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, 28 insertions, 0 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 20070b7c573d..dcf6e31970a1 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -108,6 +108,30 @@ static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
 }
 #endif
 
+#ifdef CONFIG_NONPROMISC_DEVMEM
+static inline int range_is_allowed(unsigned long from, unsigned long to)
+{
+	unsigned long cursor;
+
+	cursor = from >> PAGE_SHIFT;
+	while ((cursor << PAGE_SHIFT) < to) {
+		if (!devmem_is_allowed(cursor)) {
+			printk(KERN_INFO "Program %s tried to read /dev/mem "
+				"between %lx->%lx.\n",
+				current->comm, from, to);
+			return 0;
+		}
+		cursor++;
+	}
+	return 1;
+}
+#else
+static inline int range_is_allowed(unsigned long from, unsigned long to)
+{
+	return 1;
+}
+#endif
+
 /*
  * This funcion reads the *physical* memory. The f_pos points directly to the 
  * memory location. 
@@ -157,6 +181,8 @@ static ssize_t read_mem(struct file * file, char __user * buf,
 		 */
 		ptr = xlate_dev_mem_ptr(p);
 
+		if (!range_is_allowed(p, p+count))
+			return -EPERM;
 		if (copy_to_user(buf, ptr, sz))
 			return -EFAULT;
 		buf += sz;
@@ -214,6 +240,8 @@ static ssize_t write_mem(struct file * file, const char __user * buf,
 		 */
 		ptr = xlate_dev_mem_ptr(p);
 
+		if (!range_is_allowed(p, p+sz))
+			return -EPERM;
 		copied = copy_from_user(ptr, buf, sz);
 		if (copied) {
 			written += sz - copied;