[PATCH V31 03/25] Restrict /dev/{mem,kmem,port} when the kernel is locked down

From: Matthew Garrett
Date: Tue Mar 26 2019 - 14:30:25 EST

From: Matthew Garrett <mjg59@xxxxxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Matthew Garrett <mjg59@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
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;