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From: Paul Moore <paul@paul-moore.com>
Date: Sun, 17 Mar 2024 21:58:58 -0400
Message-ID: <CAHC9VhRsOp8-C+bEH+NnRrBu_NRjJongAXZu_P2+c_OkDBs9rw@mail.gmail.com>
Subject: Re: [RFC PATCH v15 05/21] initramfs|security: Add a security hook to do_populate_rootfs()
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Fan Wu <wufan@linux.microsoft.com>, corbet@lwn.net, zohar@linux.ibm.com, 
	jmorris@namei.org, serge@hallyn.com, tytso@mit.edu, ebiggers@kernel.org, 
	axboe@kernel.dk, agk@redhat.com, snitzer@kernel.org, eparis@redhat.com, 
	linux-doc@vger.kernel.org, linux-integrity@vger.kernel.org, 
	linux-security-module@vger.kernel.org, fsverity@lists.linux.dev, 
	linux-block@vger.kernel.org, dm-devel@lists.linux.dev, audit@vger.kernel.org, 
	linux-kernel@vger.kernel.org
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On Sun, Mar 17, 2024 at 8:29=E2=80=AFPM Casey Schaufler <casey@schaufler-ca=
com> wrote:
> On 3/15/2024 8:35 PM, Fan Wu wrote:
> > This patch introduces a new hook to notify security system that the
> > content of initramfs has been unpacked into the rootfs.
> >
> > Upon receiving this notification, the security system can activate
> > a policy to allow only files that originated from the initramfs to
> > execute or load into kernel during the early stages of booting.
> >
> > This approach is crucial for minimizing the attack surface by
> > ensuring that only trusted files from the initramfs are operational
> > in the critical boot phase.
> >
> > Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> >
> > ---
> > v1-v11:
> >   + Not present
> >
> > v12:
> >   + Introduced
> >
> > v13:
> >   + Rename the hook name to initramfs_populated()
> >
> > v14:
> >   + No changes
> >
> > v15:
> >   + No changes
> > ---
> >  include/linux/lsm_hook_defs.h |  2 ++
> >  include/linux/security.h      |  8 ++++++++
> >  init/initramfs.c              |  3 +++
> >  security/security.c           | 10 ++++++++++
> >  4 files changed, 23 insertions(+)
> >
> > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_def=
s.h
> > index 334e00efbde4..7db99ae75651 100644
> > --- a/include/linux/lsm_hook_defs.h
> > +++ b/include/linux/lsm_hook_defs.h
> > @@ -450,3 +450,5 @@ LSM_HOOK(int, 0, uring_override_creds, const struct=
 cred *new)
> >  LSM_HOOK(int, 0, uring_sqpoll, void)
> >  LSM_HOOK(int, 0, uring_cmd, struct io_uring_cmd *ioucmd)
> >  #endif /* CONFIG_IO_URING */
> > +
> > +LSM_HOOK(void, LSM_RET_VOID, initramfs_populated, void)
>
> This is an awfully expensive way to set a flag. Adding a LSM hook list
> isn't free. Isn't there a way to capture this state change through one of
> the mount hooks?

Unfortunately no, the initramfs isn't mounted like a traditional
filesystem, it is "populated" by unpacking the cpio into the initramfs
at early boot.  This LSM hook should be called exactly once during
boot, and the performance impact should be minimal; I should also be
wildly more performant than earlier revisions of this patchset that
required grabbing a single spinlock on every file access.

Of course if you have an idea on how this could be done
differently/better I think we're all open to new ideas ...

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paul-moore.com