[PATCH bpf-next v3 01/11] bpf, lsm: Annotate lsm hook return value range

[Paul Moore]

On Sun, Jun 9, 2024 at 1:39 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
> On 6/8/2024 6:54 AM, Alexei Starovoitov wrote:
> > On Sat, Jun 8, 2024 at 1:04 AM Xu Kuohai <xukuohai at huaweicloud.com> wrote:
> >> On 6/7/2024 5:53 AM, Paul Moore wrote:
> >>> On Thu, Apr 11, 2024 at 8:24 AM Xu Kuohai <xukuohai at huaweicloud.com> wrote:
> >>>> From: Xu Kuohai <xukuohai at huawei.com>
> >>>>
> >>>> Add macro LSM_RET_INT to annotate lsm hook return integer type and the
> >>>> default return value, and the expected return range.
> >>>>
> >>>> The LSM_RET_INT is declared as:
> >>>>
> >>>> LSM_RET_INT(defval, min, max)
> >>>>
> >>>> where
> >>>>
> >>>> - defval is the default return value
> >>>>
> >>>> - min and max indicate the expected return range is [min, max]
> >>>>
> >>>> The return value range for each lsm hook is taken from the description
> >>>> in security/security.c.
> >>>>
> >>>> The expanded result of LSM_RET_INT is not changed, and the compiled
> >>>> product is not changed.
> >>>>
> >>>> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
> >>>> ---
> >>>>   include/linux/lsm_hook_defs.h | 591 +++++++++++++++++-----------------
> >>>>   include/linux/lsm_hooks.h     |   6 -
> >>>>   kernel/bpf/bpf_lsm.c          |  10 +
> >>>>   security/security.c           |   1 +
> >>>>   4 files changed, 313 insertions(+), 295 deletions(-)
> >>> ...
> >>>
> >>>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> >>>> index 334e00efbde4..708f515ffbf3 100644
> >>>> --- a/include/linux/lsm_hook_defs.h
> >>>> +++ b/include/linux/lsm_hook_defs.h
> >>>> @@ -18,435 +18,448 @@
> >>>>    * The macro LSM_HOOK is used to define the data structures required by
> >>>>    * the LSM framework using the pattern:
> >>>>    *
> >>>> - *     LSM_HOOK(<return_type>, <default_value>, <hook_name>, args...)
> >>>> + *     LSM_HOOK(<return_type>, <return_description>, <hook_name>, args...)
> >>>>    *
> >>>>    * struct security_hook_heads {
> >>>> - *   #define LSM_HOOK(RET, DEFAULT, NAME, ...) struct hlist_head NAME;
> >>>> + *   #define LSM_HOOK(RET, RETVAL_DESC, NAME, ...) struct hlist_head NAME;
> >>>>    *   #include <linux/lsm_hook_defs.h>
> >>>>    *   #undef LSM_HOOK
> >>>>    * };
> >>>>    */
> >>>> -LSM_HOOK(int, 0, binder_set_context_mgr, const struct cred *mgr)
> >>>> -LSM_HOOK(int, 0, binder_transaction, const struct cred *from,
> >>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_set_context_mgr, const struct cred *mgr)
> >>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transaction, const struct cred *from,
> >>>>           const struct cred *to)
> >>>> -LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from,
> >>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transfer_binder, const struct cred *from,
> >>>>           const struct cred *to)
> >>>> -LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
> >>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transfer_file, const struct cred *from,
> >>>>           const struct cred *to, const struct file *file)
> >>> I'm not overly excited about injecting these additional return value
> >>> range annotations into the LSM hook definitions, especially since the
> >>> vast majority of the hooks "returns 0 on success, negative values on
> >>> error".  I'd rather see some effort put into looking at the
> >>> feasibility of converting some (all?) of the LSM hook return value
> >>> exceptions into the more conventional 0/-ERRNO format.  Unfortunately,
> >>> I haven't had the time to look into that myself, but if you wanted to
> >>> do that I think it would be a good thing.
> >>>
> >> I agree that keeping all hooks return a consistent range of 0/-ERRNO
> >> is more elegant than adding return value range annotations. However, there
> >> are two issues that might need to be addressed first:
> >>
> >> 1. Compatibility
> >>
> >> For instance, security_vm_enough_memory_mm() determines whether to
> >> set cap_sys_admin by checking if the hook vm_enough_memory returns
> >> a positive number. If we were to change the hook vm_enough_memory
> >> to return 0 to indicate the need for cap_sys_admin, then for the
> >> LSM BPF program currently returning 0, the interpretation of its
> >> return value would be reversed after the modification.
> >
> > This is not an issue. bpf lsm progs are no different from other lsm-s.
> > If the meaning of return value or arguments to lsm hook change
> > all lsm-s need to adjust as well. Regardless of whether they are
> > written as in-kernel lsm-s, bpf-lsm, or out-of-tree lsm-s.

Yes, the are no guarantees around compatibility in kernel/LSM
interface from one kernel release to the next.  If we need to change a
LSM hook, we can change a LSM hook; the important part is that when we
change the LSM hook we must make sure to update all of the in-tree
LSMs which make use of that hook.

> >> 2. Expressing multiple non-error states using 0/-ERRNO
> >>
> >> IIUC, although 0/-ERRNO can be used to express different errors,
> >> only 0 can be used for non-error state. If there are multiple
> >> non-error states, they cannot be distinguished. For example,
> >> security_inode_need_killpriv() returns < 0 on error, 0 if
> >> security_inode_killpriv() doesn't need to be called, and > 0
> >> if security_inode_killpriv() does need to be called.
> > This looks like a problem indeed.
>
> Hang on. There aren't really three states here. security_inode_killpriv()
> is called only on the security_inode_need_killpriv() > 0 case. I'm not
> looking at the code this instant, but adjusting the return to something
> like -ENOSYS (OK, maybe not a great choice, but you get the idea) instead
> of 0 in the don't call case and switching the positive value to 0 should
> work just fine.
>
> We're working on getting the LSM interfaces to be more consistent. This
> particular pair of hooks is an example of why we need to do that.

Yes, exactly.  Aside from the issues with BPF verification, we've seen
problems in the past with LSM hooks that differ from the usual "0 on
success, negative values on failure" pattern.  I'm not saying it is
possible to convert all of the hooks to fit this model, but even if we
can only adjust one or two I think that is still a win.

As far as security_inode_need_killpriv()/security_inode_killpriv() is
concerned, one possibility would be to shift the ATTR_KILL_PRIV
set/mask operation into the LSM hook, something like this:

[WARNING: completely untested, likely broken, yadda yadda]

/**
 * ...
 * Returns: Return 0 on success, negative values on failure.  @attrs
may be updated
 *          on success.
 */
int security_inode_need_killpriv(*dentry, attrs)
{
  int rc;
  rc = call_int_hook(inode_killpriv, dentry);
  if (rc < 0)
    return rc;
  if (rc > 0)
    attrs |= ATTR_KILL_PRIV;
  else if (rc == 0)
    attrs &= ~ATTR_KILL_PRIV;
  return 0;
}

Yes, that doesn't fix the problem for the individual LSMs, but it does
make the hook a bit more consistent from the rest of the kernel.

-- 
paul-moore.com