[RAMEN9610-21565]vfs: fix do_last() regression

commit 6404674acd596de41fd3ad5f267b4525494a891a upstream.

Brown paperbag time: fetching ->i_uid/->i_mode really should've been
done from nd->inode.  I even suggested that, but the reason for that has
slipped through the cracks and I went for dir->d_inode instead - made
for more "obvious" patch.

Analysis:

 - at the entry into do_last() and all the way to step_into(): dir (aka
   nd->path.dentry) is known not to have been freed; so's nd->inode and
   it's equal to dir->d_inode unless we are already doomed to -ECHILD.
   inode of the file to get opened is not known.

 - after step_into(): inode of the file to get opened is known; dir
   might be pointing to freed memory/be negative/etc.

 - at the call of may_create_in_sticky(): guaranteed to be out of RCU
   mode; inode of the file to get opened is known and pinned; dir might
   be garbage.

The last was the reason for the oriGinal patch.  Except that at the
do_last() entry we can be in RCU mode and it is possible that
nd->path.dentry->d_inode has already changed under us.

In that case we are going to fail with -ECHILD, but we need to be
careful; nd->inode is pointing to valid struct inode and it's the same
as nd->path.dentry->d_inode in "won't fail with -ECHILD" case, so we
should use that.

Change-Id: I3693a39152fcde59e23adc75845c1532b680167b
Reported-by: "Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@nokia.com>
Reported-by: syzbot+190005201ced78a74ad6@syzkaller.appspotmail.com
Wearing-brown-paperbag: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org
Fixes: d0cb50185ae9 ("do_last(): fetch directory ->i_mode and ->i_uid before it's too late")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit affeac77a4178d0cb586d97c851e6caf24370958)

diff --git a/fs/namei.c b/fs/namei.c
index 0c57b3cae0fe137e602200d819a8aaa952e72a77..ff07ca3602f00297bacaa436c2f717050dc98d75 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3296,8 +3296,8 @@ static int do_last(struct nameidata *nd,
                   int *opened)
 {
        struct dentry *dir = nd->path.dentry;
-       kuid_t dir_uid = dir->d_inode->i_uid;
-       umode_t dir_mode = dir->d_inode->i_mode;
+       kuid_t dir_uid = nd->inode->i_uid;
+       umode_t dir_mode = nd->inode->i_mode;
        int open_flag = op->open_flag;
        bool will_truncate = (open_flag & O_TRUNC) != 0;
        bool got_write = false;