linux-kernel - [PATCH] acpi: disallow loading configfs acpi tables when locked down

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Message-Id: <20200615104332.901519-1-Jason@zx2c4.com>
Date:   Mon, 15 Jun 2020 04:43:32 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        mjg59@...f.ucam.org, kernel-hardening@...ts.openwall.com
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>, stable@...r.kernel.org
Subject: [PATCH] acpi: disallow loading configfs acpi tables when locked down

Like other vectors already patched, this one here allows the root user
to load ACPI tables, which enables arbitrary physical address writes,
which in turn makes it possible to disable lockdown. This patch prevents
this by checking the lockdown status before allowing a new ACPI table to be
installed. The link in the trailer shows a PoC of how this might be
used.

Link: https://git.zx2c4.com/american-unsigned-language/tree/american-unsigned-language-2.sh
Cc: stable@...r.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/acpi/acpi_configfs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/acpi_configfs.c b/drivers/acpi/acpi_configfs.c
index ece8c1a921cc..88c8af455ea3 100644
--- a/drivers/acpi/acpi_configfs.c
+++ b/drivers/acpi/acpi_configfs.c
@@ -11,6 +11,7 @@
 #include <linux/module.h>
 #include <linux/configfs.h>
 #include <linux/acpi.h>
+#include <linux/security.h>
 
 #include "acpica/accommon.h"
 #include "acpica/actables.h"
@@ -28,7 +29,10 @@ static ssize_t acpi_table_aml_write(struct config_item *cfg,
 {
 	const struct acpi_table_header *header = data;
 	struct acpi_table *table;
-	int ret;
+	int ret = security_locked_down(LOCKDOWN_ACPI_TABLES);
+
+	if (ret)
+		return ret;
 
 	table = container_of(cfg, struct acpi_table, cfg);
 
-- 
2.27.0