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From: Paul Moore <paul@paul-moore.com>
Date: Mon, 5 Feb 2024 18:10:47 -0500
Message-ID: <CAHC9VhSX4iNHEw89-mpF07cSqgGd1myQ6CUfiQnA9pgg3QS7Tw@mail.gmail.com>
Subject: Re: [PATCH RFC v12 8/20] ipe: add userspace interface
To: Fan Wu <wufan@linux.microsoft.com>
Cc: 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, 
	linux-fscrypt@vger.kernel.org, linux-block@vger.kernel.org, 
	dm-devel@lists.linux.dev, audit@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Deven Bowers <deven.desai@linux.microsoft.com>
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On Mon, Feb 5, 2024 at 6:01=E2=80=AFPM Fan Wu <wufan@linux.microsoft.com> w=
rote:
> On 2/3/2024 2:25 PM, Paul Moore wrote:
> > On Jan 30, 2024 Fan Wu <wufan@linux.microsoft.com> wrote:
> >>
> >> As is typical with LSMs, IPE uses securityfs as its interface with
> >> userspace. for a complete list of the interfaces and the respective
> >> inputs/outputs, please see the documentation under
> >> admin-guide/LSM/ipe.rst
> >>
> >> Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
> >> Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> >> ---
> >> v2:
> >>    + Split evaluation loop, access control hooks,
> >>      and evaluation loop from policy parser and userspace
> >>      interface to pass mailing list character limit
> >>
> >> v3:
> >>    + Move policy load and activation audit event to 03/12
> >>    + Fix a potential panic when a policy failed to load.
> >>    + use pr_warn for a failure to parse instead of an
> >>      audit record
> >>    + Remove comments from headers
> >>    + Add lockdep assertions to ipe_update_active_policy and
> >>      ipe_activate_policy
> >>    + Fix up warnings with checkpatch --strict
> >>    + Use file_ns_capable for CAP_MAC_ADMIN for securityfs
> >>      nodes.
> >>    + Use memdup_user instead of kzalloc+simple_write_to_buffer.
> >>    + Remove strict_parse command line parameter, as it is added
> >>      by the sysctl command line.
> >>    + Prefix extern variables with ipe_
> >>
> >> v4:
> >>    + Remove securityfs to reverse-dependency
> >>    + Add SHA1 reverse dependency.
> >>    + Add versioning scheme for IPE properties, and associated
> >>      interface to query the versioning scheme.
> >>    + Cause a parser to always return an error on unknown syntax.
> >>    + Remove strict_parse option
> >>    + Change active_policy interface from sysctl, to securityfs,
> >>      and change scheme.
> >>
> >> v5:
> >>    + Cause an error if a default action is not defined for each
> >>      operation.
> >>    + Minor function renames
> >>
> >> v6:
> >>    + No changes
> >>
> >> v7:
> >>    + Propagating changes to support the new ipe_context structure in t=
he
> >>      evaluation loop.
> >>
> >>    + Further split the parser and userspace interface changes into
> >>      separate commits.
> >>
> >>    + "raw" was renamed to "pkcs7" and made read only
> >>    + "raw"'s write functionality (update a policy) moved to "update"
> >>    + introduced "version", "policy_name" nodes.
> >>    + "content" renamed to "policy"
> >>    + changes to allow the compiled-in policy to be treated
> >>      identical to deployed-after-the-fact policies.
> >>
> >> v8:
> >>    + Prevent securityfs initialization if the LSM is disabled
> >>
> >> v9:
> >>    + Switch to securityfs_recursive_remove for policy folder deletion
> >>
> >> v10:
> >>    + Simplify and correct concurrency
> >>    + Fix typos
> >>
> >> v11:
> >>    + Correct code comments
> >>
> >> v12:
> >>    + Correct locking and remove redundant code
> >> ---
> >>   security/ipe/Makefile    |   2 +
> >>   security/ipe/fs.c        | 101 +++++++++
> >>   security/ipe/fs.h        |  16 ++
> >>   security/ipe/ipe.c       |   3 +
> >>   security/ipe/ipe.h       |   2 +
> >>   security/ipe/policy.c    | 123 ++++++++++
> >>   security/ipe/policy.h    |   9 +
> >>   security/ipe/policy_fs.c | 469 +++++++++++++++++++++++++++++++++++++=
++
> >>   8 files changed, 725 insertions(+)
> >>   create mode 100644 security/ipe/fs.c
> >>   create mode 100644 security/ipe/fs.h
> >>   create mode 100644 security/ipe/policy_fs.c
> >
> > ...
