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, 1 insertions, 0 deletions

diff --git a/include/asm-x86/page.h b/include/asm-x86/page.h
index 6724a4bc6b7a..b381f4a5a0bd 100644
--- a/include/asm-x86/page.h
+++ b/include/asm-x86/page.h
@@ -47,6 +47,7 @@
 #ifndef __ASSEMBLY__
 
 extern int page_is_ram(unsigned long pagenr);
+extern int devmem_is_allowed(unsigned long pagenr);
 
 extern unsigned long max_pfn_mapped;