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Date: Mon, 15 Apr 2024 22:42:49 +0300
Message-Id: <D0KYBIKEMVBJ.3TUUWH3M4XW7I@kernel.org>
Cc: <linux-security-module@vger.kernel.org>, <linux-doc@vger.kernel.org>,
 <linux-kernel@vger.kernel.org>, <linux-kselftest@vger.kernel.org>,
 <bpf@vger.kernel.org>, <zohar@linux.ibm.com>, <dmitry.kasatkin@gmail.com>,
 <linux-integrity@vger.kernel.org>, <wufan@linux.microsoft.com>,
 <pbrobinson@gmail.com>, <zbyszek@in.waw.pl>, <hch@lst.de>,
 <mjg59@srcf.ucam.org>, <pmatilai@redhat.com>, <jannh@google.com>,
 <dhowells@redhat.com>, <jikos@kernel.org>, <mkoutny@suse.com>,
 <ppavlu@suse.com>, <petr.vorel@gmail.com>, <mzerqung@0pointer.de>,
 <kgold@linux.ibm.com>, "Roberto Sassu" <roberto.sassu@huawei.com>
Subject: Re: [PATCH v4 10/14] digest cache: Prefetch digest lists if
 requested
From: "Jarkko Sakkinen" <jarkko@kernel.org>
To: "Roberto Sassu" <roberto.sassu@huaweicloud.com>, <corbet@lwn.net>,
 <paul@paul-moore.com>, <jmorris@namei.org>, <serge@hallyn.com>,
 <akpm@linux-foundation.org>, <shuah@kernel.org>,
 <mcoquelin.stm32@gmail.com>, <alexandre.torgue@foss.st.com>,
 <mic@digikod.net>
References: <20240415142436.2545003-1-roberto.sassu@huaweicloud.com>
 <20240415142436.2545003-11-roberto.sassu@huaweicloud.com>
In-Reply-To: <20240415142436.2545003-11-roberto.sassu@huaweicloud.com>

On Mon Apr 15, 2024 at 5:24 PM EEST, Roberto Sassu wrote:
> From: Roberto Sassu <roberto.sassu@huawei.com>
>
> A desirable goal when doing integrity measurements is that they are done
> always in the same order across boots, so that the resulting PCR value
> becomes predictable and suitable for sealing policies. However, due to
> parallel execution of system services at boot, a deterministic order of
> measurements is difficult to achieve.
>
> The digest_cache LSM is not exempted from this issue. Under the assumptio=
n
> that only the digest list is measured, and file measurements are omitted =
if
> their digest is found in that digest list, a PCR can be predictable only =
if
> all files belong to the same digest list. Otherwise, it will still be
> unpredictable, since files accessed in a non-deterministic order will cau=
se
> digest lists to be measured in a non-deterministic order too.
>
> Overcome this issue, if prefetching is enabled, by searching a digest lis=
t
> file name in digest_list_dir_lookup_filename() among the entries of the
> linked list built by digest_cache_dir_create(). If the file name does not
> match, read the digest list to trigger its measurement. Otherwise, also
> create a digest cache and return that to the caller. Release the extra
> reference of the directory digest cache in digest_cache_new(), since it w=
as
> only used for the search and it is not going to be returned.
