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Subject: [PATCH 05/24] Restrict /dev/{mem, kmem,
 port} when the kernel is locked down
From:   David Howells <dhowells@redhat.com>
To:     torvalds@linux-foundation.org
Cc:     linux-man@vger.kernel.org, linux-api@vger.kernel.org,
        jmorris@namei.org, linux-kernel@vger.kernel.org,
        dhowells@redhat.com, linux-security-module@vger.kernel.org
Date:   Wed, 11 Apr 2018 17:25:12 +0100
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From: Matthew Garrett <mjg59@srcf.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@srcf.ucam.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: "Lee, Chun-Yi" <jlee@suse.com>
---

 drivers/char/mem.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index ffeb60d3434c..b2fca26e5765 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -784,6 +784,8 @@ 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;
 }