[PATCH linux dev-4.18] /dev/mem: add a devmem kernel parameter to activate the device

[Cédric Le Goater]

For security reasons, some configuration needs to run without /dev/mem
but on some occasions, to debug HW for instance, it's still useful to
be able to reboot the system with access to physical memory.

Add a kernel parameter which activates the /dev/mem device only when
'mem.devmem' is enabled.

Signed-off-by: Cédric Le Goater <clg at kaod.org>
---
 drivers/char/mem.c                              | 12 ++++++++++++
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 drivers/char/Kconfig                            |  9 +++++++++
 3 files changed, 24 insertions(+)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index df66a9dd0aae..8c021a559e6c 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -10,6 +10,7 @@
  */
 
 #include <linux/mm.h>
+#include <linux/moduleparam.h>
 #include <linux/miscdevice.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
@@ -36,6 +37,7 @@
 # include <linux/efi.h>
 #endif
 
+#define DEVMEM_MINOR	1
 #define DEVPORT_MINOR	4
 
 static inline unsigned long size_inside_page(unsigned long start,
@@ -912,6 +914,12 @@ static char *mem_devnode(struct device *dev, umode_t *mode)
 	return NULL;
 }
 
+#ifdef CONFIG_DEVMEM_BOOTPARAM
+static bool devmem;
+module_param(devmem, bool, 0444);
+MODULE_PARM_DESC(devmem, "kernel parameter to activate /dev/mem");
+#endif
+
 static struct class *mem_class;
 
 static int __init chr_dev_init(void)
@@ -930,6 +938,10 @@ static int __init chr_dev_init(void)
 		if (!devlist[minor].name)
 			continue;
 
+#ifdef CONFIG_DEVMEM_BOOTPARAM
+		if (minor == DEVMEM_MINOR && !devmem)
+			continue;
+#endif
 		/*
 		 * Create /dev/port?
 		 */
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1370b424a453..a8ed12d0d678 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2402,6 +2402,9 @@
 			deep    - Suspend-To-RAM or equivalent (if supported)
 			See Documentation/admin-guide/pm/sleep-states.rst.
 
+	mem.devmem=	Activate the /dev/mem device
+			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)
+
 	meye.*=		[HW] Set MotionEye Camera parameters
 			See Documentation/media/v4l-drivers/meye.rst.
 
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 212f447938ae..08c56148190b 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -16,6 +16,15 @@ config DEVMEM
 	  memory.
 	  When in doubt, say "Y".
 
+config DEVMEM_BOOTPARAM
+	bool "mem.devmem boot parameter"
+	depends on DEVMEM
+	default n
+	help
+	  This option adds a 'mem.devmem' kernel parameter which activates
+	  the /dev/mem device when enabled.
+	  When in doubt, say "N".
+
 config DEVKMEM
 	bool "/dev/kmem virtual device support"
 	# On arm64, VMALLOC_START < PAGE_OFFSET, which confuses kmem read/write
-- 
2.17.1