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

1 files changed, 26 insertions, 0 deletions

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