From: ebiederm@xmission.com (Eric W. Biederman)
To: Paul Moore <paul@paul-moore.com>
Cc: omosnace@redhat.com, selinux@vger.kernel.org,
trond.myklebust@primarydata.com, seth.forshee@canonical.com,
linux-fsdevel@vger.kernel.org,
<linux-security-module@vger.kernel.org>
Subject: Re: [PATCH] selinux: always allow mounting submounts
Date: Wed, 28 Nov 2018 09:40:56 -0600 [thread overview]
Message-ID: <87r2f5fbw7.fsf@xmission.com> (raw)
In-Reply-To: <CAHC9VhTvuPvnkfgQhT4+AM3iEz4_p85QgShM_oXmpVtLKuq10Q@mail.gmail.com> (Paul Moore's message of "Mon, 26 Nov 2018 18:25:30 -0500")
Paul Moore <paul@paul-moore.com> writes:
> On Wed, Nov 21, 2018 at 10:38 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>> On Wed, Nov 21, 2018 at 1:41 PM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>> > On Tue, Nov 20, 2018 at 11:09 PM Paul Moore <paul@paul-moore.com> wrote:
>> > > On Fri, Nov 16, 2018 at 8:12 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>> > > > If a superblock has the MS_SUBMOUNT flag set, we should always allow
>> > > > mounting it. These mounts are done automatically by the kernel either as
>> > > > part of mounting some parent mount (e.g. debugfs always mounts tracefs
>> > > > under "tracing" for compatibility) or they are mounted automatically as
>> > > > needed on subdirectory accesses (e.g. NFS crossmnt mounts). Since such
>> > > > automounts are either an implicit consequence of the parent mount (which
>> > > > is already checked) or they can happen during regular accesses (where it
>> > > > doesn't make sense to check against the current task's context), the
>> > > > mount permission check should be skipped for them.
>> > > >
>> > > > Without this patch, attempts to access contents of an automounted
>> > > > directory can cause unexpected SELinux denials.
>> > > >
>> > > > In the current kernel tree, the MS_SUBMOUNT flag is set only via
>> > > > vfs_submount(), which is called only from the following places:
>> > > > - AFS, when automounting special "symlinks" referencing other cells
>> > > > - CIFS, when automounting "referrals"
>> > > > - NFS, when automounting subtrees
>> > > > - debugfs, when automounting tracefs
>> > > >
>> > > > In all cases the submounts are meant to be transparent to the user and
>> > > > it makes sense that if mounting the master is allowed, then so should be
>> > > > the automounts. Note that CAP_SYS_ADMIN capability checking is already
>> > > > skipped for (SB_KERNMOUNT|SB_SUBMOUNT) in:
>> > > > - sget_userns() in fs/super.c:
>> > > > if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) &&
>> > > > !(type->fs_flags & FS_USERNS_MOUNT) &&
>> > > > !capable(CAP_SYS_ADMIN))
>> > > > return ERR_PTR(-EPERM);
>> > > > - sget() in fs/super.c:
>> > > > /* Ensure the requestor has permissions over the target filesystem */
>> > > > if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN))
>> > > > return ERR_PTR(-EPERM);
>> > > >
>> > > > Verified internally on patched RHEL 7.6 with a reproducer using
>> > > > NFS+httpd and selinux-tesuite.
>> > >
>> > > I think this all sounds reasonable, but please verify this with an
>> > > upstream kernel. Upstream our focus is on the upstream kernel
>> > > (surprise!), downstream RHEL is your responsibility, not ours :)
>> >
>> > I tested on RHEL because that's what I can do most conveniently. I
>> > don't have a very good workflow/environment for complex testing on
>> > upstream right now. I don't expect the results to be any different on
>> > the upstream kernel, but I understand your concern. I have been
>> > thinking about some patch testing automation using Fedora Rawhide (I
>> > hope that's close enough to upstream at least :), so I guess it's time
>> > to get scriptin'...
>>
>> I have now tested it on Fedora Rawhide with a scratch kernel with this
>> patch applied [1] (x86_64 only). I ran the whole selinux-testsuite
>> with the submount test [2] and everything passed (except for the known
>> overlay failures and skipped binder test) ...
>
> Merged into selinux/next, thanks.
A few late comments on this.
The change mentioned in fixes did not remove a SB_KERNMOUNT so I don't
see how it is a fix for that. That change just added SB_SUBMOUNT so you
can test for and detect this situation. Are you seeing something that I
am not in that change?
I expect what we need for the long term is to move sb_kern_mount except
for the security mount option bits into do_new_mount so security modules
don't have to perform funny checks because the security hook is in the
wrong place.
Further as far as I can tell from reading the code every filesystem that
performs submounts except for nfs is broken. As no one else calls
security_sb_clone_mnt_opts. Instead the normal mnt_opts hooks are
called with no security mount options.
Which leads me to point that smack doesn't even implement
sb_clone_mnt_opts so I expect smack gets the security mount options
wrong.
Is it common to specify the security mount options on filesystems?
I see the code. I see what needs to be done to keep them working.
(Commas in options names ick). I don't understand how they are used and
how common they are.
I care because the vfs is in the middle of some work to clean up this
side of mounting and at the very least I am review changes and spotting
bugs. Understanding how the security mount options work from the
perspective of someone who actually uses them would be a real help.
Eric
next parent reply other threads:[~2018-11-28 15:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20181116131202.26513-1-omosnace@redhat.com>
[not found] ` <CAHC9VhTn9qz77PQ3mboaCsNzK9D8hjYFg9z5sCWr8Yb7W2Pqxg@mail.gmail.com>
[not found] ` <CAFqZXNsAHnV5RzwtONF0hJRy31qb4c=sajvaEkmEjjky4duEBA@mail.gmail.com>
[not found] ` <CAFqZXNvSdKYr998v3hEhZu4pRwuQs8Q7qRWPhOAtLAc9__Bs6w@mail.gmail.com>
[not found] ` <CAHC9VhTvuPvnkfgQhT4+AM3iEz4_p85QgShM_oXmpVtLKuq10Q@mail.gmail.com>
2018-11-28 15:40 ` Eric W. Biederman [this message]
2018-11-28 16:12 ` [PATCH] selinux: always allow mounting submounts Ondrej Mosnacek
2018-11-28 17:38 ` Eric W. Biederman
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