diff mbox series

[RFC,v15,18/21] ipe: enable support for fs-verity as a trust provider

Message ID 1710560151-28904-19-git-send-email-wufan@linux.microsoft.com (mailing list archive)
State Changes Requested
Delegated to: Paul Moore
Headers show
Series Integrity Policy Enforcement LSM (IPE) | expand

Commit Message

Fan Wu March 16, 2024, 3:35 a.m. UTC 
Enable IPE policy authors to indicate trust for a singular fsverity
file, identified by the digest information, through "fsverity_digest"
and all files using fsverity's builtin signatures via
"fsverity_signature".

This enables file-level integrity claims to be expressed in IPE,
allowing individual files to be authorized, giving some flexibility
for policy authors. Such file-level claims are important to be expressed
for enforcing the integrity of packages, as well as address some of the
scalability issues in a sole dm-verity based solution (# of loop back
devices, etc).

This solution cannot be done in userspace as the minimum threat that
IPE should mitigate is an attacker downloads malicious payload with
all required dependencies. These dependencies can lack the userspace
check, bypassing the protection entirely. A similar attack succeeds if
the userspace component is replaced with a version that does not
perform the check. As a result, this can only be done in the common
entry point - the kernel.

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:
  * Undo squash of 08/12, 10/12 - separating drivers/md/ from security/
  * Use common-audit function for fsverity_signature.
  + Change fsverity implementation to use fsverity_get_digest
  + prevent unnecessary copy of fs-verity signature data, instead
    just check for presence of signature data.
  + Remove free_inode_security hook, as the digest is now acquired
    at runtime instead of via LSM blob.

v9:
  + Adapt to the new parser

v10:
  + Update the fsverity get digest call

v11:
  + No changes

v12:
  + Fix audit format
  + Simplify property evaluation

v13:
  + Remove the CONFIG_IPE_PROP_FS_VERITY dependency inside the parser
    to make the policy grammar independent of the kernel config.

v14:
  + No changes

v15:
  + Fix on grammar issue in Kconfig
  + Switch hook to security_inode_setintegrity()
---
 security/ipe/Kconfig         |  13 +++++
 security/ipe/audit.c         |  17 ++++++
 security/ipe/eval.c          | 108 ++++++++++++++++++++++++++++++++++-
 security/ipe/eval.h          |  10 ++++
 security/ipe/hooks.c         |  28 +++++++++
 security/ipe/hooks.h         |   6 ++
 security/ipe/ipe.c           |  13 +++++
 security/ipe/ipe.h           |   3 +
 security/ipe/policy.h        |   3 +
 security/ipe/policy_parser.c |   6 ++
 10 files changed, 206 insertions(+), 1 deletion(-)

Comments

Eric Biggers March 18, 2024, 5:17 a.m. UTC | #1 
On Fri, Mar 15, 2024 at 08:35:48PM -0700, Fan Wu wrote:
> +config IPE_PROP_FS_VERITY
> +	bool "Enable property for fs-verity files"
> +	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
> +	help
> +	  This option enables the usage of properties "fsverity_signature"
> +	  and "fsverity_digest". These properties evaluate to TRUE when
> +	  a file is fsverity enabled and with a signed digest

Again: why would anyone care if there is a signature, if that signature is not
checked.

I think you meant to write something like: "when a file is fsverity enabled and
has a valid builtin signature whose signing cert is in the .fs-verity keyring".

- Eric
Roberto Sassu March 18, 2024, 8:08 a.m. UTC | #2 
On Sun, 2024-03-17 at 22:17 -0700, Eric Biggers wrote:
> On Fri, Mar 15, 2024 at 08:35:48PM -0700, Fan Wu wrote:
> > +config IPE_PROP_FS_VERITY
> > +	bool "Enable property for fs-verity files"
> > +	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
> > +	help
> > +	  This option enables the usage of properties "fsverity_signature"
> > +	  and "fsverity_digest". These properties evaluate to TRUE when
> > +	  a file is fsverity enabled and with a signed digest
> 
> Again: why would anyone care if there is a signature, if that signature is not
> checked.
> 
> I think you meant to write something like: "when a file is fsverity enabled and
> has a valid builtin signature whose signing cert is in the .fs-verity keyring".

