x86: introduce /dev/mem restrictions with a config option

This patch introduces a restriction on /dev/mem: Only non-memory can be
read or written unless the newly introduced config option is set.

The X server needs access to /dev/mem for the PCI space, but it doesn't need
access to memory; both the file permissions and SELinux permissions of /dev/mem
just make X effectively super-super powerful. With the exception of the
BIOS area, there's just no valid app that uses /dev/mem on actual memory.
Other popular users of /dev/mem are rootkits and the like.
(note: mmap access of memory via /dev/mem was already not allowed since
a really long time)

People who want to use /dev/mem for kernel debugging can enable the config
option.

The restrictions of this patch have been in the Fedora and RHEL kernels for
at least 4 years without any problems.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 12 insertions, 0 deletions

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 610aaec..0c1890c 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -5,6 +5,18 @@ config TRACE_IRQFLAGS_SUPPORT
 
 source "lib/Kconfig.debug"
 
+config NONPROMISC_DEVMEM
+	bool "Disable promiscuous /dev/mem"
+	default y
+	help
+	  The /dev/mem file by default only allows userspace access to PCI
+	  space and the BIOS code and data regions. This is sufficient for
+	  dosemu and X and all common users of /dev/mem. With this config
+	  option, you allow userspace access to all of memory, including
+	  kernel and userspace memory. Accidental access to this is
+	  obviously disasterous, but specific access can be used by people
+	  debugging the kernel.
+
 config EARLY_PRINTK
 	bool "Early printk" if EMBEDDED
 	default y