[CVE-2015-1420][Precise][Trusty][Utopic][Vivid] vfs: read file_handle only once in handle_to_path

[Luis Henriques]

From: Sasha Levin <sasha.levin at oracle.com>

We used to read file_handle twice.  Once to get the amount of extra
bytes, and once to fetch the entire structure.

This may be problematic since we do size verifications only after the
first read, so if the number of extra bytes changes in userspace between
the first and second calls, we'll have an incoherent view of
file_handle.

Instead, read the constant size once, and copy that over to the final
structure without having to re-read it again.

Signed-off-by: Sasha Levin <sasha.levin at oracle.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: stable at vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 161f873b89136eb1e69477c847d5a5033239d9ba)
CVE-2015-1420
BugLink: https://bugs.launchpad.net/bugs/1416503
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/fhandle.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 999ff5c3cab0..d59712dfa3e7 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -195,8 +195,9 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		goto out_err;
 	}
 	/* copy the full handle */
-	if (copy_from_user(handle, ufh,
-			   sizeof(struct file_handle) +
+	*handle = f_handle;
+	if (copy_from_user(&handle->f_handle,
+			   &ufh->f_handle,
 			   f_handle.handle_bytes)) {
 		retval = -EFAULT;
 		goto out_handle;