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

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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.




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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