From 75b0cea7bf307f362057cc778efe89af4c615354 Mon Sep 17 00:00:00 2001
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Mon, 15 Jun 2020 04:43:32 -0600
Subject: [PATCH] ACPI: configfs: Disallow loading ACPI tables when locked down
Git-commit: 75b0cea7bf307f362057cc778efe89af4c615354
Patch-mainline: v5.8-rc3
References: CVE-2020-15780 bsc#1173573

[ backport note: Use the downstream kernel_is_locked_down() check instead ]

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.

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: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/acpi/acpi_configfs.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/acpi/acpi_configfs.c
+++ b/drivers/acpi/acpi_configfs.c
@@ -29,6 +29,9 @@ static ssize_t acpi_table_aml_write(stru
 	struct acpi_table *table;
 	int ret;
 
+	if (kernel_is_locked_down())
+		return -EPERM;
+
 	table = container_of(cfg, struct acpi_table, cfg);
 
 	if (table->header) {