Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks
From: Al Viro <viro@zeniv.linux.org.uk>
Date: 2020-01-01 00:55:01
Also in:
linux-fsdevel, lkml, stable
On Wed, Jan 01, 2020 at 12:43:24AM +0000, Al Viro wrote:
quoted hunk ↗ jump to hunk
I'm not sure if you have caught anything else, but we really, really should *NOT* consider the LAST_BIND as "maybe we should follow the result" material. So at least the following is needed; could you check if anything else remains with that applied?diff --git a/fs/namei.c b/fs/namei.c index d6c91d1e88cb..d4fbbda8a7ff 100644 --- a/fs/namei.c +++ b/fs/namei.c@@ -2656,10 +2656,7 @@ 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) {
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.