Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Jan 01, 2020 at 12:54:46AM +0000, Al Viro wrote:
> Note, BTW, that lookup_last() (aka walk_component()) does just
> that - we only hit step_into() on LAST_NORM.  The same goes
> for do_last().  mountpoint_last() not doing the same is _not_
> intentional - it's definitely a bug.
> 
> Consider your testcase; link points to . here.  So the only
> thing you could expect from trying to follow it would be
> the directory 'link' lives in.  And you don't have it
> when you reach the fscker via /proc/self/fd/3; what happens
> instead is nd->path set to ./link (by nd_jump_link()) *AND*
> step_into() called, pushing the same ./link onto stack.
> It violates all kinds of assumptions made by fs/namei.c -
> when pushing a symlink onto stack nd->path is expected to
> contain the base directory for resolving it.
> 
> I'm fairly sure that this is the cause of at least some
> of the insanity you've caught; there always could be
> something else, of course, but this hole needs to be
> closed in any case.

... and with removal of now unused local variable, that's

mountpoint_last(): fix the treatment of LAST_BIND

step_into() should be attempted only in LAST_NORM
case, when we have the parent directory (in nd->path).
We get away with that for LAST_DOT and LOST_DOTDOT,
since those can't be symlinks, making step_init() and
equivalent of path_to_nameidata() - we do a bit of
useless work, but that's it.  For LAST_BIND (i.e.
the case when we'd just followed a procfs-style
symlink) we really can't go there - result might
be a symlink and we really can't attempt following
it.

lookup_last() and do_last() do handle that properly;
mountpoint_last() should do the same.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/namei.c b/fs/namei.c
index d6c91d1e88cb..13f9f973722b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2643,7 +2643,6 @@ EXPORT_SYMBOL(user_path_at_empty);
 static int
 mountpoint_last(struct nameidata *nd)
 {
-	int error = 0;
 	struct dentry *dir = nd->path.dentry;
 	struct path path;
 
@@ -2656,10 +2655,7 @@ mountpoint_last(struct nameidata *nd)
 	nd->flags &= ~LOOKUP_PARENT;
 
 	if (unlikely(nd->last_type != LAST_NORM)) {
-		error = handle_dots(nd, nd->last_type);
-		if (error)
-			return error;
-		path.dentry = dget(nd->path.dentry);
+		return handle_dots(nd, nd->last_type);
 	} else {
 		path.dentry = d_lookup(dir, &nd->last);
 		if (!path.dentry) {