Re: [PATCH v5 06/24] fsverity: pass tree_blocksize to end_enable_verity()

["Darrick J. Wong" <djwong@kernel.org>]

On Sat, Mar 09, 2024 at 08:34:51AM +1100, Dave Chinner wrote:
> On Thu, Mar 07, 2024 at 07:46:50PM -0800, Darrick J. Wong wrote:
> > On Thu, Mar 07, 2024 at 02:02:24PM -0800, Eric Biggers wrote:
> > > On Wed, Mar 06, 2024 at 08:30:00AM -0800, Darrick J. Wong wrote:
> > > > Or you could leave the unfinished tree as-is; that will waste space, but
> > > > if userspace tries again, the xattr code will replace the old merkle
> > > > tree block contents with the new ones.  This assumes that we're not
> > > > using XATTR_CREATE during FS_IOC_ENABLE_VERITY.
> > > 
> > > This should work, though if the file was shrunk between the FS_IOC_ENABLE_VERITY
> > > that was interrupted and the one that completed, there may be extra Merkle tree
> > > blocks left over.
> > 
> > What if ->enable_begin walked the xattrs and trimmed out any verity
> > xattrs that were already there?  Though I think ->enable_end actually
> > could do this since one of the args is the tree size, right?
> 
> If we are overwriting xattrs, it's effectively a remove then a new
> create operation, so we may as well just add a XFS_ATTR_VERITY
> namespace invalidation filter that removes any xattr in that
> namespace in ->enable_begin...

Yeah, that sounds like a good idea.  One nice aspect of the generic
listxattr code (aka not the simplified one that scrub uses) is that the
cursor tracking means that we could actually iterate-and-zap old merkle
tree blocks.

If we know the size of the merkle tree ahead of time (say it's N blocks)
then we just start zapping N, then N+1, etc. until we don't find any
more.  That wouldn't be exhaustive, but it's good enough to catch most
cases.

Online fsck should, however, have a way to call ensure_verity_info() so
that it can scan the xattrs looking for merkle tree blocks beyond
tree_size, missing merkle tree blocks within tree_size, missing
descriptors, etc.  It looks like the merkle tree block contents are
entirely hashes (no sibling/child/parent pointers, block headers, etc.)
so there's not a lot to check in the tree structure.  It looks pretty
similar to flattening a heap into a linear array.

> > > BTW, is xfs_repair planned to do anything about any such extra blocks?
> > 
> > Sorry to answer your question with a question, but how much checking is
> > $filesystem expected to do for merkle trees?
> > 
> > In theory xfs_repair could learn how to interpret the verity descriptor,
> > walk the merkle tree blocks, and even read the file data to confirm
> > intactness.  If the descriptor specifies the highest block address then
> > we could certainly trim off excess blocks.  But I don't know how much of
> > libfsverity actually lets you do that; I haven't looked into that
> > deeply. :/
> 
> Perhaps a generic fsverity userspace checking library we can link in
> to fs utilities like e2fsck and xfs_repair is the way to go here.
> That way any filesystem that supports fsverity can do offline
> validation of the merkle tree after checking the metadata is OK if
> desired.

That'd be nice.  Does the above checking sound reasonable? :)

> > For xfs_scrub I guess the job is theoretically simpler, since we only
> > need to stream reads of the verity files through the page cache and let
> > verity tell us if the file data are consistent.
> 
> *nod*

I had another thought overnight -- regular read()s incur the cost of
copying pagecache contents to userspace.  Do we really care about that,
though?  In theory we could mmap verity file contents and then use
MADV_POPULATE_READ to pull in the page cache and return error codes.  No
copying, and fewer syscalls.

> > For both tools, if something finds errors in the merkle tree structure
> > itself, do we turn off verity?  Or do we do something nasty like
> > truncate the file?
> 
> Mark it as "data corrupt" in terms of generic XFS health status, and
> leave it up to the user to repair the data and/or recalc the merkle
> tree, depending on what they find when they look at the corrupt file
> status.

Is there a way to forcibly read the file contents even if it fails
verity validation?  I was assuming the only recourse in that case is to
delete the file and restore from backup/package manager/etc.

> > Is there an ioctl or something that allows userspace to validate an
> > entire file's contents?  Sort of like what BLKVERIFY would have done for
> > block devices, except that we might believe its answers?
> > 
> > Also -- inconsistencies between the file data and the merkle tree aren't
> > something that xfs can self-heal, right?
> 
> Not that I know of - the file data has to be validated before we can
> tell if the error is in the data or the merkle tree, and only the
> user can validate the data is correct.

<nod>

--D

> -Dave.
> -- 
> Dave Chinner
> david@fromorbit.com
>