linux-kernel - Re: [PATCH v3 4/5] LSM: lsm_context in security_dentry_init

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Message-ID: <CAEjxPJ6-jL=h-Djxp5MGRbTexQF1vRDPNcwpxCZwFM22Gja0dg@mail.gmail.com>
Date: Thu, 20 Feb 2025 13:02:21 -0500
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Paul Moore <paul@...l-moore.com>
Cc: Casey Schaufler <casey@...aufler-ca.com>, linux-security-module@...r.kernel.org, 
	jmorris@...ei.org, serge@...lyn.com, keescook@...omium.org, 
	john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp, 
	linux-kernel@...r.kernel.org, selinux@...r.kernel.org, mic@...ikod.net, 
	ceph-devel@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: Re: [PATCH v3 4/5] LSM: lsm_context in security_dentry_init_security

On Thu, Feb 20, 2025 at 12:54 PM Paul Moore <paul@...l-moore.com> wrote:
>
> On Thu, Feb 20, 2025 at 12:40 PM Paul Moore <paul@...l-moore.com> wrote:
> > On Thu, Feb 20, 2025 at 11:43 AM Stephen Smalley
> > <stephen.smalley.work@...il.com> wrote:
> > > On Wed, Oct 23, 2024 at 5:23 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
> > > >
> > > > Replace the (secctx,seclen) pointer pair with a single lsm_context
> > > > pointer to allow return of the LSM identifier along with the context
> > > > and context length. This allows security_release_secctx() to know how
> > > > to release the context. Callers have been modified to use or save the
> > > > returned data from the new structure.
> > > >
> > > > Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
> > > > Cc: ceph-devel@...r.kernel.org
> > > > Cc: linux-nfs@...r.kernel.org
> > > > ---
> > > >  fs/ceph/super.h               |  3 +--
> > > >  fs/ceph/xattr.c               | 16 ++++++----------
> > > >  fs/fuse/dir.c                 | 35 ++++++++++++++++++-----------------
> > > >  fs/nfs/nfs4proc.c             | 20 ++++++++++++--------
> > > >  include/linux/lsm_hook_defs.h |  2 +-
> > > >  include/linux/security.h      | 26 +++-----------------------
> > > >  security/security.c           |  9 ++++-----
> > > >  security/selinux/hooks.c      |  9 +++++----
> > > >  8 files changed, 50 insertions(+), 70 deletions(-)
> > > >
> > >
> > > > diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
> > > > index 76776d716744..0b116ef3a752 100644
> > > > --- a/fs/nfs/nfs4proc.c
> > > > +++ b/fs/nfs/nfs4proc.c
> > > > @@ -114,6 +114,7 @@ static inline struct nfs4_label *
> > > >  nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
> > > >         struct iattr *sattr, struct nfs4_label *label)
> > > >  {
> > > > +       struct lsm_context shim;
> > > >         int err;
> > > >
> > > >         if (label == NULL)
> > > > @@ -128,21 +129,24 @@ nfs4_label_init_security(struct inode *dir, struct dentry *dentry,
> > > >         label->label = NULL;
> > > >
> > > >         err = security_dentry_init_security(dentry, sattr->ia_mode,
> > > > -                               &dentry->d_name, NULL,
> > > > -                               (void **)&label->label, &label->len);
> > > > -       if (err == 0)
> > > > -               return label;
> > > > +                               &dentry->d_name, NULL, &shim);
> > > > +       if (err)
> > > > +               return NULL;
> > > >
> > > > -       return NULL;
> > > > +       label->label = shim.context;
> > > > +       label->len = shim.len;
> > > > +       return label;
> > > >  }
> > > >  static inline void
> > > >  nfs4_label_release_security(struct nfs4_label *label)
> > > >  {
> > > > -       struct lsm_context scaff; /* scaffolding */
> > > > +       struct lsm_context shim;
> > > >
> > > >         if (label) {
> > > > -               lsmcontext_init(&scaff, label->label, label->len, 0);
> > > > -               security_release_secctx(&scaff);
> > > > +               shim.context = label->label;
> > > > +               shim.len = label->len;
> > > > +               shim.id = LSM_ID_UNDEF;
> > >
> > > Is there a patch that follows this one to fix this? Otherwise, setting
> > > this to UNDEF causes SELinux to NOT free the context, which produces a
> > > memory leak for every NFS inode security context. Reported by kmemleak
> > > when running the selinux-testsuite NFS tests.
> >
> > I don't recall seeing anything related to this, but patches are
> > definitely welcome.
>
> Looking at this quickly, this is an interesting problem as I don't
> believe we have enough context in nfs4_label_release_security() to
> correctly set the shim.id value.  If there is a positive, it is that
> lsm_context is really still just a string wrapped up with some
> metadata, e.g. length/ID, so we kfree()'ing shim.context is going to
> be okay-ish, at least for the foreseeable future.
>
> I can think of two ways to fix this, but I'd love to hear other ideas too.
>
> 1. Handle the LSM_ID_UNDEF case directly in security_release_secctx()
> and skip any individual LSM processing.
>
> 2. Define a new LSM_ID_ANY value and update all of the LSMs to also
> process the ANY case as well as their own.
>
> I'm not finding either option very exciting, but option #2 looks
> particularly ugly, so I think I'd prefer to see someone draft a patch
> for option #1 assuming nothing better is presented.

We could perhaps add a u32 lsmid to struct nfs4_label, save it from
the shim.id obtained in nfs4_label_init_security(), and use it in
nfs4_label_release_security(). Not sure why that wasn't done in the
first place.