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

[Baoquan He <baoquan.he@linux.dev>]

On 07/31/26 at 11:14am, Eric Biggers wrote:
> 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);

Looks good to me, thx.

> 
> > 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.

Sounds great, thanks.