Re: [PATCH v19 12/20] dm verity: expose root hash digest and signature data to LSMs

[Mikulas Patocka <mpatocka@redhat.com>]

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>



On Fri, 24 May 2024, Fan Wu wrote:

> From: Deven Bowers <deven.desai@linux.microsoft.com>
> 
> 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 adds two security hook calls in dm-verity to expose the
> dm-verity roothash and the roothash signature to LSMs via preresume
> callback. The hook calls are depended on CONFIG_SECURITY.
> 
> 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
> 
> v12:
>   + No changes
> 
> v13:
>   + No changes
> 
> v14:
>   + Correct code format
>   + Remove unnecessary header and switch to dm_disk()
> 
> v15:
>   + Refactor security_bdev_setsecurity() to security_bdev_setintegrity()
>   + Remove unnecessary headers
> 
> v16:
>   + Use kmemdup to duplicate signature
>   + Clean up lsm blob data in error case
> 
> v17:
>   + Switch to depend on CONFIG_SECURITY
>   + Use new enum name LSM_INT_DMVERITY_SIG_VALID
> 
> v18:
>   + Amend commit title
>   + Fix incorrect error handling
>   + Make signature exposure depends on CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
>   + Fix inaccurate comment
>   + Remove include/linux/dm-verity.h
>   + use crypto_ahash_alg_name(v->tfm) instead of v->alg_name
> 
> v19:
>   + Drop finalize callback and switch to preresume callback
>   + Adding NULL check to avoid kmemdup when sig is NULL
> ---
>  drivers/md/dm-verity-target.c | 108 ++++++++++++++++++++++++++++++++++
>  drivers/md/dm-verity.h        |   6 ++
>  include/linux/security.h      |   9 ++-
>  3 files changed, 122 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index bb5da66da4c1..0a54ce02ea53 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -22,6 +22,7 @@
>  #include <linux/scatterlist.h>
>  #include <linux/string.h>
>  #include <linux/jump_label.h>
> +#include <linux/security.h>
>  
>  #define DM_MSG_PREFIX			"verity"
>  
> @@ -1017,6 +1018,41 @@ static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  	blk_limits_io_min(limits, limits->logical_block_size);
>  }
>  
> +#ifdef CONFIG_SECURITY
> +
> +static int verity_init_sig(struct dm_verity *v, const void *sig,
> +			   size_t sig_size)
> +{
> +	v->sig_size = sig_size;
> +
> +	if (sig) {
> +		v->root_digest_sig = kmemdup(sig, v->sig_size, GFP_KERNEL);
> +		if (!v->root_digest_sig)
> +			return -ENOMEM;
> +	}
> +
> +	return 0;
> +}
> +
> +static void verity_free_sig(struct dm_verity *v)
> +{
> +	kfree(v->root_digest_sig);
> +}
> +
> +#else
> +
> +static inline int verity_init_sig(struct dm_verity *v, const void *sig,
> +				  size_t sig_size)
> +{
> +	return 0;
> +}
> +
> +static inline void verity_free_sig(struct dm_verity *v)
> +{
> +}
> +
> +#endif /* CONFIG_SECURITY */
> +
>  static void verity_dtr(struct dm_target *ti)
>  {
>  	struct dm_verity *v = ti->private;
> @@ -1035,6 +1071,7 @@ 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);
> @@ -1434,6 +1471,13 @@ 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);
>  
> @@ -1584,6 +1628,67 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
>  	return 0;
>  }
>  
> +#ifdef CONFIG_SECURITY
> +
> +#ifdef CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
> +
> +static int verity_security_set_signature(struct block_device *bdev,
> +					 struct dm_verity *v)
> +{
> +	return security_bdev_setintegrity(bdev,
> +					  LSM_INT_DMVERITY_SIG_VALID,
> +					  v->root_digest_sig,
> +					  v->sig_size);
> +}
> +
> +#else
> +
> +static inline int verity_security_set_signature(struct block_device *bdev,
> +						struct dm_verity *v)
> +{
> +	return 0;
> +}
> +
> +#endif /* CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG */
> +
> +/*
> + * Expose verity target's root hash and signature data to LSMs before resume.
> + *
> + * Returns 0 on success, or -ENOMEM if the system is out of memory.
> + */
> +static int verity_preresume(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_disk(dm_table_get_md(ti->table))->part0;
> +	root_digest.digest = v->root_digest;
> +	root_digest.digest_len = v->digest_size;
> +	root_digest.alg = crypto_ahash_alg_name(v->tfm);
> +
> +	r = security_bdev_setintegrity(bdev, LSM_INT_DMVERITY_ROOTHASH, &root_digest,
> +				       sizeof(root_digest));
> +	if (r)
> +		return r;
> +
> +	r =  verity_security_set_signature(bdev, v);
> +	if (r)
> +		goto bad;
> +
> +	return 0;
> +
> +bad:
> +
> +	security_bdev_setintegrity(bdev, LSM_INT_DMVERITY_ROOTHASH, NULL, 0);
> +
> +	return r;
> +}
> +
> +#endif /* CONFIG_SECURITY */
> +
>  static struct target_type verity_target = {
>  	.name		= "verity",
>  	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
> @@ -1596,6 +1701,9 @@ static struct target_type verity_target = {
>  	.prepare_ioctl	= verity_prepare_ioctl,
>  	.iterate_devices = verity_iterate_devices,
>  	.io_hints	= verity_io_hints,
> +#ifdef CONFIG_SECURITY
> +	.preresume	= verity_preresume,
> +#endif /* CONFIG_SECURITY */
>  };
>  module_dm(verity);
>  
> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> index 20b1bcf03474..2de89e0d555b 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -43,6 +43,9 @@ 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_SECURITY
> +	u8 *root_digest_sig;	/* signature of the root digest */
> +#endif /* CONFIG_SECURITY */
>  	unsigned int salt_size;
>  	sector_t data_start;	/* data offset in 512-byte sectors */
>  	sector_t hash_start;	/* hash start in blocks */
> @@ -56,6 +59,9 @@ struct dm_verity {
>  	bool hash_failed:1;	/* set if hash of any block failed */
>  	bool use_bh_wq:1;	/* try to verify in BH wq before normal work-queue */
>  	unsigned int digest_size;	/* digest size for the current hash algorithm */
> +#ifdef CONFIG_SECURITY
> +	unsigned int sig_size;	/* root digest signature size */
> +#endif /* CONFIG_SECURITY */
>  	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/security.h b/include/linux/security.h
> index a64e83622c7c..09c80326518f 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -83,8 +83,15 @@ enum lsm_event {
>  	LSM_POLICY_CHANGE,
>  };
>  
> +struct dm_verity_digest {
> +	const char *alg;
> +	const u8 *digest;
> +	size_t digest_len;
> +};
> +
>  enum lsm_integrity_type {
> -	__LSM_INT_MAX
> +	LSM_INT_DMVERITY_SIG_VALID,
> +	LSM_INT_DMVERITY_ROOTHASH,
>  };
>  
>  /*
> -- 
> 2.44.0
>