Re: [PATCH v20 12/20] dm verity: expose root hash digest and signature data to LSMs

[Fan Wu <wufan@linux.microsoft.com>]

Hi Mikulas,

I hope you’re doing well. I wanted to thank you again for your thorough 
review for the last version. I’ve since made some minor updates for this 
version, including adding more comments and refactoring the way the hash 
algorithm name is obtained due to recent changes in dm-verity.

Would you mind if we keep the Review-by tag on the latest version since 
the changes are minor? Your feedback is greatly valued, and I’d 
appreciate it if you could take a quick look when you have a moment.

-Fan

On 8/2/2024 11:08 PM, 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.
> 
> However, critical security metadata like the dm-verity roothash and its
> signing information are not easily accessible to the LSMs.
> To address this limitation, this patch introduces a mechanism to store
> and manage these essential security details within a newly added LSM blob
> in the block_device structure.
> 
> This addition allows LSMs to make access control decisions on the integrity
> data stored within the block_device, enabling more flexible security
> policies. For instance, LSMs can now revoke access to dm-verity devices
> based on their roothashes, ensuring that only authorized and verified
> content is accessible. Additionally, LSMs can enforce policies to only
> allow files from dm-verity devices that have a valid digital signature to
> execute, effectively blocking any unsigned files from execution, thus
> enhancing security against unauthorized modifications.
> 
> The patch includes new hook calls, `security_bdev_setintegrity()`, in
> dm-verity to expose the dm-verity roothash and the roothash signature to
> LSMs via preresume() callback. By using the preresume() callback, it
> ensures that the security metadata is consistently in sync with the
> metadata of the dm-verity target in the current active mapping table.
> 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>
> ---
...
> 
> v20:
>    + Adding more documentation regarding the new setintegrity hook call
>    + Update the code for getting hash algorithm from either v->ahash_tfm
>      or v->shash_tfm
> ---
>   drivers/md/dm-verity-target.c | 118 ++++++++++++++++++++++++++++++++++
>   drivers/md/dm-verity.h        |   6 ++
>   include/linux/security.h      |   9 ++-
>   3 files changed, 132 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index cf659c8feb29..24ba9a10444c 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"
>   
> @@ -930,6 +931,41 @@ static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
>   	limits->dma_alignment = limits->logical_block_size - 1;
>   }
>   
> +#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;
> @@ -949,6 +985,7 @@ static void verity_dtr(struct dm_target *ti)
>   	kfree(v->initial_hashstate);
>   	kfree(v->root_digest);
>   	kfree(v->zero_digest);
> +	verity_free_sig(v);
>   
>   	if (v->ahash_tfm) {
>   		static_branch_dec(&ahash_enabled);
> @@ -1418,6 +1455,13 @@ 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);
>   
> @@ -1559,8 +1603,79 @@ 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)
> +{
> +	/*
> +	 * if the dm-verity target is unsigned, v->root_digest_sig will
> +	 * be NULL, and the hook call is still required to let LSMs mark
> +	 * the device as unsigned. This information is crucial for LSMs to
> +	 * block operations such as execution on unsigned files
> +	 */
> +	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;
> +	if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm)
> +		root_digest.alg = crypto_ahash_alg_name(v->ahash_tfm);
> +	else
> +		root_digest.alg = crypto_shash_alg_name(v->shash_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",
> +/* Note: the LSMs depend on the singleton and immutable features */
>   	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
>   	.version	= {1, 10, 0},
>   	.module		= THIS_MODULE,
> @@ -1571,6 +1686,9 @@ 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 aac3a1b1d94a..ea2da450f173 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -45,6 +45,9 @@ struct dm_verity {
>   	u8 *salt;		/* salt: its size is salt_size */
>   	u8 *initial_hashstate;	/* salted initial state, if shash_tfm is set */
>   	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 */
> @@ -58,6 +61,9 @@ 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 hash_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 39aec1c96d6a..0604893f2f9e 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,
>   };
>   
>   /*