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[209.132.180.67]) by mx.google.com with ESMTP id k62-v6si8378370pgc.79.2018.10.05.08.08.13; Fri, 05 Oct 2018 08:08:29 -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; 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 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728325AbeJEWGy (ORCPT + 99 others); Fri, 5 Oct 2018 18:06:54 -0400 Received: from mx1.mailbox.org ([80.241.60.212]:50822 "EHLO mx1.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726082AbeJEWGy (ORCPT ); Fri, 5 Oct 2018 18:06:54 -0400 Received: from smtp2.mailbox.org (unknown [IPv6:2001:67c:2050:105:465:1:2:0]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.mailbox.org (Postfix) with ESMTPS id 5FAAE498B3; Fri, 5 Oct 2018 17:07:43 +0200 (CEST) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp2.mailbox.org ([80.241.60.241]) by spamfilter03.heinlein-hosting.de (spamfilter03.heinlein-hosting.de [80.241.56.117]) (amavisd-new, port 10030) with ESMTP id f0UDbB4oMwaa; Fri, 5 Oct 2018 17:07:41 +0200 (CEST) Date: Sat, 6 Oct 2018 01:07:28 +1000 From: Aleksa Sarai To: Jann Horn Cc: "Eric W. Biederman" , jlayton@kernel.org, Bruce Fields , Al Viro , 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 Subject: Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution Message-ID: <20181005150728.mgqnpkbukpeu3bsm@ryuk> References: <20180929103453.12025-1-cyphar@cyphar.com> <20180929131534.24472-1-cyphar@cyphar.com> <20181004162611.vdlujbdguvagalpt@ryuk> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="oh4knwbqqdaenl7c" Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --oh4knwbqqdaenl7c Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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 *n= d) > > return -EXDEV; > > break; > > } > > + if (unlikely(nd->flags & (LOOKUP_BENEATH | LOOKUP_CHROO= T))) { > > + char *pathbuf, *pathptr; > > + > > + pathbuf =3D kmalloc(PATH_MAX, GFP_ATOMIC); > > + if (!pathbuf) > > + return -ECHILD; > > + pathptr =3D __d_path(&nd->path, &nd->root, path= buf, PATH_MAX); > > + kfree(pathbuf); > > + if (IS_ERR_OR_NULL(pathptr)) { > > + if (!pathptr) > > + pathptr =3D ERR_PTR(-EMULTIHOP); > > + return PTR_ERR(pathptr); > > + } > > + } >=20 > 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). 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. 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.) --- 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 =3D read_seqbegin(&rename_lock); + /* Cannot take m_seq here. */ + + pathbuf =3D kmalloc(PATH_MAX, GFP_ATOMIC); + if (!pathbuf) + return -ECHILD; + pathptr =3D __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); + kfree(pathbuf); + if (IS_ERR_OR_NULL(pathptr)) { + int error =3D PTR_ERR_OR_ZERO(pathptr); + + if (!error) + error =3D nd_jump_root(nd); + return error; + } + } if (nd->path.dentry !=3D nd->path.mnt->mnt_root) { struct dentry *old =3D nd->path.dentry; struct dentry *parent =3D 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 =3D read_seqbegin(&rename_lock); + /* Cannot take m_seq here. */ + + pathbuf =3D kmalloc(PATH_MAX, GFP_KERNEL); + if (!pathbuf) + return -ENOMEM; + pathptr =3D __d_path(&nd->path, &nd->root, pathbuf, PATH_MAX); + kfree(pathbuf); + if (IS_ERR_OR_NULL(pathptr)) { + int error =3D PTR_ERR_OR_ZERO(pathptr); + + if (!error) + error =3D nd_jump_root(nd); + return error; + } + } if (nd->path.dentry !=3D nd->path.mnt->mnt_root) { int ret =3D path_parent_directory(&nd->path); if (ret) @@ -2269,6 +2311,9 @@ static const char *path_init(struct nameidata *nd, un= signed flags) nd->last_type =3D LAST_ROOT; /* if there are only slashes... */ nd->flags =3D flags | LOOKUP_JUMPED | LOOKUP_PARENT; nd->depth =3D 0; + nd->m_seq =3D read_seqbegin(&mount_lock); + nd->r_seq =3D read_seqbegin(&rename_lock); + if (flags & LOOKUP_ROOT) { struct dentry *root =3D nd->root.dentry; struct inode *inode =3D root->d_inode; @@ -2279,7 +2324,6 @@ static const char *path_init(struct nameidata *nd, un= signed flags) if (flags & LOOKUP_RCU) { nd->seq =3D __read_seqcount_begin(&nd->path.dentry->d_seq); nd->root_seq =3D nd->seq; - nd->m_seq =3D read_seqbegin(&mount_lock); } else { path_get(&nd->path); } @@ -2290,7 +2334,6 @@ static const char *path_init(struct nameidata *nd, un= signed flags) nd->path.mnt =3D NULL; nd->path.dentry =3D NULL; =20 - nd->m_seq =3D read_seqbegin(&mount_lock); if (unlikely(flags & (LOOKUP_CHROOT | LOOKUP_XDEV))) { error =3D dirfd_path_init(nd); if (unlikely(error)) --=20 2.19.0 --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --oh4knwbqqdaenl7c Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEXzbGxhtUYBJKdfWmnhiqJn3bjbQFAlu3fjAACgkQnhiqJn3b jbSTfw//cFzBBCEdluqIcPeMw8ANpTy3DdtjEtEOPy/DcE4z++lDYLO5jGhSyg8e gdpEaNiiRS3BbSjxUqU9BD2EO8cg6DWtiOdAZfuJo2YKZlK/uuKnm/q+4eGSahYm aHKmDdT4OqMBZR20Cguxkz2lt8DtIGBbw6OZVtgyMZlplpAmVmSj1E+YjIZFhGVe ACr+goArC+WEuGzQiTP1fxqpi91wjpuec4HGeJTP9/296Gr8DkLVUZuZrDx7uOG3 0dznSloj6HSkvqAjam7pfyMm4W5xMgvy8WLod6lpIeM1CY577VoDnysJIYBIGRpS TApxgka9VRQGd/7qukLH5ZIAmJfFVDmOpDfxUSamXdWwFM7521L3WJdombcGS2w7 3NvZW3Ig0O4ekek1rtjRGIUDJ6sZG0PNwokJz93aiMdkp8LTwM3VtbSCPOOAOFab 609ktJefCzqVSj3qmj43lKoB8rinEDMOVcqKna9AisJSTiYXYODrbR5MrQTUV6le fLKF2UIwF4qToDnOn9o2TczI/XRZa8RCtcb3xeXGc2VL6caOQQv7puNximQLRN2H tR79LaoTbEBxNmtHZOFlm0Xt3/2ExV/gHP1KE8QFka+kZqB0iImYo9n7+rka2bEZ Sftv6wwZ2Z37aoO3xdhkhCAvIOQgujuWFRj1PVOWReJ1WtcgZSk= =jMHa -----END PGP SIGNATURE----- --oh4knwbqqdaenl7c--