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

3 files changed, 34 insertions, 0 deletions

diff --git a/src/bin/psql/common.c b/src/bin/psql/common.c
index 9b5b8439608..11200cc6a6f 100644
--- a/src/bin/psql/common.c
+++ b/src/bin/psql/common.c
@@ -942,6 +942,13 @@ 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
diff --git a/src/bin/psql/variables.c b/src/bin/psql/variables.c
index f0934426443..9fde5d86b16 100644
--- a/src/bin/psql/variables.c
+++ b/src/bin/psql/variables.c
@@ -361,6 +361,32 @@ SetVariableHooks(VariableSpace space, const char *name,
 }
 
 /*
+ * 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".
  */
 bool
diff --git a/src/bin/psql/variables.h b/src/bin/psql/variables.h
index 03af11197ca..383ee1dffb9 100644
--- a/src/bin/psql/variables.h
+++ b/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);