linux-kernel - [PATCH v2] efi/efi_test: lock down /dev/efi_test and require CAP_SYS

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Message-Id: <20191008105510.6975-1-javierm@redhat.com>
Date:   Tue,  8 Oct 2019 12:55:10 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Ivan Hu <ivan.hu@...onical.com>, Laszlo Ersek <lersek@...hat.com>,
        linux-efi@...r.kernel.org, Laura Abbott <labbott@...hat.com>,
        Josh Boyer <jwboyer@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Javier Martinez Canillas <javierm@...hat.com>,
        Janne Karhunen <janne.karhunen@...il.com>,
        Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>,
        linux-security-module@...r.kernel.org,
        Casey Schaufler <casey@...aufler-ca.com>,
        Micah Morton <mortonm@...omium.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        James Morris <jmorris@...ei.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>
Subject: [PATCH v2] efi/efi_test: lock down /dev/efi_test and require CAP_SYS_ADMIN

The driver exposes EFI runtime services to user-space through an IOCTL
interface, calling the EFI services function pointers directly without
using the efivar API.

Disallow access to the /dev/efi_test character device when the kernel is
locked down to prevent arbitrary user-space to call EFI runtime services.

Also require CAP_SYS_ADMIN to open the chardev to prevent unprivileged
users to call the EFI runtime services, instead of just relying on the
chardev file mode bits for this.

The main user of this driver is the fwts [0] tool that already checks if
the effective user ID is 0 and fails otherwise. So this change shouldn't
cause any regression to this tool.

[0]: https://wiki.ubuntu.com/FirmwareTestSuite/Reference/uefivarinfo

Signed-off-by: Javier Martinez Canillas <javierm@...hat.com>
Acked-by: Laszlo Ersek <lersek@...hat.com>

---

Changes in v2:
- Also disable /dev/efi_test access when the kernel is locked down as
  suggested by Matthew Garrett.
- Add Acked-by tag from Laszlo Ersek.

 drivers/firmware/efi/test/efi_test.c | 8 ++++++++
 include/linux/security.h             | 1 +
 security/lockdown/lockdown.c         | 1 +
 3 files changed, 10 insertions(+)

diff --git a/drivers/firmware/efi/test/efi_test.c b/drivers/firmware/efi/test/efi_test.c
index 877745c3aaf..7baf48c01e7 100644
--- a/drivers/firmware/efi/test/efi_test.c
+++ b/drivers/firmware/efi/test/efi_test.c
@@ -14,6 +14,7 @@
 #include <linux/init.h>
 #include <linux/proc_fs.h>
 #include <linux/efi.h>
+#include <linux/security.h>
 #include <linux/slab.h>
 #include <linux/uaccess.h>
 
@@ -717,6 +718,13 @@ static long efi_test_ioctl(struct file *file, unsigned int cmd,
 
 static int efi_test_open(struct inode *inode, struct file *file)
 {
+	int ret = security_locked_down(LOCKDOWN_EFI_TEST);
+
+	if (ret)
+		return ret;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EACCES;
 	/*
 	 * nothing special to do here
 	 * We do accept multiple open files at the same time as we
diff --git a/include/linux/security.h b/include/linux/security.h
index a8d59d612d2..9df7547afc0 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -105,6 +105,7 @@ enum lockdown_reason {
 	LOCKDOWN_NONE,
 	LOCKDOWN_MODULE_SIGNATURE,
 	LOCKDOWN_DEV_MEM,
+	LOCKDOWN_EFI_TEST,
 	LOCKDOWN_KEXEC,
 	LOCKDOWN_HIBERNATION,
 	LOCKDOWN_PCI_ACCESS,
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 8a10b43daf7..40b790536de 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -20,6 +20,7 @@ static const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_NONE] = "none",
 	[LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
 	[LOCKDOWN_DEV_MEM] = "/dev/mem,kmem,port",
+	[LOCKDOWN_EFI_TEST] = "/dev/efi_test access",
 	[LOCKDOWN_KEXEC] = "kexec of unsigned images",
 	[LOCKDOWN_HIBERNATION] = "hibernation",
 	[LOCKDOWN_PCI_ACCESS] = "direct PCI access",
-- 
2.21.0