[PATCH v2 1/2] evm: Alloc evm_digest in evm_verify_hmac() if CONFIG_VMAP_STACK=y

[Roberto Sassu]

From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

Commit ac4e97abce9b8 ("scatterlist: sg_set_buf() argument must be in linear
mapping") checks that both the signature and the digest reside in the
linear mapping area.

However, more recently commit ba14a194a434c ("fork: Add generic vmalloced
stack support"), made it possible to move the stack in the vmalloc area,
which is not contiguous, and thus not suitable for sg_set_buf() which needs
adjacent pages.

Fix this by checking if CONFIG_VMAP_STACK is enabled. If yes, allocate an
evm_digest structure, and use that instead of the in-stack counterpart.

Cc: stable@xxxxxxxxxxxxxxx # 4.9.x
Fixes: ba14a194a434 ("fork: Add generic vmalloced stack support")
Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
---
 security/integrity/evm/evm_main.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 23d484e05e6f..7f76d6103f2e 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -174,6 +174,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 	struct signature_v2_hdr *hdr;
 	enum integrity_status evm_status = INTEGRITY_PASS;
 	struct evm_digest digest;
+	struct evm_digest *digest_ptr = &digest;
 	struct inode *inode;
 	int rc, xattr_len, evm_immutable = 0;
 
@@ -231,14 +232,26 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 		}
 
 		hdr = (struct signature_v2_hdr *)xattr_data;
-		digest.hdr.algo = hdr->hash_algo;
+
+		if (IS_ENABLED(CONFIG_VMAP_STACK)) {
+			digest_ptr = kmalloc(sizeof(*digest_ptr), GFP_NOFS);
+			if (!digest_ptr) {
+				rc = -ENOMEM;
+				break;
+			}
+		}
+
+		digest_ptr->hdr.algo = hdr->hash_algo;
+
 		rc = evm_calc_hash(dentry, xattr_name, xattr_value,
-				   xattr_value_len, xattr_data->type, &digest);
+				   xattr_value_len, xattr_data->type,
+				   digest_ptr);
 		if (rc)
 			break;
 		rc = integrity_digsig_verify(INTEGRITY_KEYRING_EVM,
 					(const char *)xattr_data, xattr_len,
-					digest.digest, digest.hdr.length);
+					digest_ptr->digest,
+					digest_ptr->hdr.length);
 		if (!rc) {
 			inode = d_backing_inode(dentry);
 
@@ -268,8 +281,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
 		else
 			evm_status = INTEGRITY_FAIL;
 	}
-	pr_debug("digest: (%d) [%*phN]\n", digest.hdr.length, digest.hdr.length,
-		  digest.digest);
+	pr_debug("digest: (%d) [%*phN]\n", digest_ptr->hdr.length,
+		 digest_ptr->hdr.length, digest_ptr->digest);
+
+	if (digest_ptr && digest_ptr != &digest)
+		kfree(digest_ptr);
 out:
 	if (iint)
 		iint->evm_status = evm_status;
-- 
2.25.1