From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-180.mta0.migadu.com (out-180.mta0.migadu.com [91.218.175.180]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 005FD2E7BB6 for ; Sat, 1 Aug 2026 00:47:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785545241; cv=none; b=T237wCWo6NnNQypkWUkgyubKpjQUNnj02LWuK7dbKnqAX6qm/iGydLJdeZtNtYoDo7p+wVhWGXvDWgEn8q2w2FHp1FflOamyF7nPbCD9C713UjbRi3j+dwQvM8SgJ/trPBn1hzxRZNiIsueox+ZMcZ8dkJDWR8/WzpTtKefpCGk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785545241; c=relaxed/simple; bh=aJGluKLug7fUNd7JeyfrJc4LQxPaXirsOBspg/4La3c=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=YyrUjqzPPxzQxrHzeYuaet5h9Em1O+gp09Uy6JElU/H7TVmBqUA9gWAVFaVmvfY5x/dkf91TEo0rjbhlNYTpEzcgWzdDWii0T/fsBW53cGoMcZox7cABx3MGv/AsQ37NHe6b3m4X8MuGll5WBmHok8meCSbCj1QgOTbERrvbJ6c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Lp/5WC0u; arc=none smtp.client-ip=91.218.175.180 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Lp/5WC0u" Date: Sat, 1 Aug 2026 08:46:52 +0800 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785545219; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=gMek1aawQ8IyooAOw8hjJzP++g15hi+kUNuEx4O2XuA=; b=Lp/5WC0ukjnAn5o7N1qqH1jN1lRSaj6RecCQ9ONDeU+uZ3dm1b6L3FalP3D7qZ6EHAhVdC VlIh6gP28hAflRVDqF1Bq042TiBVII/vlLXde6jAxOk4MqdsHPCkYT7WPSTm3YwOPD+5px ZYNtt22HoOYMfrGxy6XWPDx99Au6IY4= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Baoquan He To: Eric Biggers Cc: linux-mm@kvack.org, Andrew Morton , Chris Li , Kairui Song , linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org, Kemeng Shi , Nhat Pham , Barry Song , Youngjun Park , stable@vger.kernel.org Subject: Re: [PATCH] mm/swap: reject swapon() on filesystem-level encrypted files Message-ID: References: <20260730184853.48347-1-ebiggers@kernel.org> <20260731181408.GA12803@sol> Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260731181408.GA12803@sol> X-Migadu-Flow: FLOW_OUT 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 > > > --- > > > 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 > > 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.