>
> Prefetching needs to be explicitly enabled by setting the new
> security.dig_prefetch xattr to 1 in the directory containing the digest
> lists. The newly introduced function digest_cache_prefetch_requested()
> checks first if the DIR_PREFETCH bit is set in dig_owner, otherwise it
> reads the xattr. digest_cache_create() sets DIR_PREFETCH in dig_owner, if
> prefetching is enabled, before declaring the digest cache as initialized.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
> ---
>  include/uapi/linux/xattr.h       |  3 +
>  security/digest_cache/dir.c      | 55 +++++++++++++++++-
>  security/digest_cache/internal.h | 11 +++-
>  security/digest_cache/main.c     | 95 +++++++++++++++++++++++++++++++-
>  security/digest_cache/populate.c |  8 ++-
>  security/digest_cache/verif.c    |  5 +-
>  6 files changed, 170 insertions(+), 7 deletions(-)
>
> diff --git a/include/uapi/linux/xattr.h b/include/uapi/linux/xattr.h
> index 8a58cf4bce65..8af33d38d9e8 100644
> --- a/include/uapi/linux/xattr.h
> +++ b/include/uapi/linux/xattr.h
> @@ -57,6 +57,9 @@
>  #define XATTR_DIGEST_LIST_SUFFIX "digest_list"
>  #define XATTR_NAME_DIGEST_LIST XATTR_SECURITY_PREFIX XATTR_DIGEST_LIST_S=
UFFIX
> =20
> +#define XATTR_DIG_PREFETCH_SUFFIX "dig_prefetch"
> +#define XATTR_NAME_DIG_PREFETCH XATTR_SECURITY_PREFIX XATTR_DIG_PREFETCH=
_SUFFIX
> +
>  #define XATTR_SELINUX_SUFFIX "selinux"
>  #define XATTR_NAME_SELINUX XATTR_SECURITY_PREFIX XATTR_SELINUX_SUFFIX
> =20
> diff --git a/security/digest_cache/dir.c b/security/digest_cache/dir.c
> index 7bfcdd5f7ef1..a7d203c15386 100644
> --- a/security/digest_cache/dir.c
> +++ b/security/digest_cache/dir.c
> @@ -54,6 +54,7 @@ static bool digest_cache_dir_iter(struct dir_context *_=
_ctx, const char *name,
>  	new_entry->seq_num =3D UINT_MAX;
>  	new_entry->digest_cache =3D NULL;
>  	mutex_init(&new_entry->digest_cache_mutex);
> +	new_entry->prefetched =3D false;
> =20
>  	if (new_entry->name[0] < '0' || new_entry->name[0] > '9')
>  		goto out;
> @@ -127,6 +128,7 @@ int digest_cache_dir_create(struct digest_cache *dige=
st_cache,
>   * @digest_cache: Digest cache
>   * @digest: Digest to search
>   * @algo: Algorithm of the digest to search
> + * @filename: File name of the digest list to search
>   *
>   * This function iterates over the linked list created by
>   * digest_cache_dir_create() and looks up the digest in the digest cache=
 of
> @@ -149,7 +151,8 @@ digest_cache_dir_lookup_digest(struct dentry *dentry,
>  		if (!dir_entry->digest_cache) {
>  			cache =3D digest_cache_create(dentry, digest_list_path,
>  						    digest_cache->path_str,
> -						    dir_entry->name);
> +						    dir_entry->name, false,
> +						    false);
>  			/* Ignore digest caches that cannot be instantiated. */
>  			if (!cache) {
>  				mutex_unlock(&dir_entry->digest_cache_mutex);
> @@ -158,6 +161,8 @@ digest_cache_dir_lookup_digest(struct dentry *dentry,
> =20
>  			/* Consume extra ref. from digest_cache_create(). */
>  			dir_entry->digest_cache =3D cache;
> +			/* Digest list was read, mark entry as prefetched. */
> +			dir_entry->prefetched =3D true;
>  		}
>  		mutex_unlock(&dir_entry->digest_cache_mutex);
> =20
> @@ -171,6 +176,54 @@ digest_cache_dir_lookup_digest(struct dentry *dentry=
,
>  	return 0UL;
>  }
> =20
> +/**
> + * digest_cache_dir_lookup_filename - Lookup a digest list
> + * @dentry: Dentry of the file whose digest list is looked up
> + * @digest_list_path: Path structure of the digest list directory
> + * @digest_cache: Digest cache
> + * @filename: File name of the digest list to search
> + *
> + * This function iterates over the linked list created by
> + * digest_cache_dir_create() and looks up a digest list with a matching =
file
> + * name among the entries. If there is no match, it prefetches (reads) t=
he
> + * current digest list. Otherwise, it returns the digest cache pointer f=
rom
> + * digest_cache_create() to the caller.
> + *
> + * Return: A digest cache pointer if the digest list if found, NULL othe=
rwise.
