From: Aleksa Sarai <cyphar@cyphar.com>
To: Stas Sergeev <stsp2@yandex.ru>
Cc: linux-kernel@vger.kernel.org,
"Stefan Metzmacher" <metze@samba.org>,
"Eric Biederman" <ebiederm@xmission.com>,
"Alexander Viro" <viro@zeniv.linux.org.uk>,
"Andy Lutomirski" <luto@kernel.org>,
"Christian Brauner" <brauner@kernel.org>,
"Jan Kara" <jack@suse.cz>, "Jeff Layton" <jlayton@kernel.org>,
"Chuck Lever" <chuck.lever@oracle.com>,
"Alexander Aring" <alex.aring@gmail.com>,
"David Laight" <David.Laight@aculab.com>,
linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Christian Göttsche" <cgzones@googlemail.com>
Subject: Re: [PATCH v6 0/3] implement OA2_CRED_INHERIT flag for openat2()
Date: Tue, 7 May 2024 17:50:58 +1000 [thread overview]
Message-ID: <20240506.071502-teak.lily.alpine.girls-aiKJgErDohK@cyphar.com> (raw)
In-Reply-To: <20240427112451.1609471-1-stsp2@yandex.ru>
[-- Attachment #1: Type: text/plain, Size: 2570 bytes --]
On 2024-04-27, Stas Sergeev <stsp2@yandex.ru> wrote:
> This patch-set implements the OA2_CRED_INHERIT flag for openat2() syscall.
> It is needed to perform an open operation with the creds that were in
> effect when the dir_fd was opened, if the dir was opened with O_CRED_ALLOW
> flag. This allows the process to pre-open some dirs and switch eUID
> (and other UIDs/GIDs) to the less-privileged user, while still retaining
> the possibility to open/create files within the pre-opened directory set.
>
> The sand-boxing is security-oriented: symlinks leading outside of a
> sand-box are rejected. /proc magic links are rejected. fds opened with
> O_CRED_ALLOW are always closed on exec() and cannot be passed via unix
> socket.
> The more detailed description (including security considerations)
> is available in the log messages of individual patches.
(I meant to reply last week but I couldn't get my mail server to send
mail...)
It seems to me that this can already be implemented using
MOUNT_ATTR_IDMAP, without creating a new form of credential overriding
within the filesystem (and with such a deceptively simple
implementation...)
If you are a privileged process which plans to change users, you can
create a detached tree with a user mapping that gives that user access
to only that tree. This is far more effective at restricting possible
attacks because id-mapped mounts don't override credentials during VFS
operations (meaning that if you miss something, you have a big problem),
instead they only affect uid-related operations within the filesystem
for that mount. Since this implementation does no inherit
CAP_DAC_OVERRIDE, being able to rewrite uid/gids is all you need.
A new attack I just thought of while writing this mail is that because
there is no RESOLVE_NO_XDEV requirement, it should be possible for the
process to get an arbitrary write primitive by creating a new
userns+mountns and then bind-mounting / underneath the directory. Since
O_CRED_INHERIT uses override_creds, it doesn't care about whether
something about the O_CRED_ALLOW directory changed afterwards. Yes, you
can "just fix this" by adding a RESOLVE_NO_XDEV requirement too, but
given that there have been 2-3 security issues with this design found
already, it makes me feel really uneasy. Using id-mapped mounts avoids
this issue because the new mount will not have the id-mapping applied
and thus there is no security issue.
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 228 bytes --]
next prev parent reply other threads:[~2024-05-07 7:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-27 11:24 [PATCH v6 0/3] implement OA2_CRED_INHERIT flag for openat2() Stas Sergeev
2024-04-27 11:24 ` [PATCH v6 1/3] fs: reorganize path_openat() Stas Sergeev
2024-04-27 11:24 ` [PATCH v6 2/3] open: add O_CRED_ALLOW flag Stas Sergeev
2024-04-27 11:24 ` [PATCH v6 3/3] openat2: add OA2_CRED_INHERIT flag Stas Sergeev
2024-05-04 20:38 ` Donald Buczek
2024-05-04 21:11 ` stsp
2024-05-07 7:50 ` Aleksa Sarai [this message]
2024-05-07 9:02 ` [PATCH v6 0/3] implement OA2_CRED_INHERIT flag for openat2() stsp
2024-05-07 11:58 ` Aleksa Sarai
2024-05-07 12:48 ` stsp
2024-05-21 19:01 ` Jann Horn
2024-05-21 20:35 ` stsp
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20240506.071502-teak.lily.alpine.girls-aiKJgErDohK@cyphar.com \
--to=cyphar@cyphar.com \
--cc=David.Laight@aculab.com \
--cc=alex.aring@gmail.com \
--cc=brauner@kernel.org \
--cc=cgzones@googlemail.com \
--cc=chuck.lever@oracle.com \
--cc=ebiederm@xmission.com \
--cc=jack@suse.cz \
--cc=jlayton@kernel.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@kernel.org \
--cc=metze@samba.org \
--cc=pbonzini@redhat.com \
--cc=stsp2@yandex.ru \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).