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>

3 files changed, 27 insertions, 22 deletions

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index f96547f83cab..ed115acb5dee 100644
--- a/fs/crypto/policy.c
+++ b/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);
 
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index 10686fd67fb4..1bb7df5e4536 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -776,7 +776,7 @@ resizefs_out:
                                    (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
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 47abb96098e4..28f4f4cbb8d8 100644
--- a/fs/f2fs/file.c
+++ b/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)