linux-kernel - Re: overlayfs vs. fscrypt

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Date:   Wed, 13 Mar 2019 09:33:49 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Richard Weinberger <richard@....at>,
        linux-fsdevel@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: overlayfs vs. fscrypt

On Wed, Mar 13, 2019 at 04:11:48PM +0000, Al Viro wrote:
> On Wed, Mar 13, 2019 at 08:01:27AM -0700, Eric Biggers wrote:
> 
> > What do you think about this?
> 
> That fscrypt might have some very deep flaws.  I'll need to RTFS and
> review its model, but what I've seen in this thread so far is not
> promising anything good.
> 
> It's not just overlayfs - there are all kinds of interesting trouble
> possible just with fscrypt, unless I'm misparsing what had been said
> so far.

FYI, there *is* a known bug I was very recently made aware of and am planning to
fix.  When ->lookup() finds the plaintext name for a directory and the
ciphertext name is already in the dcache, d_splice_alias() will __d_move() the
existing dentry to the plaintext name.  But it doesn't set
DCACHE_ENCRYPTED_WITH_KEY, so the dentry incorrectly is still marked as a
ciphertext name and will be invalidated on the next lookup.  That's especially
problematic if the lookup that caused the __d_move() came from sys_mount().

I'm thinking the best fix is to have __d_move() propagate
DCACHE_ENCRYPTED_WITH_KEY from 'target' to 'dentry'.

- Eric