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Subject[PATCH v1 4/5] ima: support fs-verity file digest based signatures
Date
Instead of calculating a file hash and verifying the signature stored
in the security.ima xattr against the calculated file hash, verify the
signature based on the fs-verity's file digest and other metadata. The
fs-verity file digest is a hash that includes the Merkle tree root hash.

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
---
Changelog v1:
- Based on Eric Bigger's review, instead of verifying the fsverity's file
digest directly, sign a hash of it with other file metadata.

security/integrity/ima/ima_api.c | 20 ++++++++++++
security/integrity/ima/ima_appraise.c | 45 ++++++++++++++++++++++++++-
2 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_api.c b/security/integrity/ima/ima_api.c
index 7505563315cb..4fe7bc99378a 100644
--- a/security/integrity/ima/ima_api.c
+++ b/security/integrity/ima/ima_api.c
@@ -14,6 +14,7 @@
#include <linux/xattr.h>
#include <linux/evm.h>
#include <linux/iversion.h>
+#include <linux/fsverity.h>

#include "ima.h"

@@ -200,6 +201,23 @@ int ima_get_action(struct user_namespace *mnt_userns, struct inode *inode,
allowed_algos);
}

+static int ima_collect_verity_digest(struct integrity_iint_cache *iint,
+ struct ima_digest_data *hash)
+{
+ u8 verity_digest[FS_VERITY_MAX_DIGEST_SIZE];
+ enum hash_algo verity_alg;
+ int rc;
+
+ rc = fsverity_collect_digest(iint->inode, verity_digest, &verity_alg);
+ if (rc)
+ return -EINVAL;
+ if (hash->algo != verity_alg)
+ return -EINVAL;
+ hash->length = hash_digest_size[verity_alg];
+ memcpy(hash->digest, verity_digest, hash->length);
+ return 0;
+}
+
/*
* ima_collect_measurement - collect file measurement
*
@@ -251,6 +269,8 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,

if (buf)
result = ima_calc_buffer_hash(buf, size, &hash.hdr);
+ else if (veritysig)
+ result = ima_collect_verity_digest(iint, &hash.hdr);
else
result = ima_calc_file_hash(file, &hash.hdr);

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 549fe051269a..53938aa0497a 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -240,6 +240,11 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
struct evm_ima_xattr_data *xattr_value, int xattr_len,
enum integrity_status *status, const char **cause)
{
+ u8 verity_digest[IMA_MAX_DIGEST_SIZE + 1];
+ struct {
+ struct ima_digest_data hdr;
+ char digest[IMA_MAX_DIGEST_SIZE];
+ } hash;
int rc = -EINVAL, hash_start = 0;

switch (xattr_value->type) {
@@ -277,7 +282,45 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
*status = INTEGRITY_PASS;
break;
case IMA_VERITY_DIGSIG:
- fallthrough;
+ set_bit(IMA_DIGSIG, &iint->atomic_flags);
+
+ /*
+ * The IMA signature is based on a hash of IMA_VERITY_DIGSIG
+ * and the fs-verity file digest, not directly on the
+ * fs-verity file digest. Both digests should probably be
+ * included in the IMA measurement list, but for now this
+ * digest is only used for verifying the IMA signature.
+ */
+ verity_digest[0] = IMA_VERITY_DIGSIG;
+ memcpy(verity_digest + 1, iint->ima_hash->digest,
+ iint->ima_hash->length);
+
+ hash.hdr.algo = iint->ima_hash->algo;
+ hash.hdr.length = iint->ima_hash->length;
+
+ rc = ima_calc_buffer_hash(verity_digest,
+ iint->ima_hash->length + 1,
+ &hash.hdr);
+ if (rc) {
+ *cause = "verity-hashing-error";
+ *status = INTEGRITY_FAIL;
+ break;
+ }
+
+ rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
+ (const char *)xattr_value,
+ xattr_len,
+ hash.hdr.digest,
+ hash.hdr.length);
+
+ if (rc) {
+ *cause = "invalid-verity-signature";
+ *status = INTEGRITY_FAIL;
+ } else {
+ *status = INTEGRITY_PASS;
+ }
+
+ break;
case EVM_IMA_XATTR_DIGSIG:
set_bit(IMA_DIGSIG, &iint->atomic_flags);
rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
--
2.27.0
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