[PATCH 10/13] fscrypto: restrict setting new policy to empty files and directories only

[Eric Biggers]

On f2fs, a user could create a regular file of small positive size and
issue FS_IOC_SET_ENCRYPTION_POLICY to set its encryption policy.
However, this did not behave as expected because the existing data was
not actually encrypted by the ioctl.

Fix this by only permitting an encryption policy to be created for empty
regular files and directories.

For a correct solution, it is necessary to conduct the operation under
the inode lock; otherwise the inode's size might be changed concurrently.

Signed-off-by: Eric Biggers <ebiggers3@xxxxxxxxx>
---
 fs/crypto/policy.c | 42 ++++++++++++++++++++++++++++++------------
 1 file changed, 30 insertions(+), 12 deletions(-)

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 93244b5..cb5ba27 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -94,23 +94,41 @@ static int create_encryption_context_from_policy(struct inode *inode,
 
 int fscrypt_set_policy(struct inode *inode, const struct fscrypt_policy *policy)
 {
+	int ret = 0;
+
 	if (policy->version != 0)
 		return -EINVAL;
 
+	inode_lock(inode);
+
 	if (!inode_has_encryption_context(inode)) {
-		if (!inode->i_sb->s_cop->empty_dir)
-			return -EOPNOTSUPP;
-		if (!inode->i_sb->s_cop->empty_dir(inode))
-			return -ENOTEMPTY;
-		return create_encryption_context_from_policy(inode, policy);
+		/* A new policy may only be set on an empty directory or an
+		 * empty regular file. */
+		ret = -EINVAL;
+		if (S_ISDIR(inode->i_mode)) {
+			if (!inode->i_sb->s_cop->empty_dir)
+				ret = -EOPNOTSUPP;
+			else if (inode->i_sb->s_cop->empty_dir(inode))
+				ret = 0;
+			else
+				ret = -ENOTEMPTY;
+		} else if (S_ISREG(inode->i_mode)) {
+			ret = -ENOTEMPTY;
+			if (inode->i_size == 0)
+				ret = 0;
+		}
+		if (!ret) {
+			ret = create_encryption_context_from_policy(inode,
+								    policy);
+		}
+	} else if (!is_encryption_context_consistent_with_policy(inode, policy)) {
+		printk(KERN_WARNING
+		       "%s: Policy inconsistent with encryption context\n",
+		       __func__);
+		ret = -EINVAL;
 	}
-
-	if (is_encryption_context_consistent_with_policy(inode, policy))
-		return 0;
-
-	printk(KERN_WARNING "%s: Policy inconsistent with encryption context\n",
-	       __func__);
-	return -EINVAL;
+	inode_unlock(inode);
+	return ret;
 }
 EXPORT_SYMBOL(fscrypt_set_policy);
 
-- 
2.7.4