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[209.132.180.67]) by mx.google.com with ESMTP id u8-v6si499919plh.376.2018.10.05.08.56.25; Fri, 05 Oct 2018 08:56:40 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=VNTKJ5px; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729039AbeJEWzM (ORCPT + 99 others); Fri, 5 Oct 2018 18:55:12 -0400 Received: from mail-ot1-f68.google.com ([209.85.210.68]:37607 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728970AbeJEWzM (ORCPT ); Fri, 5 Oct 2018 18:55:12 -0400 Received: by mail-ot1-f68.google.com with SMTP id o13-v6so13156341otl.4 for ; Fri, 05 Oct 2018 08:55:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=ryo7SsTtyQiVWhgwZQip6OGwjVNU2KZ/nkMdnpriWSM=; b=VNTKJ5pxpdzzKyPCwsG8eexUzcvbqjTKx2whTJqCD/cRAbgoIlwv1abqpTpv1n3BDm HvL5CMc/JExC6IYqueHOWF/r7x7RxXD3wXUkUTVnqEQXEnJiZfxQnDt14UaRjlTfz0rh I9DLZgclN9155owOMYlo+FsyTOj6aWQZVmN9pEkiAh1LRR3oXKayrX+ZwUiHgA5lqAJa KF+MtWTuDnX17volVh3Lwva0xC2my5Ua2QsBya1pS2MqObMbE6cisJS/QNOnZk6SZJRu Cji0DrCgqQdSdsKMhmeXHewsRyq0KpHSQSRSpNq9nrbBArAuq80h9cb3iosANHAryunY TxPg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=ryo7SsTtyQiVWhgwZQip6OGwjVNU2KZ/nkMdnpriWSM=; b=hcU7BOywc56rTw9tBNyt0eaeFsKztOQ1Rf1RP/KBEWdMg/5n6QWQv98WQoDPkqNMsw ttwiD2OP1MkBndNVrV1a4IaqGcnCPZuntwkEqT8vxt6vR7KJkW40hcRoiFc/3lq8sTGB ARnwjrspZkAA2Zi+b8olNos6/mAv+nFwrgqeRjhCGuCK6MuGIXVhva6t8P67NhMrebih vc07WBCpekvc/4yKv/Q4VMtA4nHOQSj5nLh0uCZuOkU45Pb4kNUwY5WXK7wCsdEtBNtJ 7vv0VfQIgyIHgI5j65yV5Dp5ok0lrEhARvdUzh7YD4aYMi5axoXENNtDBBQshg7q6rsP Uqtg== X-Gm-Message-State: ABuFfojtzy2GD6UUUjpGx+AmNG+/nP7WjlYFm+CIfE/ywppSsoZVTLhe SZFjQYFbM5EsIZg8AiaEXnl1XmesmdNY1ZBkWNItRQ== X-Received: by 2002:a9d:2117:: with SMTP id i23mr6627147otb.230.1538754953560; Fri, 05 Oct 2018 08:55:53 -0700 (PDT) MIME-Version: 1.0 References: <20180929103453.12025-1-cyphar@cyphar.com> <20180929131534.24472-1-cyphar@cyphar.com> <20181004162611.vdlujbdguvagalpt@ryuk> <20181005150728.mgqnpkbukpeu3bsm@ryuk> In-Reply-To: <20181005150728.mgqnpkbukpeu3bsm@ryuk> From: Jann Horn Date: Fri, 5 Oct 2018 17:55:26 +0200 Message-ID: Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution To: cyphar@cyphar.com, "Eric W. Biederman" , Al Viro Cc: jlayton@kernel.org, Bruce Fields , Arnd Bergmann , shuah@kernel.org, David Howells , Andy Lutomirski , christian@brauner.io, Tycho Andersen , kernel list , linux-fsdevel@vger.kernel.org, linux-arch , linux-kselftest@vger.kernel.org, dev@opencontainers.org, containers@lists.linux-foundation.org, Linux API Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 5, 2018 at 5:07 PM Aleksa Sarai wrote: > On 2018-10-04, Jann Horn wrote: > > On Thu, Oct 4, 2018 at 6:26 PM Aleksa Sarai wrote: > > > On 2018-09-29, Jann Horn wrote: > > > > You attempt to open "C/../../etc/passwd" under the root "/A/B". > > > > Something else concurrently moves /A/B/C to /A/C. This can result in > > > > the following: > > > > > > > > 1. You start the path walk and reach /A/B/C. > > > > 2. The other process moves /A/B/C to /A/C. Your path walk is now at /A/C. > > > > 3. Your path walk follows the first ".." up into /A. This is outside > > > > the process root, but you never actually encountered the process root, > > > > so you don't notice. > > > > 4. Your path walk follows the second ".." up to /. Again, this is > > > > outside the process root, but you don't notice. > > > > 5. Your path walk walks down to /etc/passwd, and the open completes > > > > successfully. You now have an fd pointing outside your chroot. > > > > > > I've been playing with this and I have the following patch, which > > > according to my testing protects against attacks where ".." skips over > > > nd->root. It abuses __d_path to figure out if nd->path can be resolved > > > from nd->root (obviously a proper version of this patch would refactor > > > __d_path so it could be used like this -- and would not return > > > -EMULTIHOP). > > > > > > I've also attached my reproducer. With it, I was seeing fairly constant > > > breakouts before this patch and after it I didn't see a single breakout > > > after running it overnight. Obviously this is not conclusive, but I'm > > > hoping that it can show what my idea for protecting against ".." was. > > > > > > Does this patch make sense? Or is there something wrong with it that I'm > > > not seeing? > > > > > > --8<------------------------------------------------------------------- > > > > > > There is a fairly easy-to-exploit race condition with chroot(2) (and > > > thus by extension AT_THIS_ROOT and AT_BENEATH) where a rename(2) of a > > > path can be used to "skip over" nd->root and thus escape to the > > > filesystem above nd->root. > > > > > > thread1 [attacker]: > > > for (;;) > > > renameat2(AT_FDCWD, "/a/b/c", AT_FDCWD, "/a/d", RENAME_EXCHANGE); > > > thread2 [victim]: > > > for (;;) > > > openat(dirb, "b/c/../../etc/shadow", O_THISROOT); > > > > > > With fairly significant regularity, thread2 will resolve to > > > "/etc/shadow" rather than "/a/b/etc/shadow". With this patch, such cases > > > will be detected during ".." resolution (which is the weak point of > > > chroot(2) -- since walking *into* a subdirectory tautologically cannot > > > result in you walking *outside* nd->root). > > > > > > The use of __d_path here might seem suspect, however we don't mind if a > > > path is moved from within the chroot to outside the chroot and we > > > incorrectly decide it is safe (because at that point we are still within > > > the set of files which were accessible at the beginning of resolution). > > > However, we can fail resolution on the next path component if it remains > > > outside of the root. A path which has always been outside nd->root > > > during resolution will never be resolveable from nd->root and thus will > > > always be blocked. > > > > > > DO NOT MERGE: Currently this code returns -EMULTIHOP in this case, > > > purely as a debugging measure (so that you can see that > > > the protection actually does something). Obviously in the > > > proper patch this will return -EXDEV. > > > > > > Signed-off-by: Aleksa Sarai > > > --- > > > fs/namei.c | 32 ++++++++++++++++++++++++++++++-- > > > 1 file changed, 30 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/namei.c b/fs/namei.c > > > index 6f995e6de6b1..c8349693d47b 100644 > > > --- a/fs/namei.c > > > +++ b/fs/namei.c > > > @@ -53,8 +53,8 @@ > > > * The new code replaces the old recursive symlink resolution with > > > * an iterative one (in case of non-nested symlink chains). It does > > > * this with calls to _follow_link(). > > > - * As a side effect, dir_namei(), _namei() and follow_link() are now > > > - * replaced with a single function lookup_dentry() that can handle all > > > + * As a side effect, dir_namei(), _namei() and follow_link() are now > > > + * replaced with a single function lookup_dentry() that can handle all > > > * the special cases of the former code. > > > * > > > * With the new dcache, the pathname is stored at each inode, at least as > > > @@ -1375,6 +1375,20 @@ static int follow_dotdot_rcu(struct nameidata *nd) > > > return -EXDEV; > > > break; > > > } > > > + if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT))) { > > > + char *pathbuf, *pathptr; > > > + > > > + pathbuf = kmalloc(PATH_MAX, GFP_ATOMIC); > > > + if (!pathbuf) > > > + return -ECHILD; > > > + pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); > > > + kfree(pathbuf); > > > + if (IS_ERR_OR_NULL(pathptr)) { > > > + if (!pathptr) > > > + pathptr = ERR_PTR(-EMULTIHOP); > > > + return PTR_ERR(pathptr); > > > + } > > > + } > > > > One somewhat problematic thing about this approach is that if someone > > tries to lookup > > "a/a/a/a/a/a/a/a/a/a/[...]