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

src/bin/psql/common.c

@@ -795,6 +795,13 @@ StoreQueryTuple(const PGresult *result)
 			/* concatenate prefix and column name */
 			varname = psprintf("%s%s", pset.gset_prefix, colname);
 
+			if (VariableHasHook(pset.vars, varname))
+			{
+				psql_error("attempt to \\gset into specially treated variable \"%s\" ignored\n",
+						   varname);
+				continue;
+			}
+
 			if (!PQgetisnull(result, 0, i))
 				value = PQgetvalue(result, 0, i);
 			else

src/bin/psql/variables.c

@@ -264,6 +264,24 @@ SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook
 	return true;
 }
 
+/*
+ * Return true iff the named variable has an assign hook function.
+ */
+bool
+VariableHasHook(VariableSpace space, const char *name)
+{
+	struct _variable *current;
+
+	Assert(space);
+	Assert(name);
+
+	for (current = space->next; current; current = current->next)
+		if (strcmp(current->name, name) == 0)
+			return current->assign_hook != NULL;
+
+	return false;
+}
+
 bool
 SetVariableBool(VariableSpace space, const char *name)
 {

src/bin/psql/variables.h

@@ -50,6 +50,7 @@ void		PrintVariables(VariableSpace space);
 
 bool		SetVariable(VariableSpace space, const char *name, const char *value);
 bool		SetVariableAssignHook(VariableSpace space, const char *name, VariableAssignHook hook);
+bool		VariableHasHook(VariableSpace space, const char *name);
 bool		SetVariableBool(VariableSpace space, const char *name);
 bool		DeleteVariable(VariableSpace space, const char *name);
 

src/test/regress/expected/psql.out

@@ -10,6 +10,10 @@ select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 select 10 as "bad name"
 \gset
 could not set variable "bad name"
+select 'terse' as "OSITY", 'ok' as _foo \gset VERB
+attempt to \gset into specially treated variable "VERBOSITY" ignored
+\echo :VERB_foo :VERBOSITY
+ok default
 -- 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

@@ -13,6 +13,9 @@ select 10 as test01, 20 as test02, 'Hello' as test03 \gset pref01_
 select 10 as "bad name"
 \gset
 
+select 'terse' as "OSITY", 'ok' as _foo \gset VERB
+\echo :VERB_foo :VERBOSITY
+
 -- 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