Re: [RFC PATCH v5 06/11] dm-verity: move signature check after tree validation
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(a)linux.microsoft.com>
> ---
> 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.