/../../../../../../../../../.." for some > > reason, you'll have quadratic runtime: For each "..", you'll have to > > walk up to the root. > > What if we took rename_lock (call it nd->r_seq) at the start of the > resolution, and then only tried the __d_path-style check > > if (read_seqretry(&rename_lock, nd->r_seq) || > read_seqretry(&mount_lock, nd->m_seq)) > /* do the __d_path lookup. */ > > That way you would only hit the slow path if there were concurrent > renames or mounts *and* you are doing a path resolution with > AT_THIS_ROOT or AT_BENEATH. I've attached a modified patch that does > this (and after some testing it also appears to work). Yeah, I think that might do the job. > I'm not sure if there's a way to always avoid the quadratic lookup > without (significantly and probably unreasonably) changing how dcache > invalidation works. And obviously using this slow path if there was > _any_ rename on the _entire_ system is suboptimal, but I think it is a > significant improvement. Yeah, I think this is much better. > Another possibility is to expand on Andy's suggestion to use > /proc/$pid/root, and instead require AT_THIS_ROOT to use the root of a > namespace as its dirfd (I'm not sure if there's a trivial way to detect > this though). This wouldn't help with AT_BENEATH, but it should protect > against ".." shenanigans without any ".." handling changes. (This is > less ideal because it requires a container process, but it is another > way of dealing with the issue.) (For container usecases, but not for a web server that uses AT_BENEATH.) > --- > fs/namei.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++--- > 1 file changed, 46 insertions(+), 3 deletions(-) > > diff --git a/fs/namei.c b/fs/namei.c > index 6f995e6de6b1..12c9be175cb4 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -493,7 +493,7 @@ struct nameidata { > struct path root; > struct inode *inode; /* path.dentry.d_inode */ > unsigned int flags; > - unsigned seq, m_seq; > + unsigned seq, m_seq, r_seq; > int last_type; > unsigned depth; > int total_link_count; > @@ -1375,6 +1375,27 @@ static int follow_dotdot_rcu(struct nameidata *nd) > return -EXDEV; > break; > } > + if (unlikely((nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT)) && > + (read_seqretry(&rename_lock, nd->r_seq) || > + read_seqretry(&mount_lock, nd->m_seq)))) { > + char *pathbuf, *pathptr; > + > + nd->r_seq = read_seqbegin(&rename_lock); > + /* Cannot take m_seq here. */ > + > + pathbuf = kmalloc(PATH_MAX, GFP_ATOMIC); > + if (!pathbuf) > + return -ECHILD; > + pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); > + kfree(pathbuf); You're doing this check before actually looking up the parent, right? So as long as I don't trigger the "path_equal(&nd->path, &nd->root)" check that you do for O_BENEATH, escaping up by one level is possible, right? You should probably move this check so that it happens after following "..". (Also: I assume that you're going to get rid of that memory allocation in a future version.) > + if (IS_ERR_OR_NULL(pathptr)) { > + int error = PTR_ERR_OR_ZERO(pathptr); > + > + if (!error) > + error = nd_jump_root(nd); > + return error; > + } > + } > if (nd->path.dentry != nd->path.mnt->mnt_root) { > struct dentry *old = nd->path.dentry; > struct dentry *parent = old->d_parent; > @@ -1510,6 +1531,27 @@ static int follow_dotdot(struct nameidata *nd) > return -EXDEV; > break; > } > + if (unlikely((nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROOT)) && > + (read_seqretry(&rename_lock, nd->r_seq) || > + read_seqretry(&mount_lock, nd->m_seq)))) { > + char *pathbuf, *pathptr; > + > + nd->r_seq = read_seqbegin(&rename_lock); > + /* Cannot take m_seq here. */ > + > + pathbuf = kmalloc(PATH_MAX, GFP_KERNEL); > + if (!pathbuf) > + return -ENOMEM; > + pathptr = __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); > + kfree(pathbuf); > + if (IS_ERR_OR_NULL(pathptr)) { > + int error = PTR_ERR_OR_ZERO(pathptr); > + > + if (!error) > + error = nd_jump_root(nd); > + return error; > + } > + } Same problem as in the RCU case above. > if (nd->path.dentry != nd->path.mnt->mnt_root) { > int ret = path_parent_directory(&nd->path); > if (ret) > @@ -2269,6 +2311,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags) > nd->last_type = LAST_ROOT; /* if there are only slashes... */ > nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT; > nd->depth = 0; > + nd->m_seq = read_seqbegin(&mount_lock); > + nd->r_seq = read_seqbegin(&rename_lock); This means that now, attempting to perform a lookup while something is holding the rename_lock will spin on the lock. I don't know whether that's a problem in practice though. Does anyone on this thread know whether this is problematic?