Re: [PATCH RFC v11 13/19] dm verity: consume root hash digest and signature data via LSM hook

[Paul Moore <paul@paul-moore.com>]

On Oct  4, 2023 Fan Wu <wufan@linux.microsoft.com> wrote:
> 
> dm-verity provides a strong guarantee of a block device's integrity. As
> a generic way to check the integrity of a block device, it provides
> those integrity guarantees to its higher layers, including the filesystem
> level.
> 
> An LSM that control access to a resource on the system based on the
> available integrity claims can use this transitive property of
> dm-verity, by querying the underlying block_device of a particular
> file.
> 
> The digest and signature information need to be stored in the block
> device to fulfill the next requirement of authorization via LSM policy.
> This will enable the LSM to perform revocation of devices that are still
> mounted, prohibiting execution of files that are no longer authorized
> by the LSM in question.
> 
> This patch added two security hook calls in dm-verity to save the
> dm-verity roothash and the roothash signature to the block device's
> LSM blobs.
> 
> Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
> Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> ---
> v2:
>   + No Changes
> 
> v3:
>   + No changes
> 
> v4:
>   + No changes
> 
> v5:
>   + No changes
> 
> v6:
>   + Fix an improper cleanup that can result in
>     a leak
> 
> v7:
>   + Squash patch 08/12, 10/12 to [11/16]
>   + Use part0 for block_device, to retrieve the block_device, when
>     calling security_bdev_setsecurity
> 
> v8:
>   + Undo squash of 08/12, 10/12 - separating drivers/md/ from
>     security/ & block/
>   + Use common-audit function for dmverity_signature.
>   + Change implementation for storing the dm-verity digest to use the
>     newly introduced dm_verity_digest structure introduced in patch
>     14/20.
>   + Create new structure, dm_verity_digest, containing digest algorithm,
>     size, and digest itself to pass to the LSM layer. V7 was missing the
>     algorithm.
>   + Create an associated public header containing this new structure and
>     the key values for the LSM hook, specific to dm-verity.
>   + Additional information added to commit, discussing the layering of
>     the changes and how the information passed will be used.
> 
> v9:
>   + No changes
> 
> v10:
>   + No changes
> 
> v11:
>   + Add an optional field to save signature
>   + Move the security hook call to the new finalize hook
> ---
>  drivers/md/dm-verity-target.c | 71 +++++++++++++++++++++++++++++++++++
>  drivers/md/dm-verity.h        |  6 +++
>  include/linux/dm-verity.h     | 19 ++++++++++
>  3 files changed, 96 insertions(+)
>  create mode 100644 include/linux/dm-verity.h

We need an ACK from Alasdair and/or Mike on this too.

> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 80673b66c194..db58b53649e3 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -13,6 +13,7 @@
>   * access behavior.
>   */
>  
> +#include "dm-core.h"
>  #include "dm-verity.h"
>  #include "dm-verity-fec.h"
>  #include "dm-verity-verify-sig.h"
> @@ -22,6 +23,9 @@
>  #include <linux/scatterlist.h>
>  #include <linux/string.h>
>  #include <linux/jump_label.h>
> +#include <linux/security.h>
> +#include <linux/dm-verity.h>
> +#include <crypto/hash_info.h>
>  
>  #define DM_MSG_PREFIX			"verity"
>  
> @@ -952,6 +956,17 @@ static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  	blk_limits_io_min(limits, limits->logical_block_size);
>  }
>  
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +static void verity_free_sig(struct dm_verity *v)
> +{
> +	kfree(v->root_digest_sig);
> +}
> +#else
> +static inline void verity_free_sig(struct dm_verity *v)
> +{
> +}
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
> +
>  static void verity_dtr(struct dm_target *ti)
>  {
>  	struct dm_verity *v = ti->private;
> @@ -966,6 +981,7 @@ static void verity_dtr(struct dm_target *ti)
>  	kfree(v->salt);
>  	kfree(v->root_digest);
>  	kfree(v->zero_digest);
> +	verity_free_sig(v);
>  
>  	if (v->tfm)
>  		crypto_free_ahash(v->tfm);
> @@ -1157,6 +1173,25 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
>  	return r;
>  }
>  
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +static int verity_init_sig(struct dm_verity *v, const void *sig,
> +			   size_t sig_size)
> +{
> +	v->sig_size = sig_size;
> +	v->root_digest_sig = kmalloc(v->sig_size, GFP_KERNEL);
> +	if (!v->root_digest)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +#else
> +static inline int verity_init_sig(struct dm_verity *v, const void *sig,
> +				  size_t sig_size)
> +{
> +	return 0;
> +}
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
> +
>  /*
>   * Target parameters:
>   *	<version>	The current format is version 1.
> @@ -1365,6 +1400,13 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
>  		ti->error = "Root hash verification failed";
>  		goto bad;
>  	}
> +
> +	r = verity_init_sig(v, verify_args.sig, verify_args.sig_size);
> +	if (r < 0) {
> +		ti->error = "Cannot allocate root digest signature";
> +		goto bad;
> +	}
> +
>  	v->hash_per_block_bits =
>  		__fls((1 << v->hash_dev_block_bits) / v->digest_size);
>  
> @@ -1501,6 +1543,32 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
>  	return 0;
>  }
>  
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +static int verity_finalize(struct dm_target *ti)
> +{
> +	struct block_device *bdev;
> +	struct dm_verity_digest root_digest;
> +	struct dm_verity *v;
> +	int r;
> +
> +	v = ti->private;
> +	bdev = dm_table_get_md(ti->table)->disk->part0;
> +	root_digest.digest = v->root_digest;
> +	root_digest.digest_len = v->digest_size;
> +	root_digest.algo = v->alg_name;
> +
> +	r = security_bdev_setsecurity(bdev, DM_VERITY_ROOTHASH_SEC_NAME, &root_digest,
> +				      sizeof(root_digest));
> +	if (r)
> +		return r;
> +
> +	return security_bdev_setsecurity(bdev,
> +					 DM_VERITY_SIGNATURE_SEC_NAME,
> +					 v->root_digest_sig,
> +					 v->sig_size);
> +}
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
> +
>  static struct target_type verity_target = {
>  	.name		= "verity",
>  	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
> @@ -1513,6 +1581,9 @@ static struct target_type verity_target = {
>  	.prepare_ioctl	= verity_prepare_ioctl,
>  	.iterate_devices = verity_iterate_devices,
>  	.io_hints	= verity_io_hints,
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +	.finalize	= verity_finalize,
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
>  };
>  module_dm(verity);
>  
> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> index 2f555b420367..a093d4a54615 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -42,6 +42,9 @@ struct dm_verity {
>  	u8 *root_digest;	/* digest of the root block */
>  	u8 *salt;		/* salt: its size is salt_size */
>  	u8 *zero_digest;	/* digest for a zero block */
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +	u8 *root_digest_sig;	/* digest signature of the root block */
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
>  	unsigned int salt_size;
>  	sector_t data_start;	/* data offset in 512-byte sectors */
>  	sector_t hash_start;	/* hash start in blocks */
> @@ -55,6 +58,9 @@ struct dm_verity {
>  	bool hash_failed:1;	/* set if hash of any block failed */
>  	bool use_tasklet:1;	/* try to verify in tasklet before work-queue */
>  	unsigned int digest_size;	/* digest size for the current hash algorithm */
> +#ifdef CONFIG_IPE_PROP_DM_VERITY
> +	unsigned int sig_size;	/* digest signature size */
> +#endif /* CONFIG_IPE_PROP_DM_VERITY */
>  	unsigned int ahash_reqsize;/* the size of temporary space for crypto */
>  	enum verity_mode mode;	/* mode for handling verification errors */
>  	unsigned int corrupted_errs;/* Number of errors for corrupted blocks */
> diff --git a/include/linux/dm-verity.h b/include/linux/dm-verity.h
> new file mode 100644
> index 000000000000..bb0413d55d72
> --- /dev/null
> +++ b/include/linux/dm-verity.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_DM_VERITY_H
> +#define _LINUX_DM_VERITY_H
> +
> +#include <linux/types.h>
> +#include <crypto/hash_info.h>
> +#include <linux/device-mapper.h>
> +
> +struct dm_verity_digest {
> +	const char *algo;
> +	const u8 *digest;
> +	size_t digest_len;
> +};
> +
> +#define DM_VERITY_SIGNATURE_SEC_NAME DM_NAME	".verity-signature"
> +#define DM_VERITY_ROOTHASH_SEC_NAME  DM_NAME	".verity-roothash"
> +
> +#endif /* _LINUX_DM_VERITY_H */
> -- 
> 2.25.1

--
paul-moore.com