[RFC PATCH v10 13/17] fsverity: consume builtin signature via LSM hook

Fan Wu
2023-06-28T21:10:45Z

fsverity represents a mechanism to support both integrity and
authenticity protection of a file, supporting both signed and unsigned
digests.

An LSM which controls access to a resource based on authenticity and
integrity of said resource, can then use this data to make an informed
decision on the authorization (provided by the LSM's policy) of said
claim.

This effectively allows the extension of a policy enforcement layer in
LSM for fsverity, allowing for more granular control of how a
particular authenticity claim can be used. For example, "all (built-in)
signed fsverity files should be allowed to execute, but only these
hashes are allowed to be loaded as kernel modules".

This enforcement must be done in kernel space, as a userspace only
solution would fail a simple litmus test: Download a self-contained
malicious binary that never touches the userspace stack. This
binary would still be able to execute.

Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
---
v1-v6:
+ Not present

v7:
Introduced

v8:
+ Split fs/verity/ changes and security/ changes into separate patches
+ Change signature of fsverity_create_info to accept non-const inode
+ Change signature of fsverity_verify_signature to accept non-const inode
+ Don't cast-away const from inode.
+ Digest functionality dropped in favor of:
("fs-verity: define a function to return the integrity protected
file digest")
+ Reworded commit description and title to match changes.
+ Fix a bug wherein no LSM implements the particular fsverity @name
(or LSM is disabled), and returns -EOPNOTSUPP, causing errors.

v9:
+ No changes

v10:
+ Rename the signature blob key
+ Cleanup redundant code
+ Make the hook call depends on CONFIG_FS_VERITY_BUILTIN_SIGNATURES
---
fs/verity/fsverity_private.h | 2 +-
fs/verity/open.c | 26 +++++++++++++++++++++++++-
include/linux/fsverity.h | 2 ++
3 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index 49bf3a1eb2a0..1f06dd6b1c6e 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -108,7 +108,7 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
unsigned int log_blocksize,
const u8 *salt, size_t salt_size);

-struct fsverity_info *fsverity_create_info(const struct inode *inode,
+struct fsverity_info *fsverity_create_info(struct inode *inode,
struct fsverity_descriptor *desc);

void fsverity_set_info(struct inode *inode, struct fsverity_info *vi);
diff --git a/fs/verity/open.c b/fs/verity/open.c
index 1db5106a9c38..91f632df08b8 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -8,6 +8,7 @@
#include "fsverity_private.h"

#include <linux/mm.h>
+#include <linux/security.h>
#include <linux/slab.h>

static struct kmem_cache *fsverity_info_cachep;
@@ -172,12 +173,28 @@ static int compute_file_digest(const struct fsverity_hash_alg *hash_alg,
return err;
}

+#ifdef CONFIG_FS_VERITY_BUILTIN_SIGNATURES
+static int fsverity_inode_setsecurity(struct inode *inode,
+ struct fsverity_descriptor *desc)
+{
+ return security_inode_setsecurity(inode, FS_VERITY_INODE_SEC_NAME,
+ desc->signature,
+ le32_to_cpu(desc->sig_size), 0);
+}
+#else
+static inline int fsverity_inode_setsecurity(struct inode *inode,
+ struct fsverity_descriptor *desc)
+{
+ return 0;
+}
+#endif /* CONFIG_IPE_PROP_FS_VERITY*/
+
/*
* Create a new fsverity_info from the given fsverity_descriptor (with optional
* appended builtin signature), and check the signature if present. The
* fsverity_descriptor must have already undergone basic validation.
*/
-struct fsverity_info *fsverity_create_info(const struct inode *inode,
+struct fsverity_info *fsverity_create_info(struct inode *inode,
struct fsverity_descriptor *desc)
{
struct fsverity_info *vi;
@@ -242,6 +259,13 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
spin_lock_init(&vi->hash_page_init_lock);
}

+ err = fsverity_inode_setsecurity(inode, desc);
+ if (err == -EOPNOTSUPP)
+ err = 0;
+
+ if (err)
+ goto fail;
+
return vi;

fail:
diff --git a/include/linux/fsverity.h b/include/linux/fsverity.h
index 1eb7eae580be..9666721baf15 100644
--- a/include/linux/fsverity.h
+++ b/include/linux/fsverity.h
@@ -319,4 +319,6 @@ static inline int fsverity_prepare_setattr(struct dentry *dentry,
return 0;
}

+#define FS_VERITY_INODE_SEC_NAME "fsverity.builtin-sig"
+
#endif /* _LINUX_FSVERITY_H */
--
2.25.1