Received-SPF: pass (google.com: domain of linux-kernel+bounces-92391-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Message-ID: <3098aef3e5f924e5717b4ba4a34817d9f22ec479.camel@huaweicloud.com>
Subject: Re: [PATCH v2 24/25] commoncap: use vfs fscaps interfaces
From: Roberto Sassu <roberto.sassu@huaweicloud.com>
To: Christian Brauner <brauner@kernel.org>, "Seth Forshee (DigitalOcean)"
	 <sforshee@kernel.org>
Cc: Serge Hallyn <serge@hallyn.com>, Paul Moore <paul@paul-moore.com>, Eric
 Paris <eparis@redhat.com>, James Morris <jmorris@namei.org>, Alexander Viro
 <viro@zeniv.linux.org.uk>, Jan Kara <jack@suse.cz>, Stephen Smalley
 <stephen.smalley.work@gmail.com>, Ondrej Mosnacek <omosnace@redhat.com>, 
 Casey Schaufler <casey@schaufler-ca.com>, Mimi Zohar <zohar@linux.ibm.com>,
 Roberto Sassu <roberto.sassu@huawei.com>,  Dmitry Kasatkin
 <dmitry.kasatkin@gmail.com>, Eric Snowberg <eric.snowberg@oracle.com>,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>, Jonathan Corbet
 <corbet@lwn.net>, Miklos Szeredi <miklos@szeredi.hu>, Amir Goldstein
 <amir73il@gmail.com>,  linux-kernel@vger.kernel.org,
 linux-fsdevel@vger.kernel.org,  linux-security-module@vger.kernel.org,
 audit@vger.kernel.org,  selinux@vger.kernel.org,
 linux-integrity@vger.kernel.org,  linux-doc@vger.kernel.org,
 linux-unionfs@vger.kernel.org
Date: Tue, 05 Mar 2024 13:46:56 +0100
In-Reply-To: <20240305-fachjargon-abmontieren-75b1d6c67a83@brauner>
References: <20240221-idmap-fscap-refactor-v2-0-3039364623bd@kernel.org>
	 <20240221-idmap-fscap-refactor-v2-24-3039364623bd@kernel.org>
	 <dcbd9e7869d2fcce69546b53851d694b8ebad54e.camel@huaweicloud.com>
	 <ZeXpbOsdRTbLsYe9@do-x1extreme>
	 <a7124afa6bed2fcadcb66efa08e256828cd6f8ab.camel@huaweicloud.com>
	 <ZeX9MRhU/EGhHkCY@do-x1extreme>
	 <20240305-fachjargon-abmontieren-75b1d6c67a83@brauner>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.44.4-0ubuntu2 
Precedence: bulk
MIME-Version: 1.0

On Tue, 2024-03-05 at 10:12 +0100, Christian Brauner wrote:
> On Mon, Mar 04, 2024 at 10:56:17AM -0600, Seth Forshee (DigitalOcean) wro=
te:
> > On Mon, Mar 04, 2024 at 05:17:57PM +0100, Roberto Sassu wrote:
> > > On Mon, 2024-03-04 at 09:31 -0600, Seth Forshee (DigitalOcean) wrote:
> > > > On Mon, Mar 04, 2024 at 11:19:54AM +0100, Roberto Sassu wrote:
> > > > > On Wed, 2024-02-21 at 15:24 -0600, Seth Forshee (DigitalOcean) wr=
ote:
> > > > > > Use the vfs interfaces for fetching file capabilities for killp=
riv
> > > > > > checks and from get_vfs_caps_from_disk(). While there, update t=
he
> > > > > > kerneldoc for get_vfs_caps_from_disk() to explain how it is dif=
ferent
> > > > > > from vfs_get_fscaps_nosec().
> > > > > >=20
> > > > > > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org=
>
> > > > > > ---
> > > > > >  security/commoncap.c | 30 +++++++++++++-----------------
> > > > > >  1 file changed, 13 insertions(+), 17 deletions(-)
> > > > > >=20
> > > > > > diff --git a/security/commoncap.c b/security/commoncap.c
> > > > > > index a0ff7e6092e0..751bb26a06a6 100644
> > > > > > --- a/security/commoncap.c
> > > > > > +++ b/security/commoncap.c
> > > > > > @@ -296,11 +296,12 @@ int cap_capset(struct cred *new,
> > > > > >   */
> > > > > >  int cap_inode_need_killpriv(struct dentry *dentry)
> > > > > >  {
> > > > > > -	struct inode *inode =3D d_backing_inode(dentry);
> > > > > > +	struct vfs_caps caps;
> > > > > >  	int error;
> > > > > > =20
> > > > > > -	error =3D __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL=
, 0);
> > > > > > -	return error > 0;
> > > > > > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimpo=
rtant */
> > > > > > +	error =3D vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps)=
;
> > > > > > +	return error =3D=3D 0;
> > > > > >  }
> > > > > > =20
> > > > > >  /**
> > > > > > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *id=
map, struct dentry *dentry)
> > > > > >  {
> > > > > >  	int error;
> > > > > > =20
> > > > > > -	error =3D __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > > > > > +	error =3D vfs_remove_fscaps_nosec(idmap, dentry);
> > > > >=20
> > > > > Uhm, I see that the change is logically correct... but the origin=
al
> > > > > code was not correct, since the EVM post hook is not called (thus=
 the
> > > > > HMAC is broken, or an xattr change is allowed on a portable signa=
ture
> > > > > which should be not).
> > > > >=20
> > > > > For completeness, the xattr change on a portable signature should=
 not
> > > > > happen in the first place, so cap_inode_killpriv() would not be c=
alled.
> > > > > However, since EVM allows same value change, we are here.
> > > >=20
> > > > I really don't understand EVM that well and am pretty hesitant to t=
ry an
> > > > change any of the logic around it. But I'll hazard a thought: shoul=
d EVM
> > > > have a inode_need_killpriv hook which returns an error in this
> > > > situation?
> > >=20
> > > Uhm, I think it would not work without modifying
> > > security_inode_need_killpriv() and the hook definition.
> > >=20
> > > Since cap_inode_need_killpriv() returns 1, the loop stops and EVM wou=
ld
> > > not be invoked. We would need to continue the loop and let EVM know
> > > what is the current return value. Then EVM can reject the change.
> > >=20
> > > An alternative way would be to detect that actually we are setting th=
e
> > > same value for inode metadata, and maybe not returning 1 from
> > > cap_inode_need_killpriv().
> > >=20
> > > I would prefer the second, since EVM allows same value change and we
> > > would have an exception if there are fscaps.
> > >=20
> > > This solves only the case of portable signatures. We would need to
> > > change cap_inode_need_killpriv() anyway to update the HMAC for mutabl=
e
> > > files.
> >=20
> > I see. In any case this sounds like a matter for a separate patch
> > series.
>=20
> Agreed.

Christian, how realistic is that we don't kill priv if we are setting
the same owner?

Serge, would we be able to replace __vfs_removexattr() (or now
vfs_get_fscaps_nosec()) with a security-equivalent alternative?

Thanks

Roberto