Re: [PATCH 28/29] xfs: allow verity files to be opened even if the fsverity metadata is damaged

Alexander Larsson
2024-04-03T08:35:35Z

On Tue, 2024-04-02 at 20:10 -0400, Colin Walters wrote:
> [cc alexl@, retained quotes for context]
>
> On Tue, Apr 2, 2024, at 6:52 PM, Darrick J. Wong wrote:
> > On Tue, Apr 02, 2024 at 04:00:06PM -0400, Colin Walters wrote:
> > >
> > >
> > > On Fri, Mar 29, 2024, at 8:43 PM, Darrick J. Wong wrote:
> > > > From: Darrick J. Wong <djwong@kernel.org>
> > > >
> > > > There are more things that one can do with an open file
> > > > descriptor on
> > > > XFS -- query extended attributes, scan for metadata damage,
> > > > repair
> > > > metadata, etc.  None of this is possible if the fsverity
> > > > metadata are
> > > > damaged, because that prevents the file from being opened.
> > > >
> > > > Ignore a selective set of error codes that we know
> > > > fsverity_file_open to
> > > > return if the verity descriptor is nonsense.
> > > >
> > > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > > ---
> > > >  fs/iomap/buffered-io.c |    8 ++++++++
> > > >  fs/xfs/xfs_file.c      |   19 ++++++++++++++++++-
> > > >  2 files changed, 26 insertions(+), 1 deletion(-)
> > > >
> > > >
> > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > > index 9f9d929dfeebc..e68a15b72dbdd 100644
> > > > --- a/fs/iomap/buffered-io.c
> > > > +++ b/fs/iomap/buffered-io.c
> > > > @@ -487,6 +487,14 @@ static loff_t iomap_readpage_iter(const
> > > > struct
> > > > iomap_iter *iter,
> > > >   size_t poff, plen;
> > > >   sector_t sector;
> > > >
> > > > + /*
> > > > + * If this verity file hasn't been activated, fail
> > > > read attempts.  This
> > > > + * can happen if the calling filesystem allows files
> > > > to be opened even
> > > > + * with damaged verity metadata.
> > > > + */
> > > > + if (IS_VERITY(iter->inode) && !fsverity_active(iter-
> > > > >inode))
> > > > + return -EIO;
> > > > +
> > > >   if (iomap->type == IOMAP_INLINE)
> > > >   return iomap_read_inline_data(iter, folio);
> > > >
> > > > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > > > index c0b3e8146b753..36034eaefbf55 100644
> > > > --- a/fs/xfs/xfs_file.c
> > > > +++ b/fs/xfs/xfs_file.c
> > > > @@ -1431,8 +1431,25 @@ xfs_file_open(
> > > >   FMODE_DIO_PARALLEL_WRITE |
> > > > FMODE_CAN_ODIRECT;
> > > >
> > > >   error = fsverity_file_open(inode, file);
> > > > - if (error)
> > > > + switch (error) {
> > > > + case -EFBIG:
> > > > + case -EINVAL:
> > > > + case -EMSGSIZE:
> > > > + case -EFSCORRUPTED:
> > > > + /*
> > > > + * Be selective about which fsverity errors we
> > > > propagate to
> > > > + * userspace; we still want to be able to open
> > > > this file even
> > > > + * if reads don't work.  Someone might want to
> > > > perform an
> > > > + * online repair.
> > > > + */
> > > > + if (has_capability_noaudit(current,
> > > > CAP_SYS_ADMIN))
> > > > + break;
> > >
> > > As I understand it, fsverity (and dm-verity) are desirable in
> > > high-safety and integrity requirement cases where the goal is for
> > > the
> > > system to "fail closed" if errors in general are detected;
> > > anything
> > > that would have the system be in an ill-defined state.
> >
> > Is "open() fails if verity metadata are trashed" a hard
> > requirement?
>
> I can't say authoritatively, but I do want to ensure we've dug into
> the semantics here, and I agree with Eric that it would make the most
> sense to have this be consistent across filesystems.

In terms of userspace I think this semantic change is fine. Even if the
metadata is broken we will still not see any non-validated data. It's
as if we didn't try to use the broken fsverity metadata until it needed
to be used. I agree with others though that having the same behavior
across all filesystems would make sense. Also, it might be useful
information that the filesystem has an error, so maybe we should log
the swallowed errors.

For kernel use, in overlayfs when using verity_mode=require, we do use
open() (in ovl_validate_verity) to trigger the initialization of
fsverity_info . However I took a look at this code, and it seems to
properly handle (i.e. fail) the case where IS_VERITY(inode) is true but
there is no fsverity_info after open.

Similarly, IMA (in ima_get_verity_digest) relies on the digest loaded
from the header. But it also seems to handle this case correctly.

> > Reads will still fail due to (iomap) readahead returning EIO for a
> > file
> > that is IS_VERITY() && !fsverity_active().  This is (afaict) the
> > state
> > you end up with when the fsverity open fails.  ext4/f2fs don't do
> > that,
> > but they also don't have online fsck so once a file's dead it's
> > dead.
>
> OK, right.  Allowing an open() but having read() fail seems like it
> doesn't weaken things too much in reality.  I think what makes me
> uncomfortable is the error-swallowing; but yes, in theory we should
> get the same or similar error on a subsequent read().

If anything the explicit error list seems a bit fragile to me. What if
the underlying fs reported some new error when reading the metadata,
should we then suddenly fail here when we didn't before?

>
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Alexander Larsson Red Hat,
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alexl@redhat.com alexander.larsson@gmail.com
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