[PATCH] lockdown: Fix kexec lockdown bypass with ima policy

[Eric Snowberg]

The lockdown LSM is primarily used in conjunction with UEFI Secure Boot.
This LSM may also be used on machines without UEFI.  It can also be enabled
when UEFI Secure Boot is disabled. One of lockdown's features is to prevent
kexec from loading untrusted kernels. Lockdown can be enabled through a
bootparam or after the kernel has booted through securityfs.

If IMA appraisal is used with the "ima_appraise=log" boot param,
lockdown can be defeated with kexec on any machine when Secure Boot is
disabled or unavailable. IMA prevents setting "ima_appraise=log"
from the boot param when Secure Boot is enabled, but this does not cover
cases where lockdown is used without Secure Boot.

To defeat lockdown, boot without Secure Boot and add ima_appraise=log
to the kernel command line; then:

$ echo "integrity" > /sys/kernel/security/lockdown
$ echo "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig" > \
  /sys/kernel/security/ima/policy
$ kexec -ls unsigned-kernel

Add a call to verify ima appraisal is set to "enforce" whenever lockdown
is enabled. This fixes CVE-2022-21505.

Fixes: 29d3c1c8dfe7 ("kexec: Allow kexec_file() with appropriate IMA policy when locked down")
Signed-off-by: Eric Snowberg <eric.snowberg at oracle.com>
Acked-by: Mimi Zohar <zohar at linux.ibm.com>
---
 security/integrity/ima/ima_policy.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 73917413365b..a8802b8da946 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -2247,6 +2247,10 @@ bool ima_appraise_signature(enum kernel_read_file_id id)
 	if (id >= READING_MAX_ID)
 		return false;
 
+	if (id == READING_KEXEC_IMAGE && !(ima_appraise & IMA_APPRAISE_ENFORCE)
+	    && security_locked_down(LOCKDOWN_KEXEC))
+		return false;
+
 	func = read_idmap[id] ?: FILE_CHECK;
 
 	rcu_read_lock();
-- 
2.27.0