ima: measure userspace policy writes before parsing

When a signed policy is not mandatory, userspace can write IMA policy rules
directly to the securityfs policy file:

echo -e "measure func=BPRM_CHECK mask=MAY_EXEC\n" \
        "audit func=BPRM_CHECK mask=MAY_EXEC\n" \
     > /sys/kernel/security/ima/policy

or by cat'ing the entire IMA custom policy file:

cat ima-policy-file > /sys/kernel/security/ima/policy

Because these rules originate from userspace and cross the userspace/kernel
trust boundary, measure the raw write buffer before parsing, regardless of
whether the new policy will be accepted or not. This can be caught when
'measure func=POLICY_CHECK' is enabled (e.g., ima_policy=tcb). The
measurement template is forced to ima-buf.
This follows the "measure & load" paradigm, exposing potential bugs in
the policy code and detecting attempts to corrupt IMA. It also completes
the POLICY_CHECK hook, which already measures partial policy load by file.

To verify the template data hash value, convert the buffer policy data
to binary:
grep "ima_policy_written" \
	/sys/kernel/security/integrity/ima/ascii_runtime_measurements | \
	tail -1 | cut -d' ' -f 6 | xxd -r -p | sha256sum

Suggested-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Enrico Bravi <enrico.bravi@polito.it>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

0 files changed, 0 insertions, 0 deletions