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

[Alexei Starovoitov]

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.

> 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. Converting all hooks to 0/-errno
doesn't look practical.