linux-kernel - Re: [PATCH RFC v10 7/17] ipe: add userspace interface

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Message-ID: <CAHC9VhQryLtJZ1W1ogyVuojnq0-ZAU-hfZLwpzUb=bobko9LsA@mail.gmail.com>
Date:   Tue, 1 Aug 2023 15:29:40 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Fan Wu <wufan@...ux.microsoft.com>
Cc:     corbet@....net, zohar@...ux.ibm.com, jmorris@...ei.org,
        serge@...lyn.com, tytso@....edu, ebiggers@...nel.org,
        axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org,
        eparis@...hat.com, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, audit@...r.kernel.org,
        roberto.sassu@...wei.com, linux-kernel@...r.kernel.org,
        Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [PATCH RFC v10 7/17] ipe: add userspace interface

On Fri, Jul 14, 2023 at 11:26 PM Fan Wu <wufan@...ux.microsoft.com> wrote:
> On Sat, Jul 08, 2023 at 12:23:04AM -0400, Paul Moore wrote:
> > On Jun 28, 2023 Fan Wu <wufan@...ux.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@...ux.microsoft.com>
> > > Signed-off-by: Fan Wu <wufan@...ux.microsoft.com>
> > > ---
> > >  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    | 111 +++++++++
> > >  security/ipe/policy.h    |   9 +
> > >  security/ipe/policy_fs.c | 481 +++++++++++++++++++++++++++++++++++++++
> > >  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

...

> > > @@ -39,6 +67,65 @@ static int set_pkcs7_data(void *ctx, const void *data, size_t len,
> > >     return 0;
> > >  }
> > >
> > > +/**
> > > + * ipe_update_policy - parse a new policy and replace @old with it.
> >
> > What does "@old" refer to?  I'm guessing you want to drop the "@".
> >
> Yes it shouldn't be here, sorry confusion.
>
> > > + * @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/@...tlen is mutually exclusive with @pkcs7/@...s7len - see
> > > + * ipe_new_policy.
> > > + *
> > > + * Return:
> > > + * * !IS_ERR       - The old policy
> >
> > "The old policy" is what?
> >
> Let me try to pharse it in another way, how about the existing policy
> saved in the inode before update?

That sounds better, thanks.

> > > diff --git a/security/ipe/policy_fs.c b/security/ipe/policy_fs.c
> > > new file mode 100644
> > > index 000000000000..52a120118cda
> > > --- /dev/null
> > > +++ b/security/ipe/policy_fs.c
> > > @@ -0,0 +1,481 @@
> >
> > ...
> >
> > > +/**
> > > + * getactive - Read handler for "ipe/policies/$name/active".
> > > + * @f: Supplies a file structure representing the securityfs node.
> > > + * @data: Suppleis a buffer passed to the write syscall.
> > > + * @len: Supplies the length of @data.
> > > + * @offset: unused.
> > > + *
> > > + * @data will be populated with the 1 or 0 depending on if the
> > > + * corresponding policy is active.
> > > + *
> > > + * Return:
> > > + * * >0    - Success, Length of buffer written
> > > + * * <0    - Error
> > > + */
> > > +static ssize_t getactive(struct file *f, char __user *data,
> > > +                    size_t len, loff_t *offset)
> > > +{
> > > +   int rc = 0;
> > > +   const char *str;
> > > +   struct inode *root = NULL;
> > > +   const struct ipe_policy *p = NULL;
> > > +
> > > +   root = d_inode(f->f_path.dentry->d_parent);
> > > +
> > > +   inode_lock_shared(root);
> > > +   p = (struct ipe_policy *)root->i_private;
> > > +   if (!p) {
> > > +           inode_unlock_shared(root);
> > > +           return -ENOENT;
> > > +   }
> > > +   inode_unlock_shared(root);
> > > +
> > > +   str = (p == rcu_access_pointer(ipe_active_policy)) ? "1" : "0";
> >
> > The line above should be wrapped with a RCU lock.
>
> This call only checks the value inside the pointer but doesn't dereference it.
> Also from https://lwn.net/Articles/652156/ I found it says "The call to
> rcu_access_pointer() need not be protected. In contrast, rcu_dereference() must
> either be within an RCU read-side critical section", so I didn't add the lock
> here, is this article outdated?

No, I believe you are correct.  There is always something new to learn
with RCU, thanks ;)

-- 
paul-moore.com