From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks Date: Wed, 1 Jan 2020 03:08:15 +0000 Message-ID: <20200101030815.GA17593@ZenIV.linux.org.uk> References: <20191230052036.8765-1-cyphar@cyphar.com> <20191230054413.GX4203@ZenIV.linux.org.uk> <20191230054913.c5avdjqbygtur2l7@yavin.dot.cyphar.com> <20191230072959.62kcojxpthhdwmfa@yavin.dot.cyphar.com> <20200101004324.GA11269@ZenIV.linux.org.uk> <20200101005446.GH4203@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200101005446.GH4203@ZenIV.linux.org.uk> Sender: linux-kernel-owner@vger.kernel.org To: Aleksa Sarai Cc: David Howells , Eric Biederman , Linus Torvalds , stable@vger.kernel.org, Christian Brauner , Serge Hallyn , dev@opencontainers.org, containers@lists.linux-foundation.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-api@vger.kernel.org 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 --- 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) {