Message-ID: <CAHC9VhRaKtsM=CuNhDy0Kx0NGSUrVhG+MhwKnHiyJxfgUwx7nA@mail.gmail.com>
Date:   Mon, 27 Mar 2023 17:02:14 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>
Cc:     mark@...heh.com, jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
        zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, casey@...aufler-ca.com,
        ocfs2-devel@....oracle.com, reiserfs-devel@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: Re: [PATCH v8 4/6] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

On Mon, Mar 27, 2023 at 3:30 AM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
> On Fri, 2023-03-24 at 17:39 -0400, Paul Moore wrote:
> > On Fri, Mar 24, 2023 at 9:26 AM Roberto Sassu
> > <roberto.sassu@...weicloud.com> wrote:
> > > On Fri, 2023-03-24 at 11:18 +0100, Roberto Sassu wrote:
> > > > On Thu, 2023-03-23 at 20:09 -0400, Paul Moore wrote:
> > > > > On Tue, Mar 14, 2023 at 4:19 AM Roberto Sassu
> > > > > <roberto.sassu@...weicloud.com> wrote:
> > > > > > From: Roberto Sassu <roberto.sassu@...wei.com>
> > > > > >
> > > > > > Currently, security_inode_init_security() supports only one LSM providing
> > > > > > an xattr and EVM calculating the HMAC on that xattr, plus other inode
> > > > > > metadata.
> > > > > >
> > > > > > Allow all LSMs to provide one or multiple xattrs, by extending the security
> > > > > > blob reservation mechanism. Introduce the new lbs_xattr field of the
> > > > > > lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
> > > > > > needs, and the LSM infrastructure knows how many xattr slots it should
> > > > > > allocate.
> > > > > >
> > > > > > Dynamically allocate the xattrs array to be populated by LSMs with the
> > > > > > inode_init_security hook, and pass it to the latter instead of the
> > > > > > name/value/len triple. Update the documentation accordingly, and fix the
> > > > > > description of the xattr name, as it is not allocated anymore.
> > > > > >
> > > > > > Since the LSM infrastructure, at initialization time, updates the number of
> > > > > > the requested xattrs provided by each LSM with a corresponding offset in
> > > > > > the security blob (in this case the xattr array), it makes straightforward
> > > > > > for an LSM to access the right position in the xattr array.
> > > > > >
> > > > > > There is still the issue that an LSM might not fill the xattr, even if it
> > > > > > requests it (legitimate case, for example it might have been loaded but not
> > > > > > initialized with a policy). Since users of the xattr array (e.g. the
> > > > > > initxattrs() callbacks) detect the end of the xattr array by checking if
> > > > > > the xattr name is NULL, not filling an xattr would cause those users to
> > > > > > stop scanning xattrs prematurely.
> > > > > >
> > > > > > Solve that issue by introducing security_check_compact_filled_xattrs(),
> > > > > > which does a basic check of the xattr array (if the xattr name is filled,
> > > > > > the xattr value should be too, and viceversa), and compacts the xattr array
> > > > > > by removing the holes.
> > > > > >
> > > > > > An alternative solution would be to let users of the xattr array know the
> > > > > > number of elements of that array, so that they don't have to check the
> > > > > > termination. However, this seems more invasive, compared to a simple move
> > > > > > of few array elements.
> > > > > >
> > > > > > security_check_compact_filled_xattrs() also determines how many xattrs in
> > > > > > the xattr array have been filled. If there is none, skip
> > > > > > evm_inode_init_security() and initxattrs(). Skipping the former also avoids
> > > > > > EVM to crash the kernel, as it is expecting a filled xattr.
> > > > > >
> > > > > > Finally, adapt both SELinux and Smack to use the new definition of the
> > > > > > inode_init_security hook, and to correctly fill the designated slots in the
> > > > > > xattr array. For Smack, reserve space for the other defined xattrs although
> > > > > > they are not set yet in smack_inode_init_security().
> > > > > >
> > > > > > Reported-by: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org> (EVM crash)
> > > > > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> > > > > > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> > > > > > Reviewed-by: Casey Schaufler <casey@...aufler-ca.com>
> > > > > > Reviewed-by: Mimi Zohar <zohar@...ux.ibm.com>
> > > > > > ---
> > > > > >  include/linux/lsm_hook_defs.h |   3 +-
> > > > > >  include/linux/lsm_hooks.h     |   1 +
> > > > > >  security/security.c           | 119 +++++++++++++++++++++++++++++-----
> > > > > >  security/selinux/hooks.c      |  19 ++++--
> > > > > >  security/smack/smack_lsm.c    |  33 ++++++----
> > > > > >  5 files changed, 137 insertions(+), 38 deletions(-)
> >
> > ...
> >
> > > > > > @@ -1604,33 +1654,66 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
> > > > > >                                  const struct qstr *qstr,
> > > > > >                                  const initxattrs initxattrs, void *fs_data)
> > > > > >  {
> > > > > > -       struct xattr new_xattrs[MAX_LSM_EVM_XATTR + 1];
> > > > > > -       struct xattr *lsm_xattr, *evm_xattr, *xattr;
> > > > > > -       int ret;
> > > > > > +       struct security_hook_list *P;
> > > > > > +       struct xattr *new_xattrs;
> > > > > > +       struct xattr *xattr;
> > > > > > +       int ret = -EOPNOTSUPP, num_filled_xattrs = 0;