I was also thinking the same. I didn't follow the recent development
closely, but unless IPE locks somehow the .fs-verity keyring, the
property you suggested would not be immutable. Meaning that someone can
add/remove a key in that keyring, making the property true or false.

Roberto
Fan Wu March 18, 2024, 8:40 p.m. UTC | #3 
On 3/17/2024 10:17 PM, Eric Biggers wrote:
> On Fri, Mar 15, 2024 at 08:35:48PM -0700, Fan Wu wrote:
>> +config IPE_PROP_FS_VERITY
>> +	bool "Enable property for fs-verity files"
>> +	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
>> +	help
>> +	  This option enables the usage of properties "fsverity_signature"
>> +	  and "fsverity_digest". These properties evaluate to TRUE when
>> +	  a file is fsverity enabled and with a signed digest
> 
> Again: why would anyone care if there is a signature, if that signature is not
> checked.
> 
> I think you meant to write something like: "when a file is fsverity enabled and
> has a valid builtin signature whose signing cert is in the .fs-verity keyring".
> 
> - Eric

Thanks for the suggestion. I agree this is a more accurate description. 
I'll update the description to include these details.

-Fan
Fan Wu March 18, 2024, 8:58 p.m. UTC | #4 
On 3/18/2024 1:08 AM, Roberto Sassu wrote:
> On Sun, 2024-03-17 at 22:17 -0700, Eric Biggers wrote:
>> On Fri, Mar 15, 2024 at 08:35:48PM -0700, Fan Wu wrote:
>>> +config IPE_PROP_FS_VERITY
>>> +	bool "Enable property for fs-verity files"
>>> +	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
>>> +	help
>>> +	  This option enables the usage of properties "fsverity_signature"
>>> +	  and "fsverity_digest". These properties evaluate to TRUE when
>>> +	  a file is fsverity enabled and with a signed digest
>>
>> Again: why would anyone care if there is a signature, if that signature is not
>> checked.
>>
>> I think you meant to write something like: "when a file is fsverity enabled and
>> has a valid builtin signature whose signing cert is in the .fs-verity keyring".
> 
> I was also thinking the same. I didn't follow the recent development
> closely, but unless IPE locks somehow the .fs-verity keyring, the
> property you suggested would not be immutable. Meaning that someone can
> add/remove a key in that keyring, making the property true or false.
> 
> Roberto

Yes, the .fs-verity keyring's mutability could affect the property's 
immutability. However, we are not planing to "lock" the keyrings, but we 
would like to use policies languages to express what certificate can be 
trusted.

For example, we can have a rule like this:

#Certificate declaration
CERTIFICATE=MyCertificate CertThumbprint=DummyThumbprint
op=EXECUTE fsverity_signature=MyCertificate action=ALLOW

This will be our immediate next work after the initial version is accepted.

-Fan
diff mbox series

Patch

diff --git a/security/ipe/Kconfig b/security/ipe/Kconfig
index 5433ca3fc855..2003c01767ad 100644
--- a/security/ipe/Kconfig
+++ b/security/ipe/Kconfig
@@ -30,6 +30,19 @@  config IPE_PROP_DM_VERITY
 	  that was mounted with a signed root-hash or the volume's
 	  root hash matches the supplied value in the policy.
 
+	  If unsure, answer Y.
+
+config IPE_PROP_FS_VERITY
+	bool "Enable property for fs-verity files"
+	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
+	help
+	  This option enables the usage of properties "fsverity_signature"
+	  and "fsverity_digest". These properties evaluate to TRUE when
+	  a file is fsverity enabled and with a signed digest or its
+	  digest matches the supplied value in the policy.
+
+	  if unsure, answer Y.
+
 endmenu
 
 endif
diff --git a/security/ipe/audit.c b/security/ipe/audit.c
index 3acbfecc9f43..f508944a3ed1 100644
--- a/security/ipe/audit.c
+++ b/security/ipe/audit.c
@@ -53,6 +53,9 @@  static const char *const audit_prop_names[__IPE_PROP_MAX] = {
 	"dmverity_roothash=",
 	"dmverity_signature=FALSE",
 	"dmverity_signature=TRUE",
+	"fsverity_digest=",
+	"fsverity_signature=FALSE",
+	"fsverity_signature=TRUE",
 };
 
