Re: [PATCH v8 4/6] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Paul Moore]

On Fri, Mar 24, 2023 at 9:26 AM Roberto Sassu
<roberto.sassu@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> wrote:
> > > > From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxx> (EVM crash)
> > > > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > > > Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> > > > ---
> > > >  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?

-- 
paul-moore.com