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: David Howells <dhowells@redhat.com>
Signed-off-by: Matthew Garrett <redacted>
Cc: x86@kernel.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..67b85939b1bd 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", LOCKDOWN_INTEGRITY))
+		return -EPERM;
 	return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
 }
 

-- 
2.21.0.392.gf8f6787159e-goog