linux-kernel - [PATCH 03/27] Restrict /dev/{mem,kmem,port} when the kernel is locked down

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Message-Id: <20190325220954.29054-4-matthewgarrett@google.com>
Date:   Mon, 25 Mar 2019 15:09:30 -0700
From:   Matthew Garrett <matthewgarrett@...gle.com>
To:     jmorris@...ei.org
Cc:     linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, dhowells@...hat.com,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Matthew Garrett <matthewgarrett@...gle.com>, x86@...nel.org
Subject: [PATCH 03/27] Restrict /dev/{mem,kmem,port} when the kernel is locked down

From: Matthew Garrett <mjg59@...f.ucam.org>

Allowing users to read and write to core kernel memory makes it possible
for the kernel to be subverted, avoiding module loading restrictions, and
also to steal cryptographic information.

Disallow /dev/mem and /dev/kmem from being opened this when the kernel has
been locked down to prevent this.

Also disallow /dev/port from being opened to prevent raw ioport access and
thus DMA from being used to accomplish the same thing.

Signed-off-by: Matthew Garrett <mjg59@...f.ucam.org>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Matthew Garrett <matthewgarrett@...gle.com>
Cc: x86@...nel.org
---
 drivers/char/mem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index b08dc50f9f26..0a2f2e75d5f4 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -786,6 +786,8 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
 
 static int open_port(struct inode *inode, struct file *filp)
 {
+	if (kernel_is_locked_down("/dev/mem,kmem,port"))
+		return -EPERM;
 	return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
 }
 
-- 
2.21.0.392.gf8f6787159e-goog