hulk inclusion category: bugfix
---------------------------
IMA and EVM have been designed as two independent subsystems: the first for checking the integrity of file data; the second for checking file metadata. Making them independent allows users to adopt them incrementally.
The point of intersection is in IMA-Appraise, which calls evm_verifyxattr() to ensure that security.ima wasn't modified during an offline attack. The design choice, to ensure incremental adoption, was to continue appraisal verification if evm_verifyxattr() returns INTEGRITY_UNKNOWN. This value is returned when EVM is not enabled in the kernel configuration, or if the HMAC key has not been loaded yet.
Although this choice appears legitimate, it might not be suitable for hardened systems, where the administrator expects that access is denied if there is any error. An attacker could intentionally delete the EVM keys from the system and set the file digest in security.ima to the actual file digest so that the final appraisal status is INTEGRITY_PASS.
This patch allows such hardened systems to strictly enforce an access control policy based on the validity of signatures/HMACs, by introducing two new values for the ima_appraise= kernel option: enforce-evm and log-evm.
Cc: stable@vger.kernel.org Fixes: 2fe5d6def1672 ("ima: integrity appraisal extension") Signed-off-by: Roberto Sassu roberto.sassu@huawei.com --- Documentation/admin-guide/kernel-parameters.txt | 3 ++- security/integrity/ima/ima_appraise.c | 8 ++++++++ 2 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index e67621c68dc2..7b4487f08ea1 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -1553,7 +1553,8 @@ Set number of hash buckets for inode cache.
ima_appraise= [IMA] appraise integrity measurements - Format: { "off" | "enforce" | "fix" | "log" } + Format: { "off" | "enforce" | "fix" | "log" | + "enforce-evm" | "log-evm" } default: "enforce"
ima_appraise_tcb [IMA] diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c index d5e62385d891..fb9f7c8deda6 100644 --- a/security/integrity/ima/ima_appraise.c +++ b/security/integrity/ima/ima_appraise.c @@ -18,6 +18,7 @@
#include "ima.h"
+static bool ima_appraise_req_evm __ro_after_init; static int __init default_appraise_setup(char *str) { #ifdef CONFIG_IMA_APPRAISE_BOOTPARAM @@ -28,6 +29,9 @@ static int __init default_appraise_setup(char *str) else if (strncmp(str, "fix", 3) == 0) ima_appraise = IMA_APPRAISE_FIX; #endif + if (strcmp(str, "enforce-evm") == 0 || + strcmp(str, "log-evm") == 0) + ima_appraise_req_evm = true; return 1; }
@@ -244,7 +248,11 @@ int ima_appraise_measurement(enum ima_hooks func, switch (status) { case INTEGRITY_PASS: case INTEGRITY_PASS_IMMUTABLE: + break; case INTEGRITY_UNKNOWN: + if (ima_appraise_req_evm && + xattr_value->type != EVM_IMA_XATTR_DIGSIG) + goto out; break; case INTEGRITY_NOXATTRS: /* No EVM protected xattrs. */ case INTEGRITY_NOLABEL: /* No security.evm xattr. */