+ mm-swap-reject-swapon-on-filesystem-level-encrypted-files.patch added to mm-unstable branch

The patch titled
     Subject: mm/swap: reject swapon() on filesystem-level encrypted files
has been added to the -mm mm-unstable branch.  Its filename is
     mm-swap-reject-swapon-on-filesystem-level-encrypted-files.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-swap-reject-swapon-on-filesystem-level-encrypted-files.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Eric Biggers <[email protected]>
Subject: mm/swap: reject swapon() on filesystem-level encrypted files
Date: Mon, 3 Aug 2026 11:04:26 -0700

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.

Link: https://lore.kernel.org/[email protected]
Fixes: 9bd8212f981e ("ext4 crypto: add encryption policy and password salt support")
Fixes: f424f664f0e8 ("f2fs crypto: add encryption policy and password salt support")
Signed-off-by: Eric Biggers <[email protected]>
Reviewed-by: Baoquan He <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Kemeng Shi <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
---

 Documentation/filesystems/fscrypt.rst |    4 ++++
 mm/swapfile.c                         |    7 +++++++
 2 files changed, 11 insertions(+)

--- a/Documentation/filesystems/fscrypt.rst~mm-swap-reject-swapon-on-filesystem-level-encrypted-files
+++ a/Documentation/filesystems/fscrypt.rst
@@ -1238,6 +1238,10 @@ astute users may notice some differences
 
 - 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.
+
 - 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
--- a/mm/swapfile.c~mm-swap-reject-swapon-on-filesystem-level-encrypted-files
+++ a/mm/swapfile.c
@@ -3662,6 +3662,13 @@ SYSCALL_DEFINE2(swapon, const char __use
 		error = -EBUSY;
 		goto bad_swap_unlock_inode;
 	}
+	if (IS_ENCRYPTED(inode)) {
+		pr_warn_once(
+			"Filesystem-level encrypted swapfile '%s' is unsupported. Create a loop device over it, or use dm-crypt\n",
+			name->name);
+		error = -EINVAL;
+		goto bad_swap_unlock_inode;
+	}
 
 	/*
 	 * The swap subsystem needs a major overhaul to support this.
_

Patches currently in -mm which might be from [email protected] are

mm-swap-reject-swapon-on-filesystem-level-encrypted-files.patch