Re: [PATCH] Revert "ext4 crypto: fix to check feature status before get policy"

[Eric Biggers]

On Wed, Oct 30, 2019 at 10:51:20AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Wed, Oct 30, 2019 at 10:38 AM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
> >
> > Hi Douglas,
> >
> > On Wed, Oct 30, 2019 at 10:06:25AM -0700, Douglas Anderson wrote:
> > > This reverts commit 0642ea2409f3 ("ext4 crypto: fix to check feature
> > > status before get policy").
> > >
> > > The commit made a clear and documented ABI change that is not backward
> > > compatible.  There exists userspace code [1] that relied on the old
> > > behavior and is now broken.
> > >
> > > While we could entertain the idea of updating the userspace code to
> > > handle the ABI change, it's my understanding that in general ABI
> > > changes that break userspace are frowned upon (to put it nicely).
> > >
> > > NOTE: if we for some reason do decide to entertain the idea of
> > > allowing the ABI change and updating userspace, I'd appreciate any
> > > help on how we should make the change.  Specifically the old code
> > > relied on the different return values to differentiate between
> > > "KeyState::NO_KEY" and "KeyState::NOT_SUPPORTED".  I'm no expert on
> > > the ext4 encryption APIs (I just ended up here tracking down the
> > > regression [2]) so I'd need a bit of handholding from someone.
> > >
> > > [1] https://chromium.googlesource.com/chromiumos/platform2/+/refs/heads/master/cryptohome/dircrypto_util.cc#73
> > > [2] https://crbug.com/1018265
> > >
> > > Fixes: 0642ea2409f3 ("ext4 crypto: fix to check feature status before get policy")
> > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > ---
> > >
> > >  Documentation/filesystems/fscrypt.rst | 3 +--
> > >  fs/ext4/ioctl.c                       | 2 --
> > >  2 files changed, 1 insertion(+), 4 deletions(-)
> > >
> > > diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
> > > index 8a0700af9596..4289c29d7c5a 100644
> > > --- a/Documentation/filesystems/fscrypt.rst
> > > +++ b/Documentation/filesystems/fscrypt.rst
> > > @@ -562,8 +562,7 @@ FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
> > >    or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
> > >    (try FS_IOC_GET_ENCRYPTION_POLICY instead)
> > >  - ``EOPNOTSUPP``: the kernel was not configured with encryption
> > > -  support for this filesystem, or the filesystem superblock has not
> > > -  had encryption enabled on it
> > > +  support for this filesystem
> > >  - ``EOVERFLOW``: the file is encrypted and uses a recognized
> > >    encryption policy version, but the policy struct does not fit into
> > >    the provided buffer
> > > diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> > > index 0b7f316fd30f..13d97fb797b4 100644
> > > --- a/fs/ext4/ioctl.c
> > > +++ b/fs/ext4/ioctl.c
> > > @@ -1181,8 +1181,6 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> > >  #endif
> > >       }
> > >       case EXT4_IOC_GET_ENCRYPTION_POLICY:
> > > -             if (!ext4_has_feature_encrypt(sb))
> > > -                     return -EOPNOTSUPP;
> > >               return fscrypt_ioctl_get_policy(filp, (void __user *)arg);
> > >
> >
> > Thanks for reporting this.  Can you elaborate on exactly why returning
> > EOPNOTSUPP breaks things in the Chrome OS code?  Since encryption is indeed not
> > supported, why isn't "KeyState::NOT_SUPPORTED" correct?
> 
> I guess all I know is from the cryptohome source code I sent a link
> to, which I'm not a super expert in.  Did you get a chance to take a
> look at that?  As far as I can tell the code is doing something like
> this:
> 
> 1. If I see EOPNOTSUPP then this must be a kernel without ext4 crypto.
> Fallback to using the old-style ecryptfs.
> 
> 2. If I see ENODATA then this is a kernel with ext4 crypto but there's
> no key yet.  We should set a key and (if necessarily) enable crypto on
> the filesystem.
> 
> 3. If I see no error then we're already good.
> 
> > Note that the state after this revert will be:
> >
> > - FS_IOC_GET_ENCRYPTION_POLICY on ext4 => ENODATA
> > - FS_IOC_GET_ENCRYPTION_POLICY on f2fs => EOPNOTSUPP
> > - FS_IOC_GET_ENCRYPTION_POLICY_EX on ext4 => EOPNOTSUPP
> > - FS_IOC_GET_ENCRYPTION_POLICY_EX on f2fs => EOPNOTSUPP
> >
> > So if this code change is made, the documentation would need to be updated to
> > explain that the error code from FS_IOC_GET_ENCRYPTION_POLICY is
> > filesystem-specific (which we'd really like to avoid...), and that
> > FS_IOC_GET_ENCRYPTION_POLICY_EX handles this case differently.  Or else the
> > other three would need to be changed to ENODATA -- which for
> > FS_IOC_GET_ENCRYPTION_POLICY on f2fs would be an ABI break in its own right,
> > though it's possible that no one would notice.
> >
> > Is your proposal to keep the error filesystem-specific for now?
> 
> I guess I'd have to leave it up to the people who know this better.
> Mostly I just saw this as an ABI change breaking userspace which to me
> means revert.  I have very little background here to make good
> decisions about the right way to move forward.
> 

Okay, that makes sense -- cryptohome assumes that ENODATA means the kernel
supports encryption, even if the encrypt ext4 feature flag isn't set yet.

The way it's really supposed to work (IMO) is that all fscrypt ioctls
consistently return EOPNOTSUPP if the feature is off, and then if userspace
really needs to know if encryption can nevertheless still be enabled and used on
the filesystem, it can check for the presence of
/sys/fs/ext4/features/encryption (or /sys/fs/f2fs/features/encryption).  Or the
feature flag can just be set by configuration before any of the fscrypt ioctls
are attempted (this is what Android does).

I guess we're stuck with the existing ext4 FS_IOC_GET_ENCRYPTION_POLICY behavior
though, so we need to take this revert for 5.4.

For 5.5 I think we should try to make things slightly more sane by removing the
same check from f2fs and fixing the documentation, so that at least each ioctl
will behave consistently across filesystems and be correctly documented.

Ted, Jaegeuk, Chao, do you agree?

- Eric