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

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Jul 31, 2026 at 11:11:02PM +0800, Baoquan He wrote:
> On 07/30/26 at 11:48am, Eric Biggers wrote:
> > 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;
> > +	}
> 
> Sounds reasonable to me.
> 
> One minor suggestion: it might be helpful to emit a pr_warn_once()
> explaining *why* the swapon was rejected and pointing users to the
> dm-crypt / loopback alternatives, so the -EINVAL isn't opaque.
> Optional though -- the check itself is what matters.

How's this?

    pr_warn_once("Filesystem-level encrypted swapfile '%s' is unsupported. Create a loop device over it, or use dm-crypt\n", name->name);

> And this patch updates fscrypt.rst. Would it be good to also
> update the swapon(8) man page to document the new -EINVAL
> condition?
> 
> Other than these,
> 
> Reviewed-by: Baoquan He <baoquan.he@linux.dev>

swapon(8) is for the util-linux program, not the syscall, so it doesn't
document error codes specifically.  It does have a NOTES section that
incompletely documents filesystem-specific constraints on swap files.
It's missing most of the current constraints.  But sure, I'll send a
patch that adds a note to there about this one.  That will be a
util-linux patch.

- Eric