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From: Eric Biggers <ebiggers@kernel.org>
To: Deven Bowers <deven.desai@linux.microsoft.com>
Cc: agk@redhat.com, axboe@kernel.dk, snitzer@redhat.com,
	jmorris@namei.org, serge@hallyn.com, zohar@linux.ibm.com,
	viro@zeniv.linux.org.uk, paul@paul-moore.com, eparis@redhat.com,
	jannh@google.com, dm-devel@redhat.com,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
	linux-audit@redhat.com, tyhicks@linux.microsoft.com,
	linux-kernel@vger.kernel.org, corbet@lwn.net, sashal@kernel.org,
	jaskarankhurana@linux.microsoft.com, mdsakib@microsoft.com,
	nramas@linux.microsoft.com, pasha.tatashin@soleen.com
Subject: Re: [RFC PATCH v5 06/11] dm-verity: move signature check after tree validation
Date: Tue, 28 Jul 2020 14:50:41 -0700
Message-ID: <20200728215041.GF4053562@gmail.com> (raw)
In-Reply-To: <20200728213614.586312-7-deven.desai@linux.microsoft.com>

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@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

  reply index

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-28 21:36 [RFC PATCH v5 00/11] Integrity Policy Enforcement LSM (IPE) Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 01/11] scripts: add ipe tooling to generate boot policy Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 02/11] security: add ipe lsm evaluation loop and audit system Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 03/11] security: add ipe lsm policy parser and policy loading Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 04/11] ipe: add property for trust of boot volume Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 05/11] fs: add security blob and hooks for block_device Deven Bowers
2020-07-28 22:22   ` Casey Schaufler
2020-07-28 22:40     ` Al Viro
2020-07-28 23:55       ` Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 06/11] dm-verity: move signature check after tree validation Deven Bowers
2020-07-28 21:50   ` Eric Biggers [this message]
2020-07-28 23:55     ` Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 07/11] dm-verity: add bdev_setsecurity hook for dm-verity signature Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 08/11] ipe: add property for signed dmverity volumes Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 09/11] dm-verity: add bdev_setsecurity hook for root-hash Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 10/11] documentation: add ipe documentation Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 10/12] ipe: add property for dmverity roothash Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 11/11] cleanup: uapi/linux/audit.h Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 11/12] documentation: add ipe documentation Deven Bowers
2020-07-28 21:36 ` [RFC PATCH v5 12/12] cleanup: uapi/linux/audit.h Deven Bowers
2020-08-02 11:55 ` [RFC PATCH v5 00/11] Integrity Policy Enforcement LSM (IPE) Pavel Machek
2020-08-02 14:03   ` Sasha Levin
2020-08-02 14:31     ` Pavel Machek
2020-08-02 16:43       ` [dm-devel] " James Bottomley
2020-08-04 16:07         ` Deven Bowers
2020-08-05 15:01           ` James Bottomley
2020-08-05 16:59             ` James Morris
2020-08-05 18:15               ` Mimi Zohar
2020-08-05 23:51                 ` James Morris
2020-08-06 14:33                   ` Mimi Zohar
2020-08-07 16:41                     ` James Morris
2020-08-07 17:31                       ` Mimi Zohar
2020-08-07 18:40                         ` Mimi Zohar
2020-08-10 20:29                           ` James Morris
2020-08-08 17:47                 ` Chuck Lever
2020-08-09 17:16                   ` Mimi Zohar
2020-08-10 15:35                     ` James Bottomley
2020-08-10 16:35                       ` Mimi Zohar
2020-08-10 17:13                         ` James Bottomley
2020-08-10 17:57                           ` Mimi Zohar
2020-08-10 23:36                       ` Chuck Lever
2020-08-11  5:43                         ` James Bottomley
2020-08-11 14:48                           ` Chuck Lever
2020-08-11 15:32                             ` James Bottomley
2020-08-11 19:30                               ` Pavel Machek
2020-08-12 14:45                               ` Chuck Lever
2020-08-11 15:53                             ` James Bottomley
2020-08-12 14:15                               ` Chuck Lever
2020-08-12 15:51                                 ` James Bottomley
2020-08-13 14:42                                   ` Chuck Lever
2020-08-13 15:10                                     ` James Bottomley
2020-08-14 14:21                                       ` Chuck Lever
2020-08-11 18:28                             ` James Bottomley
2020-08-12 13:56                               ` Chuck Lever
2020-08-12 15:42                                 ` James Bottomley
2020-08-13 14:21                                   ` Chuck Lever
2020-08-13 14:42                                     ` James Bottomley
2020-08-13 14:56                                       ` Chuck Lever
2020-08-11 21:03                   ` James Morris
2020-08-12 14:18                     ` Chuck Lever
2020-08-12 17:07                       ` Deven Bowers

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