Re: [PATCH v4 10/14] digest cache: Prefetch digest lists if requested

Jarkko Sakkinen
2024-04-16T14:47:52Z

On Tue Apr 16, 2024 at 1:34 PM EEST, Roberto Sassu wrote:
> On 4/15/2024 9:42 PM, Jarkko Sakkinen wrote:
> > 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 assumption
> >> 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 cause
> >> digest lists to be measured in a non-deterministic order too.
> >>
> >> Overcome this issue, if prefetching is enabled, by searching a digest list
> >> 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 was
> >> 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_SUFFIX
> >>
> >> +#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
> >>
> >> 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 = UINT_MAX;
> >> new_entry->digest_cache = NULL;
> >> mutex_init(&new_entry->digest_cache_mutex);
> >> + new_entry->prefetched = false;
> >>
> >> 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 *digest_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 = 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,
> >>
> >> /* Consume extra ref. from digest_cache_create(). */
> >> dir_entry->digest_cache = cache;
> >> + /* Digest list was read, mark entry as prefetched. */
> >> + dir_entry->prefetched = true;
> >> }
> >> mutex_unlock(&dir_entry->digest_cache_mutex);
> >>
> >> @@ -171,6 +176,54 @@ digest_cache_dir_lookup_digest(struct dentry *dentry,
> >> return 0UL;
> >> }
> >>
> >> +/**
> >> + * 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) the
> >> + * current digest list. Otherwise, it returns the digest cache pointer from
> >> + * digest_cache_create() to the caller.
> >> + *
> >> + * Return: A digest cache pointer if the digest list if found, NULL otherwise.
> >> + */
> >> +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 = !strcmp(dir_entry->name, filename);
> >> + if (!filename_found && dir_entry->prefetched) {
> >> + mutex_unlock(&dir_entry->digest_cache_mutex);
> >> + continue;
> >> + }
> >> +
> >> + cache = digest_cache_create(dentry, digest_list_path,
> >> + digest_cache->path_str,
> >> + dir_entry->name, false,
> >> + filename_found ? false : true);
> >> +
> >> + dir_entry->prefetched = 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 caches
> >> * @digest_cache: Digest cache
> >> diff --git a/security/digest_cache/internal.h b/security/digest_cache/internal.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. */
> >>
> >> /**
> >> * struct readdir_callback - Structure to store information for dir iteration
> >> @@ -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 created
> >> @@ -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;
> >>
> >> @@ -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);
> >>
> >> /* 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);
> >>
> >> #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 *digest_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 inode
> >> * from which the digest cache will be created or retrieved (if it already
> >> @@ -109,7 +111,8 @@ static void digest_cache_free(struct digest_cache *digest_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"
> > 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.
>
> Uhm, seems still good (6 parameters). Sure, I can make a structure to
> encapsulate the parameters, but will be used only for this purpose. Not
> sure the code understandability will improve.

I don't want to dictate this! So if you think it is best like this I'm
cool too. So make your call and I won't come back to nag further :-)

>
> Thanks
>
> Roberto
>
> >> {
> >> struct path file_path;
> >> struct digest_cache *digest_cache = NULL;
> >> @@ -148,6 +151,16 @@ struct digest_cache *digest_cache_create(struct dentry *dentry,
> >> dentry->d_name.name);
> >> goto out;
> >> }
> >> +
> >> + if (prefetch) {
> >> + /* Fine to fail, we are just prefetching. */
> >> + ret = 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;
> >> + }
> >> }
> >>
> >> dig_sec = digest_cache_get_security(inode);
> >> @@ -176,6 +189,11 @@ struct digest_cache *digest_cache_create(struct dentry *dentry,
> >>
> >> /* 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);
> >>
> >> if (S_ISREG(inode->i_mode)) {
> >> @@ -220,6 +238,52 @@ struct digest_cache *digest_cache_create(struct dentry *dentry,
> >> return digest_cache;
> >> }
> >>
> >> +/**
> >> + * 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 requested.
> >> + * If dig_owner exists in the inode security blob, it checks the DIR_PREFETCH
> >> + * 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_path,
> >> + char *path_str)
> >> +{
> >> + struct digest_cache_security *dig_sec;
> >> + bool prefetch_req = false;
> >> + char prefetch_value;
> >> + struct inode *inode;
> >> + int ret;
> >> +
> >> + inode = d_backing_inode(digest_list_path->dentry);
> >> + dig_sec = 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 = 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 = vfs_getxattr(&nop_mnt_idmap, digest_list_path->dentry,
> >> + XATTR_NAME_DIG_PREFETCH, &prefetch_value, 1);
> >> + if (ret == 1 && prefetch_value == '1') {
> >> + pr_debug("Prefetching has been enabled for directory %s\n",
> >> + path_str);
> >> + prefetch_req = 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 dentry *dentry,
> >> * with that file name. If security.digest_list is not found, this function
> >> * 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 directory
> >> + * digest cache, and then calls digest_cache_dir_lookup_filename() to retrieve
> >> + * 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] = { 0 };
> >> - struct digest_cache *digest_cache = NULL;
> >> + struct digest_cache *digest_cache = NULL, *found;
> >> struct path default_path;
> >> + bool prefetch_req = false;
> >> int ret;
> >>
> >> ret = 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 = digest_cache_prefetch_requested(&default_path,
> >> + default_path_str);
> >> create:
> >> + /* On prefetching, retrieve the directory digest cache. */
> >> digest_cache = 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 = 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 = found;
> >> + }
> >> out:
> >> path_put(&default_path);
> >> return digest_cache;
> >> diff --git a/security/digest_cache/populate.c b/security/digest_cache/populate.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 *digest_cache,
> >> return ret;
> >> }
> >>
> >> + /* The caller wants just to read digest lists. */
> >> + if (!digest_cache) {
> >> + ret = 0;
> >> + goto out_vfree;
> >> + }
> >> +
> >> data_len = digest_cache_strip_modsig(data, ret);
> >>
> >> /* 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 prefetching.
> >> */
> >> 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 char *verif_id, void *data,
> >> struct digest_cache *digest_cache = digest_cache_from_file_sec(file);
> >> struct digest_cache_verif *new_verif;
> >>
> >> + 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

BR, Jarkko