From: Aleksa Sarai <cyphar@cyphar.com>
To: stsp <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 21:58:20 +1000 [thread overview]
Message-ID: <20240507.110127-muggy.duff.trained.hobby-u9ZNUZ9CW5k@cyphar.com> (raw)
In-Reply-To: <5b5cc31f-a5be-4f64-a97b-7708466ace82@yandex.ru>
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On 2024-05-07, stsp <stsp2@yandex.ru> wrote:
> 07.05.2024 10:50, Aleksa Sarai пишет:
> > If you are a privileged process which plans to change users,
>
> Not privileged at all. But I think what you say is still possible with
> userns?
It is possible to configure MOUNT_ATTR_IDMAP in a user namespace but
there are some restrictions that I suspect will make this complicated.
If you try to do something with a regular filesystem you'll probably run
into issues because you won't have CAP_SYS_ADMIN in the super block's
userns. But you could probably do it with tmpfs.
> > 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.
> Doesn't this need a write perm to a
> directory? In his case this is not a threat,
> because you are not supposed to have a
> write perm to that dir. OA2_CRED_INHERIT
> is the only way to write.
No, bind-mounts don't require write permission. As long as you can
resolve the target path you can bind-mount on top of it, so if there's a
subdirectory you can bind-mount / underneath (and if there is only a
file you can bind-mount any file you want to access/overwrite instead).
There are restrictions on mounting through /proc/self/fd/... but they
don't apply here (all files opened by a process doing setns/unshare have
their vfsmounts updated to be from the new mount namespace, meaning you
can do mounts through them with /proc/self/fd/... without issue.)
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2024-05-07 11:58 UTC|newest]
Thread overview: 10+ 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 ` [PATCH v6 0/3] implement OA2_CRED_INHERIT flag for openat2() Aleksa Sarai
2024-05-07 9:02 ` stsp
2024-05-07 11:58 ` Aleksa Sarai [this message]
2024-05-07 12:48 ` stsp
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