ima: Directly assign the ima_default_policy pointer to ima_rules

stable inclusion
from stable-v4.19.129
commit d52a190318f0901965f200f76eb28dd8426bbb25
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I69WIO
CVE: NA

--------------------------------

commit 067a436b upstream.

This patch prevents the following oops:

[   10.771813] BUG: kernel NULL pointer dereference, address: 0000000000000
[...]
[   10.779790] RIP: 0010:ima_match_policy+0xf7/0xb80
[...]
[   10.798576] Call Trace:
[   10.798993]  ? ima_lsm_policy_change+0x2b0/0x2b0
[   10.799753]  ? inode_init_owner+0x1a0/0x1a0
[   10.800484]  ? _raw_spin_lock+0x7a/0xd0
[   10.801592]  ima_must_appraise.part.0+0xb6/0xf0
[   10.802313]  ? ima_fix_xattr.isra.0+0xd0/0xd0
[   10.803167]  ima_must_appraise+0x4f/0x70
[   10.804004]  ima_post_path_mknod+0x2e/0x80
[   10.804800]  do_mknodat+0x396/0x3c0

It occurs when there is a failure during IMA initialization, and
ima_init_policy() is not called. IMA hooks still call ima_match_policy()
but ima_rules is NULL. This patch prevents the crash by directly assigning
the ima_default_policy pointer to ima_rules when ima_rules is defined. This
wouldn't alter the existing behavior, as ima_rules is always set at the end
of ima_init_policy().

Cc: stable@vger.kernel.org # 3.7.x
Fixes: 07f6a794 ("ima: add appraise action keywords and default rules")
Reported-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NRoberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLu Jialin <lujialin4@huawei.com>
Reviewed-by: NGUO zihua <guozihua@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: NWang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

security/integrity/ima/ima_policy.c

@@ -196,7 +196,7 @@ static struct ima_rule_entry secure_boot_rules[] __ro_after_init = {
 static LIST_HEAD(ima_default_rules);
 static LIST_HEAD(ima_policy_rules);
 static LIST_HEAD(ima_temp_rules);
-static struct list_head *ima_rules;
+static struct list_head *ima_rules = &ima_default_rules;
 
 static int ima_policy __initdata;
 
@@ -655,7 +655,6 @@ void __init ima_init_policy(void)
 			temp_ima_appraise |= IMA_APPRAISE_POLICY;
 	}
 
-	ima_rules = &ima_default_rules;
 	ima_update_policy_flag();
 }