selinux: do not cancel a policy conversion that never started

sel_write_load() calls selinux_policy_cancel() when sel_make_policy_nodes()
fails, and that helper dereferences the outgoing policy to cancel its
sidtab conversion. On the first policy load there is no outgoing policy:
security_load_policy() returns early for that case, before it converts
anything, and state->policy is still NULL. A first load that fails while
building the selinuxfs tree therefore takes a NULL dereference in
selinux_policy_cancel(), reached from a write(2) to /sys/fs/selinux/load.

Skip the cancel when there is no old policy, mirroring the check
security_load_policy() already makes before it converts.

Cc: stable@vger.kernel.org
Fixes: 02a52c5c8c3b ("selinux: move policy commit after updating selinuxfs")
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 2d828548f3db6aab69e86eb9f1a7d77901c7a20b..90e81186cb2ee348b249273649194d995a900722 100644 (file)
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -2221,7 +2221,9 @@ void selinux_policy_cancel(struct selinux_load_state *load_state)
        oldpolicy = rcu_dereference_protected(state->policy,
                                        lockdep_is_held(&state->policy_mutex));
 
-       sidtab_cancel_convert(oldpolicy->sidtab);
+       /* a first load has no outgoing policy and converted nothing */
+       if (oldpolicy)
+               sidtab_cancel_convert(oldpolicy->sidtab);
        selinux_policy_free(load_state->policy);
        kfree(load_state->convert_data);
 }