devmem: check vmalloc address on kmem read/write

Otherwise vmalloc_to_page() will BUG().

This also makes the kmem read/write implementation aligned with mem(4):
"References to nonexistent locations cause errors to be returned." Here we
return -ENXIO (inspired by Hugh) if no bytes have been transfered to/from
user space, otherwise return partial read/write results.
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/mem.c

@@ -395,6 +395,7 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
 	unsigned long p = *ppos;
 	ssize_t low_count, read, sz;
 	char * kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
+	int err = 0;
 
 	read = 0;
 	if (p < (unsigned long) high_memory) {
@@ -441,12 +442,16 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
 			return -ENOMEM;
 		while (count > 0) {
 			sz = size_inside_page(p, count);
+			if (!is_vmalloc_or_module_addr((void *)p)) {
+				err = -ENXIO;
+				break;
+			}
 			sz = vread(kbuf, (char *)p, sz);
 			if (!sz)
 				break;
 			if (copy_to_user(buf, kbuf, sz)) {
-				free_page((unsigned long)kbuf);
-				return -EFAULT;
+				err = -EFAULT;
+				break;
 			}
 			count -= sz;
 			buf += sz;
@@ -456,7 +461,7 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
 		free_page((unsigned long)kbuf);
 	}
 	*ppos = p;
- 	return read;
+	return read ? read : err;
 }
 
 
@@ -520,6 +525,7 @@ static ssize_t write_kmem(struct file * file, const char __user * buf,
 	ssize_t wrote = 0;
 	ssize_t virtr = 0;
 	char * kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
+	int err = 0;
 
 	if (p < (unsigned long) high_memory) {
 		unsigned long to_write = min_t(unsigned long, count,
@@ -540,12 +546,14 @@ static ssize_t write_kmem(struct file * file, const char __user * buf,
 			unsigned long sz = size_inside_page(p, count);
 			unsigned long n;
 
+			if (!is_vmalloc_or_module_addr((void *)p)) {
+				err = -ENXIO;
+				break;
+			}
 			n = copy_from_user(kbuf, buf, sz);
 			if (n) {
-				if (wrote + virtr)
+				err = -EFAULT;
 				break;
-				free_page((unsigned long)kbuf);
-				return -EFAULT;
 			}
 			sz = vwrite(kbuf, (char *)p, sz);
 			count -= sz;
@@ -557,7 +565,7 @@ static ssize_t write_kmem(struct file * file, const char __user * buf,
 	}
 
 	*ppos = p;
- 	return virtr + wrote;
+	return virtr + wrote ? : err;
 }
 #endif