> > > > > >
> > > > > >         if (unlikely(IS_PRIVATE(inode)))
> > > > > >                 return 0;
> > > > > >
> > > > > > +       if (!blob_sizes.lbs_xattr)
> > > > > > +               return 0;
> > > > > > +
> > > > > >         if (!initxattrs)
> > > > > >                 return call_int_hook(inode_init_security, -EOPNOTSUPP, inode,
> > > > > > -                                    dir, qstr, NULL, NULL, NULL);
> > > > > > -       memset(new_xattrs, 0, sizeof(new_xattrs));
> > > > > > -       lsm_xattr = new_xattrs;
> > > > > > -       ret = call_int_hook(inode_init_security, -EOPNOTSUPP, inode, dir, qstr,
> > > > > > -                           &lsm_xattr->name,
> > > > > > -                           &lsm_xattr->value,
> > > > > > -                           &lsm_xattr->value_len);
> > > > > > -       if (ret)
> > > > > > +                                   dir, qstr, NULL);
> > > > > > +       /* Allocate +1 for EVM and +1 as terminator. */
> > > > > > +       new_xattrs = kcalloc(blob_sizes.lbs_xattr + 2, sizeof(*new_xattrs),
> > > > > > +                            GFP_NOFS);
> > > > > > +       if (!new_xattrs)
> > > > > > +               return -ENOMEM;
> > > > > > +
> > > > > > +       hlist_for_each_entry(P, &security_hook_heads.inode_init_security,
> > > > > > +                            list) {
> > > > > > +               ret = P->hook.inode_init_security(inode, dir, qstr, new_xattrs);
> > > > > > +               if (ret && ret != -EOPNOTSUPP)
> > > > > > +                       goto out;
> > > > > > +               /*
> > > > > > +                * As documented in lsm_hooks.h, -EOPNOTSUPP in this context
> > > > > > +                * means that the LSM is not willing to provide an xattr, not
> > > > > > +                * that it wants to signal an error. Thus, continue to invoke
> > > > > > +                * the remaining LSMs.
> > > > > > +                */
> > > > > > +               if (ret == -EOPNOTSUPP)
> > > > > > +                       continue;
> > > > > > +               /*
> > > > > > +                * As the number of xattrs reserved by LSMs is not directly
> > > > > > +                * available, directly use the total number blob_sizes.lbs_xattr
> > > > > > +                * to keep the code simple, while being not the most efficient
> > > > > > +                * way.
> > > > > > +                */
> > > > >
> > > > > Is there a good reason why the LSM can't return the number of xattrs
> > > > > it is adding to the xattr array?  It seems like it should be fairly
> > > > > trivial for the individual LSMs to determine and it could save a lot
> > > > > of work.  However, given we're at v8 on this patchset I'm sure I'm
> > > > > missing something obvious, can you help me understand why the idea
> > > > > above is crazy stupid? ;)
> > >
> > > Much simple answer. Yes, LSMs could return the number of xattrs set,
> > > but security_check_compact_filled_xattrs() also needs to know from
> > > which offset (the lbs_xattr of each LSM) it should start compacting.
> > >
> > > Example: suppose that you have three LSMs with:
> > >
> > > LSM#1: lbs_xattr 1
> > > LSM#2: lbs_xattr 2 (disabled)
> > > LSM#3: lbs_xattr 1
> > >
> > > The current compaction interval is: already compacted xattrs - end of
> > > new_xattr array.
> > >
> > > When the security_inode_init_security() loop calls LSM#3, the
> > > compaction interval is: 1 - 2 (LSM#2 returns 0), which clearly isn't
> > > right. The correct compaction interval should be: 3 - 4.
> > >
> > > Going to the end of new_xattrs is an approximation, but it ensures
> > > that security_check_compact_filled_xattrs() reaches the xattr set by
> > > LSM#3.
> > >
> > > The alternative I was mentioning of passing num_filled_xattrs to LSMs
> > > goes again in the direction of doing on-the-fly compaction, while LSMs
> > > are more familiar with using the lbs_* fields.
> >
> > I guess I was thinking of the case where the LSM layer, i.e.
> > security_inode_init_security(), allocates an xattr array like it does
> > now based on the maximum number of xattrs possible using the
> > lsm_blob_sizes values and passes a pointer to the individual LSMs
> > which is incremented based on how many xattrs are created by the
> > individual LSMs.  Here is some *very* rough pseudo code:
> >
> > int security_inode_init_security(...)
> > {
> >
> >   /* allocate an xattr array */
> >   xattrs = kcalloc(blob_sizes, sizeof(*xattrs), GFP_BLAH);
> >
> >   /* loop on the lsms */
> >   xa_cnt = 0;
> >   while (lsm_hooks) {
> >     rc = call_hook(lsm_hook, &xattrs[xa_cnt]);
> >     if (rc > 0)
> >       xa_cnt += rc;
> >   }
> >
> >   /* evm magic */
> >   evm_inode_init_security(...)
> > }
> >
> > Does that work?  Am I missing something?
>
> Oh, unfortunately not. EVM needs to see all xattrs (when it is moved to
> the LSM infrastructure).

Okay, that's fair, but we could still pass the full xattrs array and a
reference to the current count which could be both read and updated by
the individual LSMs, right?

The issue is that the separate compaction stage is not something we
want to have to do if we can avoid it.  Maybe we're stuck with it, but
I'm not yet convinced that we can't make some minor changes to the
LSMs to avoid the compaction step.

-- 
paul-moore.com

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