> >
> >> diff --git a/security/ipe/policy.c b/security/ipe/policy.c
> >> index f22a576a6d68..61fea3e38e11 100644
> >> --- a/security/ipe/policy.c
> >> +++ b/security/ipe/policy.c
> >> @@ -43,6 +71,68 @@ static int set_pkcs7_data(void *ctx, const void *da=
ta, size_t len,
> >>      return 0;
> >>   }
> >>
> >> +/**
> >> + * ipe_update_policy - parse a new policy and replace old with it.
> >> + * @root: Supplies a pointer to the securityfs inode saved the policy=
.
> >> + * @text: Supplies a pointer to the plain text policy.
> >> + * @textlen: Supplies the length of @text.
> >> + * @pkcs7: Supplies a pointer to a buffer containing a pkcs7 message.
> >> + * @pkcs7len: Supplies the length of @pkcs7len.
> >> + *
> >> + * @text/@textlen is mutually exclusive with @pkcs7/@pkcs7len - see
> >> + * ipe_new_policy.
> >> + *
> >> + * Context: Requires root->i_rwsem to be held.
> >> + * Return:
> >> + * * !IS_ERR        - The existing policy saved in the inode before u=
pdate
> >> + * * -ENOENT        - Policy doesn't exist
> >> + * * -EINVAL        - New policy is invalid
> >> + */
> >> +struct ipe_policy *ipe_update_policy(struct inode *root,
> >> +                                 const char *text, size_t textlen,
> >> +                                 const char *pkcs7, size_t pkcs7len)
> >> +{
> >> +    int rc =3D 0;
> >> +    struct ipe_policy *old, *ap, *new =3D NULL;
> >> +
> >> +    old =3D (struct ipe_policy *)root->i_private;
> >> +    if (!old)
> >> +            return ERR_PTR(-ENOENT);
> >> +
> >> +    new =3D ipe_new_policy(text, textlen, pkcs7, pkcs7len);
> >> +    if (IS_ERR(new))
> >> +            return new;
> >> +
> >> +    if (strcmp(new->parsed->name, old->parsed->name)) {
> >> +            rc =3D -EINVAL;
> >> +            goto err;
> >> +    }
> >> +
> >> +    if (ver_to_u64(old) > ver_to_u64(new)) {
> >> +            rc =3D -EINVAL;
> >> +            goto err;
> >> +    }
> >> +
> >> +    root->i_private =3D new;
> >> +    swap(new->policyfs, old->policyfs);
> >
> > Should the swap() take place with @ipe_policy_lock held?
> >
> I think we are safe here because root->i_rwsem is held. Other two
> operations set_active and delete are also depending on the inode lock.
> >> +    mutex_lock(&ipe_policy_lock);
> >> +    ap =3D rcu_dereference_protected(ipe_active_policy,
> >> +                                   lockdep_is_held(&ipe_policy_lock))=
;
> >> +    if (old =3D=3D ap) {
> >> +            rcu_assign_pointer(ipe_active_policy, new);
> >> +            mutex_unlock(&ipe_policy_lock);
> >> +            synchronize_rcu();
> >
> > I'm guessing you are forcing a synchronize_rcu() here because you are
> > free()'ing @old in the caller, yes?  Looking at the code, I only see
> > one caller, update_policy().  With only one caller, why not free @old
> > directly in ipe_update_policy()?  Do you see others callers that would
> > do something different?
> >
> The call of synchronize_rcu() is because we are updating the current
> active policy so we need to set the new policy as active.

Unless I'm mistaken, a syncronize_rcu() call only ensures that the
current task will see the updated value by waiting until all current
RCU critical sections have finished.  Given the mutex involved here I
don't believe this is necessary, but please correct me if I'm wrong.

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