefix and column name */
 			varname = psprintf("%s%s", pset.gset_prefix, colname);
 
+			if (VariableHasHook(pset.vars, varname))
+			{
+				pg_log_warning("attempt to \\gset into specially treated variable \"%s\" ignored",
+							   varname);
+				continue;
+			}
+
 			if (!PQgetisnull(result, 0, i))
 				value = PQgetvalue(result, 0, i);
 			else

src/bin/psql/variables.c

@@ -360,6 +360,32 @@ SetVariableHooks(VariableSpace space, const char *name,
 		(void) (*ahook) (current->value);
 }
 
+/*
+ * Return true iff the named variable has substitute and/or assign hook
+ * functions.
+ */
+bool
+VariableHasHook(VariableSpace space, const char *name)
+{
+	struct _variable *current;
+
+	Assert(space);
+	Assert(name);
+
+	for (current = space->next; current; current = current->next)
+	{
+		int			cmp = strcmp(current->name, name);
+
+		if (cmp == 0)
+			return (current->substitute_hook != NULL ||
+					current->assign_hook != NULL);
+		if (cmp > 0)
+			break;				/* it's not there */
+	}
+
+	return false;
+}
+
 /*
  * Convenience function to set a variable's value to "on".
  */

src/bin/psql/variables.h

@@ -90,6 +90,7 @@ bool		DeleteVariable(VariableSpace space, const char *name);
 void		SetVariableHooks(VariableSpace space, const char *name,
 							 VariableSubstituteHook shook,
 							 VariableAssignHook ahook);
+bool		VariableHasHook(VariableSpace space, const char *name);
 
 void		PsqlVarEnumError(const char *name, const char *value, const char *suggestions);
 

src/test/regress/expected/psql.out

@@ -106,6 +106,10 @@ select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 select 10 as "bad name"
 \gset
 invalid variable name: "bad name"
+select 97 as "EOF", 'ok' as _foo \gset IGNORE
+attempt to \gset into specially treated variable "IGNOREEOF" ignored
+\echo :IGNORE_foo :IGNOREEOF
+ok 0
 -- multiple backslash commands in one line
 select 1 as x, 2 as y \gset pref01_ \\ \echo :pref01_x
 1

src/test/regress/sql/psql.sql

@@ -55,6 +55,9 @@ select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 select 10 as "bad name"
 \gset
 
+select 97 as "EOF", 'ok' as _foo \gset IGNORE
+\echo :IGNORE_foo :IGNOREEOF
+
 -- multiple backslash commands in one line
 select 1 as x, 2 as y \gset pref01_ \\ \echo :pref01_x
 select 3 as x, 4 as y \gset pref01_ \echo :pref01_x \echo :pref01_y