linux-kernel - [RFC][PATCH v2 09/13] ima: Use digest cache for appraisal

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Message-Id: <20230812104616.2190095-10-roberto.sassu@huaweicloud.com>
Date:   Sat, 12 Aug 2023 12:46:12 +0200
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     corbet@....net, zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
        paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, bpf@...r.kernel.org,
        jarkko@...nel.org, pbrobinson@...il.com, zbyszek@...waw.pl,
        hch@....de, mjg59@...f.ucam.org, pmatilai@...hat.com,
        jannh@...gle.com, Roberto Sassu <roberto.sassu@...wei.com>
Subject: [RFC][PATCH v2 09/13] ima: Use digest cache for appraisal

From: Roberto Sassu <roberto.sassu@...wei.com>

If the digest of the accessed file is found in the digest cache, pass the
ANDed masks from the IMA policy and from the digest cache to
ima_appraise_measurement().

If the DIGEST_CACHE_APPRAISE_CONTENT flag is set in the mask, security.ima
is not available, and the modsig method is disabled, grant access in
read-only mode (except for new files).

Since xattrs were not verified with EVM, writes need to be prevented to
avoid the HMAC to be updated from an unverified one.

Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
---
 security/integrity/ima/ima.h          |  6 ++++--
 security/integrity/ima/ima_appraise.c | 14 +++++++++++++-
 security/integrity/ima/ima_main.c     |  3 ++-
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index bb75cc3d2fd..06887f8f1bc 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -322,7 +322,8 @@ int ima_appraise_measurement(enum ima_hooks func,
 			     struct integrity_iint_cache *iint,
 			     struct file *file, const unsigned char *filename,
 			     struct evm_ima_xattr_data *xattr_value,
-			     int xattr_len, const struct modsig *modsig);
+			     int xattr_len, const struct modsig *modsig,
+			     u64 digest_cache_mask);
 int ima_must_appraise(struct mnt_idmap *idmap, struct inode *inode,
 		      int mask, enum ima_hooks func);
 void ima_update_xattr(struct integrity_iint_cache *iint, struct file *file);
@@ -346,7 +347,8 @@ static inline int ima_appraise_measurement(enum ima_hooks func,
 					   const unsigned char *filename,
 					   struct evm_ima_xattr_data *xattr_value,
 					   int xattr_len,
-					   const struct modsig *modsig)
+					   const struct modsig *modsig,
+					   u8 digest_cache_mask)
 {
 	return INTEGRITY_UNKNOWN;
 }
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 10dbafdae3d..969a02802b9 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -479,7 +479,8 @@ int ima_appraise_measurement(enum ima_hooks func,
 			     struct integrity_iint_cache *iint,
 			     struct file *file, const unsigned char *filename,
 			     struct evm_ima_xattr_data *xattr_value,
-			     int xattr_len, const struct modsig *modsig)
+			     int xattr_len, const struct modsig *modsig,
+			     u64 digest_cache_mask)
 {
 	static const char op[] = "appraise_data";
 	const char *cause = "unknown";
@@ -514,6 +515,17 @@ int ima_appraise_measurement(enum ima_hooks func,
 		    (!(iint->flags & IMA_DIGSIG_REQUIRED) ||
 		     (inode->i_size == 0)))
 			status = INTEGRITY_PASS;
+		/*
+		 * Except for new files, use the digest cache to appraise the
+		 * file content and, at the same time, mark the file as
+		 * immutable to prevent file updates and transitioning from an
+		 * unverified HMAC to a valid HMAC.
+		 */
+		if (status != INTEGRITY_PASS &&
+		    (digest_cache_mask & DIGEST_CACHE_APPRAISE_CONTENT)) {
+			set_bit(IMA_DIGSIG, &iint->atomic_flags);
+			status = INTEGRITY_PASS;
+		}
 		goto out;
 	}
 
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 54d006fc490..b458ef62c4c 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -383,7 +383,8 @@ static int process_measurement(struct file *file, const struct cred *cred,
 			inode_lock(inode);
 			rc = ima_appraise_measurement(func, iint, file,
 						      pathname, xattr_value,
-						      xattr_len, modsig);
+						      xattr_len, modsig,
+						      digest_cache_mask);
 			inode_unlock(inode);
 		}
 		if (!rc)
-- 
2.34.1