linux-kernel - Re: [PATCH v2 24/25] commoncap: use vfs fscaps interfaces

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Message-ID: <ZeXpbOsdRTbLsYe9@do-x1extreme>
Date: Mon, 4 Mar 2024 09:31:56 -0600
From: "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: Christian Brauner <brauner@...nel.org>, Serge Hallyn <serge@...lyn.com>,
	Paul Moore <paul@...l-moore.com>, Eric Paris <eparis@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	Stephen Smalley <stephen.smalley.work@...il.com>,
	Ondrej Mosnacek <omosnace@...hat.com>,
	Casey Schaufler <casey@...aufler-ca.com>,
	Mimi Zohar <zohar@...ux.ibm.com>,
	Roberto Sassu <roberto.sassu@...wei.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
	Eric Snowberg <eric.snowberg@...cle.com>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Jonathan Corbet <corbet@....net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Amir Goldstein <amir73il@...il.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, audit@...r.kernel.org,
	selinux@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v2 24/25] commoncap: use vfs fscaps interfaces

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) wrote:
> > Use the vfs interfaces for fetching file capabilities for killpriv
> > checks and from get_vfs_caps_from_disk(). While there, update the
> > kerneldoc for get_vfs_caps_from_disk() to explain how it is different
> > from vfs_get_fscaps_nosec().
> > 
> > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> > ---
> >  security/commoncap.c | 30 +++++++++++++-----------------
> >  1 file changed, 13 insertions(+), 17 deletions(-)
> > 
> > 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 = d_backing_inode(dentry);
> > +	struct vfs_caps caps;
> >  	int error;
> >  
> > -	error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> > -	return error > 0;
> > +	/* Use nop_mnt_idmap for no mapping here as mapping is unimportant */
> > +	error = vfs_get_fscaps_nosec(&nop_mnt_idmap, dentry, &caps);
> > +	return error == 0;
> >  }
> >  
> >  /**
> > @@ -323,7 +324,7 @@ int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry)
> >  {
> >  	int error;
> >  
> > -	error = __vfs_removexattr(idmap, dentry, XATTR_NAME_CAPS);
> > +	error = vfs_remove_fscaps_nosec(idmap, dentry);
> 
> Uhm, I see that the change is logically correct... but the original
> 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 signature
> which should be not).
> 
> For completeness, the xattr change on a portable signature should not
> happen in the first place, so cap_inode_killpriv() would not be called.
> However, since EVM allows same value change, we are here.

I really don't understand EVM that well and am pretty hesitant to try an
change any of the logic around it. But I'll hazard a thought: should EVM
have a inode_need_killpriv hook which returns an error in this
situation?