pagemap: do not leak physical addresses to non-privileged userspace

commit ab676b7d upstream.

As pointed by recent post[1] on exploiting DRAM physical imperfection,
/proc/PID/pagemap exposes sensitive information which can be used to do
attacks.

This disallows anybody without CAP_SYS_ADMIN to read the pagemap.

[1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html

[ Eventually we might want to do anything more finegrained, but for now
  this is the simple model.   - Linus ]
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mark Seaborn <mseaborn@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[mancha security: Backported to 3.10]
Signed-off-by: mancha security <mancha1@zoho.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
(cherry picked from commit 1ffc3cd9)
Signed-off-by: Willy Tarreau <w@1wt.eu>

fs/proc/task_mmu.c

@@ -773,9 +773,19 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
 	return ret;
 }
 
+static int pagemap_open(struct inode *inode, struct file *file)
+{
+	/* do not disclose physical addresses to unprivileged
+	   userspace (closes a rowhammer attack vector) */
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+	return 0;
+}
+
 const struct file_operations proc_pagemap_operations = {
 	.llseek		= mem_lseek, /* borrow this */
 	.read		= pagemap_read,
+	.open		= pagemap_open,
 };
 #endif /* CONFIG_PROC_PAGE_MONITOR */