fscrypto: require write access to mount to set encryption policy

Since setting an encryption policy requires writing metadata to the
filesystem, it should be guarded by mnt_want_write/mnt_drop_write.
Otherwise, a user could cause a write to a frozen or readonly
filesystem.  This was handled correctly by f2fs but not by ext4.  Make
fscrypt_process_policy() handle it rather than relying on the filesystem
to get it right.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: stable@vger.kernel.org # 4.1+; check fs/{ext4,f2fs}
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/crypto/policy.c

@@ -11,6 +11,7 @@
 #include <linux/random.h>
 #include <linux/string.h>
 #include <linux/fscrypto.h>
+#include <linux/mount.h>
 
 static int inode_has_encryption_context(struct inode *inode)
 {
@@ -92,31 +93,42 @@ static int create_encryption_context_from_policy(struct inode *inode,
 	return inode->i_sb->s_cop->set_context(inode, &ctx, sizeof(ctx), NULL);
 }
 
-int fscrypt_process_policy(struct inode *inode,
+int fscrypt_process_policy(struct file *filp,
 				const struct fscrypt_policy *policy)
 {
+	struct inode *inode = file_inode(filp);
+	int ret;
+
 	if (!inode_owner_or_capable(inode))
 		return -EACCES;
 
 	if (policy->version != 0)
 		return -EINVAL;
 
+	ret = mnt_want_write_file(filp);
+	if (ret)
+		return ret;
+
 	if (!inode_has_encryption_context(inode)) {
 		if (!S_ISDIR(inode->i_mode))
-			return -EINVAL;
-		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);
+			ret = -EINVAL;
+		else if (!inode->i_sb->s_cop->empty_dir)
+			ret = -EOPNOTSUPP;
+		else if (!inode->i_sb->s_cop->empty_dir(inode))
+			ret = -ENOTEMPTY;
+		else
+			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;
+	mnt_drop_write_file(filp);
+	return ret;
 }
 EXPORT_SYMBOL(fscrypt_process_policy);
 

fs/ext4/ioctl.c

@@ -776,7 +776,7 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 				   (struct fscrypt_policy __user *)arg,
 				   sizeof(policy)))
 			return -EFAULT;
-		return fscrypt_process_policy(inode, &policy);
+		return fscrypt_process_policy(filp, &policy);
 #else
 		return -EOPNOTSUPP;
 #endif

fs/f2fs/file.c

@@ -1757,21 +1757,14 @@ static int f2fs_ioc_set_encryption_policy(struct file *filp, unsigned long arg)
 {
 	struct fscrypt_policy policy;
 	struct inode *inode = file_inode(filp);
-	int ret;
 
 	if (copy_from_user(&policy, (struct fscrypt_policy __user *)arg,
 							sizeof(policy)))
 		return -EFAULT;
 
-	ret = mnt_want_write_file(filp);
-	if (ret)
-		return ret;
-
 	f2fs_update_time(F2FS_I_SB(inode), REQ_TIME);
-	ret = fscrypt_process_policy(inode, &policy);
 
-	mnt_drop_write_file(filp);
-	return ret;
+	return fscrypt_process_policy(filp, &policy);
 }
 
 static int f2fs_ioc_get_encryption_policy(struct file *filp, unsigned long arg)

include/linux/fscrypto.h

@@ -274,8 +274,7 @@ extern void fscrypt_restore_control_page(struct page *);
 extern int fscrypt_zeroout_range(struct inode *, pgoff_t, sector_t,
 						unsigned int);
 /* policy.c */
-extern int fscrypt_process_policy(struct inode *,
-					const struct fscrypt_policy *);
+extern int fscrypt_process_policy(struct file *, const struct fscrypt_policy *);
 extern int fscrypt_get_policy(struct inode *, struct fscrypt_policy *);
 extern int fscrypt_has_permitted_context(struct inode *, struct inode *);
 extern int fscrypt_inherit_context(struct inode *, struct inode *,
@@ -345,7 +344,7 @@ static inline int fscrypt_notsupp_zeroout_range(struct inode *i, pgoff_t p,
 }
 
 /* policy.c */
-static inline int fscrypt_notsupp_process_policy(struct inode *i,
+static inline int fscrypt_notsupp_process_policy(struct file *f,
 				const struct fscrypt_policy *p)
 {
 	return -EOPNOTSUPP;