Re: [RFC PATCH] selinux: split no transition execve check

[Paul Moore]

On Thu, Jan 27, 2022 at 8:42 AM Chris PeBenito <pebenito@xxxxxxxx> wrote:
> On 1/26/22 17:51, Paul Moore wrote:
> > On Tue, Jan 25, 2022 at 9:59 AM Christian Göttsche
> > <cgzones@xxxxxxxxxxxxxx> wrote:
> >>
> >> In case a setuid or setgid binary is mislabeled with a generic context,
> >> either via a policy mistake or a move by the distribution package,
> >> executing it will be checked by the file permission execute_no_trans on
> >> the generic file context (e.g. bin_t).  The setuid(2)/setgid(2) syscall
> >> within will then be checked against the unchanged caller process
> >> context, which might have been granted the capability permission setuid/
> >> setgid to initially drop privileges.  To avoid that scenario split the
> >> execute_no_trans permission in case of a setuid/setgid binary into a new
> >> permission execute_sxid_no_trans.
> >>
> >> For backward compatibility this behavior is contained in a new policy
> >> capability.
> >>
> >> Signed-off-by: Christian Göttsche <cgzones@xxxxxxxxxxxxxx>
> >> ---
> >>   security/selinux/hooks.c                   | 9 ++++++++-
> >>   security/selinux/include/classmap.h        | 2 +-
> >>   security/selinux/include/policycap.h       | 1 +
> >>   security/selinux/include/policycap_names.h | 3 ++-
> >>   security/selinux/include/security.h        | 8 ++++++++
> >>   5 files changed, 20 insertions(+), 3 deletions(-)
> >
> > Adding the refpolicy list to this thread as their opinion seems
> > particularly relevant to this discussion.
> >
> > FWIW, this looks reasonable to me but I would like to hear what others
> > have to say.
>
> I think this a band-aid to cover up the real problem, which is the mislabeled files.

It's hard to disagree with that, and the kernel is probably the wrong
place to apply a band-aid unless it is the only option left.

> >> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> >> index 5b6895e4fc29..b825fee39a70 100644
> >> --- a/security/selinux/hooks.c
> >> +++ b/security/selinux/hooks.c
> >> @@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
> >>          ad.u.file = bprm->file;
> >>
> >>          if (new_tsec->sid == old_tsec->sid) {
> >> +               u32 perm;
> >> +
> >> +               if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
> >> +                       perm = FILE__EXECUTE_SXID_NO_TRANS;
> >> +               else
> >> +                       perm = FILE__EXECUTE_NO_TRANS;
> >> +
> >>                  rc = avc_has_perm(&selinux_state,
> >>                                    old_tsec->sid, isec->sid,
> >> -                                 SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> >> +                                 SECCLASS_FILE, perm, &ad);
> >>                  if (rc)
> >>                          return rc;
> >>          } else {
> >> diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> >> index 35aac62a662e..53a1eeeb86fb 100644
> >> --- a/security/selinux/include/classmap.h
> >> +++ b/security/selinux/include/classmap.h
> >> @@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
> >>              "quotaget", "watch", NULL } },
> >>          { "file",
> >>            { COMMON_FILE_PERMS,
> >> -           "execute_no_trans", "entrypoint", NULL } },
> >> +           "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
> >>          { "dir",
> >>            { COMMON_FILE_PERMS, "add_name", "remove_name",
> >>              "reparent", "search", "rmdir", NULL } },
> >> diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
> >> index 2ec038efbb03..23929dc3e1db 100644
> >> --- a/security/selinux/include/policycap.h
> >> +++ b/security/selinux/include/policycap.h
> >> @@ -11,6 +11,7 @@ enum {
> >>          POLICYDB_CAPABILITY_CGROUPSECLABEL,
> >>          POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
> >>          POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
> >> +       POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
> >>          __POLICYDB_CAPABILITY_MAX
> >>   };
> >>   #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
> >> diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
> >> index b89289f092c9..4c014c2cf352 100644
> >> --- a/security/selinux/include/policycap_names.h
> >> +++ b/security/selinux/include/policycap_names.h
> >> @@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
> >>          "always_check_network",
> >>          "cgroup_seclabel",
> >>          "nnp_nosuid_transition",
> >> -       "genfs_seclabel_symlinks"
> >> +       "genfs_seclabel_symlinks",
> >> +       "execute_sxid_no_trans",
> >>   };
> >>
> >>   #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
> >> diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
> >> index ac0ece01305a..ab95241b6b7b 100644
> >> --- a/security/selinux/include/security.h
> >> +++ b/security/selinux/include/security.h
> >> @@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
> >>          return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
> >>   }
> >>
> >> +static inline bool selinux_policycap_execute_sxid_no_trans(void)
> >> +{
> >> +       struct selinux_state *state = &selinux_state;
> >> +
> >> +       return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
> >> +}
> >> +
> >> +
> >>   struct selinux_policy_convert_data;
> >>
> >>   struct selinux_load_state {
> >> --
> >> 2.34.1
> >>
> >
> >
>
>
> --
> Chris PeBenito



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