Re: [RFC PATCH v5 06/11] dm-verity: move signature check after tree validation

[Deven Bowers]

On 7/28/2020 2:50 PM, Eric Biggers wrote:
> On Tue, Jul 28, 2020 at 02:36:06PM -0700, Deven Bowers wrote:
> > The CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG introduced by Jaskaran was
> > intended to be used to allow an LSM to enforce verifications for all
> > dm-verity volumes.
> >
> > However, with it's current implementation, this signature verification
> > occurs after the merkel-tree is validated, as a result the signature can
> > pass initial verification by passing a matching root-hash and signature.
> > This results in an unreadable block_device, but that has passed signature
> > validation (and subsequently, would be marked as verified).
> >
> > This change moves the signature verification to after the merkel-tree has
> > finished validation.
> >
> > Signed-off-by: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/md/dm-verity-target.c     |  44 ++++------
> >   drivers/md/dm-verity-verify-sig.c | 140 ++++++++++++++++++++++--------
> >   drivers/md/dm-verity-verify-sig.h |  24 +++--
> >   drivers/md/dm-verity.h            |   2 +-
> >   4 files changed, 134 insertions(+), 76 deletions(-)
> >
> > diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> > index eec9f252e935..fabc173aa7b3 100644
> > --- a/drivers/md/dm-verity-target.c
> > +++ b/drivers/md/dm-verity-target.c
> > @@ -471,9 +471,9 @@ static int verity_verify_io(struct dm_verity_io *io)
> >   	struct bvec_iter start;
> >   	unsigned b;
> >   	struct crypto_wait wait;
> > +	int r;
> >   
> >   	for (b = 0; b < io->n_blocks; b++) {
> > -		int r;
> >   		sector_t cur_block = io->block + b;
> >   		struct ahash_request *req = verity_io_hash_req(v, io);
> >   
> > @@ -530,6 +530,16 @@ static int verity_verify_io(struct dm_verity_io *io)
> >   			return -EIO;
> >   	}
> >   
> > +	/*
> > +	 * At this point, the merkel tree has finished validating.
> > +	 * if signature was specified, validate the signature here.
> > +	 */
> > +	r = verity_verify_root_hash(v);
> > +	if (r < 0) {
> > +		DMERR_LIMIT("signature mismatch");
> > +		return r;
> > +	}
> > +
> >   	return 0;
> >   }
>
> This doesn't make any sense.
>
> This just moves the signature verification to some random I/O.
>
> The whole point of dm-verity is that data is verified on demand.  You can't know
> whether any particular data or hash block is consistent with the root hash or
> not until it is read and verified.
>
> When the first I/O completes it might have just checked one block of a billion.
>
> Not to mention that you didn't consider locking at all.
>
> - Eric

I appear to have dangerously misunderstood how dm-verity works under the
covers. What I thought was happening here was that *this* would be where
the first I/O that completes validation and has been verified - the root
hash signature could then be checked against the root hash, and then
no-op for remaining blocks, provided the signature validates.

The reason why I was proposing moving the signature check, is that I was 
afraid of the block_device being created in dm-verity with a root-hash 
that belongs to a different device + a signature that verifies that
root-hash, would get past verity_ctr, as despite the root hash not
matching the hash tree, the signature and the root hash will be 
verified. At this point, a block_device structure would be resident in
the kernel with the security attributes I propose in the next patch in 
the series. This device would never be read successfully, but the
structure with the attribute would exist.

This felt odd because there would be a structure in the kernel with an
attribute that says it passed a security check, but the block_device is
effectively invalid.

I realize now that that's a pretty ridiculous situation because the 
theoretical attack with access to manipulate the kernel memory in such a 
way to make it viable could just override whatever is needed to make the 
exploit work, and isn't unique to dm-verity.

I'm going to drop this patch in the next iteration of this series.