> + */
> +struct digest_cache *
> +digest_cache_dir_lookup_filename(struct dentry *dentry,
> +				 struct path *digest_list_path,
> +				 struct digest_cache *digest_cache,
> +				 char *filename)
> +{
> +	struct digest_cache *cache;
> +	struct dir_entry *dir_entry;
> +	bool filename_found;
> +
> +	list_for_each_entry(dir_entry, &digest_cache->dir_entries, list) {
> +		mutex_lock(&dir_entry->digest_cache_mutex);
> +		filename_found =3D !strcmp(dir_entry->name, filename);
> +		if (!filename_found && dir_entry->prefetched) {
> +			mutex_unlock(&dir_entry->digest_cache_mutex);
> +			continue;
> +		}
> +
> +		cache =3D digest_cache_create(dentry, digest_list_path,
> +					    digest_cache->path_str,
> +					    dir_entry->name, false,
> +					    filename_found ? false : true);
> +
> +		dir_entry->prefetched =3D true;
> +		mutex_unlock(&dir_entry->digest_cache_mutex);
> +
> +		if (filename_found)
> +			return cache;
> +	}
> +
> +	return NULL;
> +}
> +
>  /**
>   * digest_cache_dir_free - Free the stored file list and put digest cach=
es
>   * @digest_cache: Digest cache
> diff --git a/security/digest_cache/internal.h b/security/digest_cache/int=
ernal.h
> index b7afca8e04da..c13b35f6b2c0 100644
> --- a/security/digest_cache/internal.h
> +++ b/security/digest_cache/internal.h
> @@ -17,6 +17,7 @@
>  #define INIT_IN_PROGRESS	0	/* Digest cache being initialized. */
>  #define INVALID			1	/* Digest cache marked as invalid. */
>  #define IS_DIR			2	/* Digest cache created from dir. */
> +#define DIR_PREFETCH		3	/* Prefetching requested for dir. */
> =20
>  /**
>   * struct readdir_callback - Structure to store information for dir iter=
ation
> @@ -37,6 +38,7 @@ struct readdir_callback {
>   * @digest_cache: Digest cache associated to the directory entry
>   * @digest_cache_mutex: Protects @digest_cache
>   * @seq_num: Sequence number of the directory entry from file name
> + * @prefetched: Whether the digest list has been already prefetched
>   * @name: File name of the directory entry
>   *
>   * This structure represents a directory entry with a digest cache creat=
ed
> @@ -47,6 +49,7 @@ struct dir_entry {
>  	struct digest_cache *digest_cache;
>  	struct mutex digest_cache_mutex;
>  	unsigned int seq_num;
> +	bool prefetched;
>  	char name[];
>  } __packed;
> =20
> @@ -205,7 +208,8 @@ digest_cache_from_file_sec(const struct file *file)
>  /* main.c */
>  struct digest_cache *digest_cache_create(struct dentry *dentry,
>  					 struct path *digest_list_path,
> -					 char *path_str, char *filename);
> +					 char *path_str, char *filename,
> +					 bool prefetch_req, bool prefetch);
> =20
>  /* htable.c */
>  int digest_cache_htable_init(struct digest_cache *digest_cache, u64 num_=
digests,
> @@ -236,6 +240,11 @@ digest_cache_dir_lookup_digest(struct dentry *dentry=
,
>  			       struct path *digest_list_path,
>  			       struct digest_cache *digest_cache, u8 *digest,
>  			       enum hash_algo algo);
> +struct digest_cache *
> +digest_cache_dir_lookup_filename(struct dentry *dentry,
> +				 struct path *digest_list_path,
> +				 struct digest_cache *digest_cache,
> +				 char *filename);
>  void digest_cache_dir_free(struct digest_cache *digest_cache);
> =20
>  #endif /* _DIGEST_CACHE_INTERNAL_H */
> diff --git a/security/digest_cache/main.c b/security/digest_cache/main.c
> index 15f1486610a3..a5616fd07c1d 100644
> --- a/security/digest_cache/main.c
> +++ b/security/digest_cache/main.c
> @@ -83,6 +83,8 @@ static void digest_cache_free(struct digest_cache *dige=
st_cache)
>   * @digest_list_path: Path structure of the digest list
>   * @path_str: Path string of the digest list
>   * @filename: Digest list file name (can be an empty string)
> + * @prefetch_req: Whether prefetching has been requested
> + * @prefetch: Whether prefetching of a digest list is being done
>   *
>   * This function first locates, from the passed path, the digest list in=
ode
>   * from which the digest cache will be created or retrieved (if it alrea=
dy
> @@ -109,7 +111,8 @@ static void digest_cache_free(struct digest_cache *di=
gest_cache)
>   */
>  struct digest_cache *digest_cache_create(struct dentry *dentry,
>  					 struct path *digest_list_path,
> -					 char *path_str, char *filename)
> +					 char *path_str, char *filename,
> +					 bool prefetch_req, bool prefetch)


This has enough parameters to consider separate "digest_cache_descriptor"=
=20
or whatever. In some architectures this is beyond register parameters,
which is IMHO good threshold to consider that.

