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

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, Oct 30, 2019 at 10:38 AM Eric Biggers <ebiggers@kernel.org> 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@chromium.org>
> > ---
> >
> >  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.


> BTW, the crbug.com link is not publicly viewable, so should not be included in
> the commit message.

My apologies.  It's public now.  Annoyingly they've been experimenting
with making bugs on crbug.com private by default (argh) and I didn't
notice.

-Doug