[PATCH] mm/swap: reject swapon() on filesystem-level encrypted files

[Eric Biggers <ebiggers@kernel.org>]

ext4 and f2fs don't prevent filesystem-level encrypted files from being
set up directly as swap files.  In this case, encryption is bypassed.

No one should be doing this, vs. the methods of encrypted swap that
actually do work (such as swapping to a dm-crypt device, or swapping to
a loopback device on top of a filesystem-level encrypted file).

Nevertheless, to prevent user error, make swapon() explicitly reject
this case.  Document this behavior in fscrypt.rst as well.

Fixes: 9bd8212f981e ("ext4 crypto: add encryption policy and password salt support")
Fixes: f424f664f0e8 ("f2fs crypto: add encryption policy and password salt support")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
---
 Documentation/filesystems/fscrypt.rst | 4 ++++
 mm/swapfile.c                         | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index c0dd35f1af12..cba1989777da 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -1238,6 +1238,10 @@ astute users may notice some differences in behavior:
 
 - DAX (Direct Access) is not supported on encrypted files.
 
+- Encrypted files cannot be used directly as swap files.  To swap to
+  an encrypted file, set up a loopback device on top of it.
+  Alternatively, encrypted swap can use a dm-crypt device instead.
+
 - The maximum length of an encrypted symlink is 2 bytes shorter than
   the maximum length of an unencrypted symlink.  For example, on an
   EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 78b49b0658ad..e4991da81b5f 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3650,6 +3650,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		error = -EBUSY;
 		goto bad_swap_unlock_inode;
 	}
+	if (IS_ENCRYPTED(inode)) {
+		error = -EINVAL;
+		goto bad_swap_unlock_inode;
+	}
 
 	/*
 	 * The swap subsystem needs a major overhaul to support this.

base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
-- 
2.55.0