This will make e.g. tracing easier as you have to map only one parameter
to a known struct to inspect the values.


>  {
>  	struct path file_path;
>  	struct digest_cache *digest_cache =3D NULL;
> @@ -148,6 +151,16 @@ struct digest_cache *digest_cache_create(struct dent=
ry *dentry,
>  				 dentry->d_name.name);
>  			goto out;
>  		}
> +
> +		if (prefetch) {
> +			/* Fine to fail, we are just prefetching. */
> +			ret =3D digest_cache_populate(NULL, digest_list_path,
> +						    path_str, filename);
> +			pr_debug("Digest list %s/%s %s prefetched\n",
> +				 path_str, filename,
> +				 !ret ? "has been" : "cannot be");
> +			goto out;
> +		}
>  	}
> =20
>  	dig_sec =3D digest_cache_get_security(inode);
> @@ -176,6 +189,11 @@ struct digest_cache *digest_cache_create(struct dent=
ry *dentry,
> =20
>  	/* Make the other lock contenders wait until creation complete. */
>  	set_bit(INIT_IN_PROGRESS, &dig_sec->dig_owner->flags);
> +
> +	/* Set DIR_PREFETCH if prefetching was requested. */
> +	if (prefetch_req)
> +		set_bit(DIR_PREFETCH, &digest_cache->flags);
> +
>  	mutex_unlock(&dig_sec->dig_owner_mutex);
> =20
>  	if (S_ISREG(inode->i_mode)) {
> @@ -220,6 +238,52 @@ struct digest_cache *digest_cache_create(struct dent=
ry *dentry,
>  	return digest_cache;
>  }
> =20
> +/**
> + * digest_cache_prefetch_requested - Verify if prefetching is requested
> + * @digest_list_path: Path structure of the digest list directory
> + * @path_str: Path string of the digest list directory
> + *
> + * This function verifies whether or not digest list prefetching is requ=
ested.
> + * If dig_owner exists in the inode security blob, it checks the DIR_PRE=
FETCH
> + * bit (faster). Otherwise, it reads the new security.dig_prefetch xattr=
.
> + *
> + * Return: True if prefetching is requested, false otherwise.
> + */
> +static bool digest_cache_prefetch_requested(struct path *digest_list_pat=
h,
> +					    char *path_str)
> +{
> +	struct digest_cache_security *dig_sec;
> +	bool prefetch_req =3D false;
> +	char prefetch_value;
> +	struct inode *inode;
> +	int ret;
> +
> +	inode =3D d_backing_inode(digest_list_path->dentry);
> +	dig_sec =3D digest_cache_get_security(inode);
> +	if (unlikely(!dig_sec))
> +		return false;
> +
> +	mutex_lock(&dig_sec->dig_owner_mutex);
> +	if (dig_sec->dig_owner) {
> +		/* Reliable test: DIR_PREFETCH set with dig_owner_mutex held. */
> +		prefetch_req =3D test_bit(DIR_PREFETCH,
> +					&dig_sec->dig_owner->flags);
> +		mutex_unlock(&dig_sec->dig_owner_mutex);
> +		return prefetch_req;
> +	}
> +	mutex_unlock(&dig_sec->dig_owner_mutex);
> +
> +	ret =3D vfs_getxattr(&nop_mnt_idmap, digest_list_path->dentry,
> +			   XATTR_NAME_DIG_PREFETCH, &prefetch_value, 1);
> +	if (ret =3D=3D 1 && prefetch_value =3D=3D '1') {
> +		pr_debug("Prefetching has been enabled for directory %s\n",
> +			 path_str);
> +		prefetch_req =3D true;
> +	}
> +
> +	return prefetch_req;
> +}
> +
>  /**
>   * digest_cache_new - Retrieve digest list file name and request digest =
cache
>   * @dentry: Dentry of the inode for which the digest cache will be used
> @@ -230,13 +294,19 @@ struct digest_cache *digest_cache_create(struct den=
try *dentry,
>   * with that file name. If security.digest_list is not found, this funct=
ion
>   * requests the creation of a digest cache on the parent directory.
>   *
> + * On prefetching, if the default path is a directory and if
> + * security.digest_list is found, this function first retrieves the dire=
ctory
> + * digest cache, and then calls digest_cache_dir_lookup_filename() to re=
trieve
> + * the desired digest cache in that directory.
> + *
>   * Return: A new digest cache on success, NULL on error.
>   */
>  static struct digest_cache *digest_cache_new(struct dentry *dentry)
>  {
>  	char filename[NAME_MAX + 1] =3D { 0 };
> -	struct digest_cache *digest_cache =3D NULL;
> +	struct digest_cache *digest_cache =3D NULL, *found;
>  	struct path default_path;
> +	bool prefetch_req =3D false;
>  	int ret;
> =20
>  	ret =3D kern_path(default_path_str, 0, &default_path);
> @@ -273,9 +343,28 @@ static struct digest_cache *digest_cache_new(struct =
dentry *dentry)
>  	pr_debug("Found %s xattr in %s, default path: %s, digest list: %s\n",
>  		 XATTR_NAME_DIGEST_LIST, dentry->d_name.name, default_path_str,
>  		 filename);
> +
> +	if (filename[0])
> +		prefetch_req =3D digest_cache_prefetch_requested(&default_path,
> +							default_path_str);
>  create:
> +	/* On prefetching, retrieve the directory digest cache. */
>  	digest_cache =3D digest_cache_create(dentry, &default_path,
> -					   default_path_str, filename);
> +					   default_path_str,
> +					   !prefetch_req ? filename : "",
> +					   prefetch_req, false);
> +	if (!digest_cache)
> +		goto out;
> +
> +	if (prefetch_req) {
> +		/* Find the digest cache with a matching file name. */
> +		found =3D digest_cache_dir_lookup_filename(dentry, &default_path,
> +							 digest_cache,
> +							 filename);
> +		/* Release ref. to the directory digest cache. */
> +		digest_cache_put(digest_cache);
> +		digest_cache =3D found;
> +	}
>  out:
>  	path_put(&default_path);
>  	return digest_cache;
> diff --git a/security/digest_cache/populate.c b/security/digest_cache/pop=
ulate.c
> index 9c2fc2295310..17e7b011c367 100644
> --- a/security/digest_cache/populate.c
> +++ b/security/digest_cache/populate.c
> @@ -143,6 +143,12 @@ int digest_cache_populate(struct digest_cache *diges=
t_cache,
>  		return ret;
>  	}
> =20
> +	/* The caller wants just to read digest lists. */
> +	if (!digest_cache) {
> +		ret =3D 0;
> +		goto out_vfree;
> +	}
> +
>  	data_len =3D digest_cache_strip_modsig(data, ret);
> =20
>  	/* Digest list parsers initialize the hash table and add the digests. *=
/
> @@ -151,7 +157,7 @@ int digest_cache_populate(struct digest_cache *digest=
_cache,
>  	if (ret < 0)
>  		pr_debug("Error parsing digest list %s%s%s, ret: %d\n",
>  			 path_str, filename[0] ? "/" : "", filename, ret);
> -
> +out_vfree:
>  	vfree(data);
>  	return ret;
>  }
> diff --git a/security/digest_cache/verif.c b/security/digest_cache/verif.=
c
> index 04023240d3b4..c42ae93261e2 100644
> --- a/security/digest_cache/verif.c
> +++ b/security/digest_cache/verif.c
> @@ -33,7 +33,7 @@ static void free_verif(struct digest_cache_verif *verif=
)
>   * This function lets a verifier supply verification data about a digest=
 list
>   * being read to populate the digest cache.
>   *
> - * Return: Zero on success, -ENOMEM if out of memory.
> + * Return: Zero on success, -ENOMEM if out of memory, -ENOENT on prefetc=
hing.
>   */
>  int digest_cache_verif_set(struct file *file, const char *verif_id, void=
 *data,
>  			   size_t size)
> @@ -41,6 +41,9 @@ int digest_cache_verif_set(struct file *file, const cha=
r *verif_id, void *data,
>  	struct digest_cache *digest_cache =3D digest_cache_from_file_sec(file);
>  	struct digest_cache_verif *new_verif;
> =20
> +	if (!digest_cache)
> +		return -ENOENT;
> +
>  	/*
>  	 * All allocations must be atomic (non-sleepable) since kprobe does not
>  	 * allow otherwise (kprobe is needed for testing).


BR, Jarkko