 /**
@@ -66,6 +69,17 @@  static void audit_dmv_roothash(struct audit_buffer *ab, const void *rh)
 	ipe_digest_audit(ab, rh);
 }
 
+/**
+ * audit_fsv_digest - audit the digest of a fsverity_digest property.
+ * @ab: Supplies a pointer to the audit_buffer to append to.
+ * @d: Supplies a pointer to the digest structure.
+ */
+static void audit_fsv_digest(struct audit_buffer *ab, const void *d)
+{
+	audit_log_format(ab, "%s", audit_prop_names[IPE_PROP_FSV_DIGEST]);
+	ipe_digest_audit(ab, d);
+}
+
 /**
  * audit_rule - audit an IPE policy rule approximation.
  * @ab: Supplies a pointer to the audit_buffer to append to.
@@ -82,6 +96,9 @@  static void audit_rule(struct audit_buffer *ab, const struct ipe_rule *r)
 		case IPE_PROP_DMV_ROOTHASH:
 			audit_dmv_roothash(ab, ptr->value);
 			break;
+		case IPE_PROP_FSV_DIGEST:
+			audit_fsv_digest(ab, ptr->value);
+			break;
 		default:
 			audit_log_format(ab, "%s", audit_prop_names[ptr->type]);
 			break;
diff --git a/security/ipe/eval.c b/security/ipe/eval.c
index 8dcf7809c9fc..8cb8c64221e0 100644
--- a/security/ipe/eval.c
+++ b/security/ipe/eval.c
@@ -10,6 +10,7 @@ 
 #include <linux/sched.h>
 #include <linux/rcupdate.h>
 #include <linux/moduleparam.h>
+#include <linux/fsverity.h>
 
 #include "ipe.h"
 #include "eval.h"
@@ -51,6 +52,23 @@  static void build_ipe_bdev_ctx(struct ipe_eval_ctx *ctx, const struct inode *con
 }
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
 
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+/**
+ * build_ipe_inode_ctx - Build inode fields of an evaluation context.
+ * @ctx: Supplies a pointer to the context to be populated.
+ * @ino: Supplies the inode struct of the file triggered IPE event.
+ */
+static void build_ipe_inode_ctx(struct ipe_eval_ctx *ctx, const struct inode *const ino)
+{
+	ctx->ino = ino;
+	ctx->ipe_inode = ipe_inode(ctx->ino);
+}
+#else
+static void build_ipe_inode_ctx(struct ipe_eval_ctx *ctx, const struct inode *const ino)
+{
+}
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
+
 /**
  * build_eval_ctx - Build an evaluation context.
  * @ctx: Supplies a pointer to the context to be populated.
@@ -63,13 +81,17 @@  void build_eval_ctx(struct ipe_eval_ctx *ctx,
 		    enum ipe_op_type op,
 		    enum ipe_hook_type hook)
 {
+	struct inode *ino;
+
 	ctx->file = file;
 	ctx->op = op;
 	ctx->hook = hook;
 
 	if (file) {
 		build_ipe_sb_ctx(ctx, file);
-		build_ipe_bdev_ctx(ctx, d_real_inode(file->f_path.dentry));
+		ino = d_real_inode(file->f_path.dentry);
+		build_ipe_bdev_ctx(ctx, ino);
+		build_ipe_inode_ctx(ctx, ino);
 	}
 }
 
@@ -148,6 +170,84 @@  static bool evaluate_dmv_sig_true(const struct ipe_eval_ctx *const ctx)
 }
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
 
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+/**
+ * evaluate_fsv_digest - Analyze @ctx against a fsv digest property.
+ * @ctx: Supplies a pointer to the context being evaluated.
+ * @p: Supplies a pointer to the property being evaluated.
+ *
+ * Return:
+ * * true	- The current @ctx match the @p
+ * * false	- The current @ctx doesn't match the @p
+ */
+static bool evaluate_fsv_digest(const struct ipe_eval_ctx *const ctx,
+				struct ipe_prop *p)
+{
+	enum hash_algo alg;
+	u8 digest[FS_VERITY_MAX_DIGEST_SIZE];
+	struct digest_info info;
+
+	if (!ctx->ino)
+		return false;
+	if (!fsverity_get_digest((struct inode *)ctx->ino,
+				 digest,
+				 NULL,
+				 &alg))
+		return false;
+
+	info.alg = hash_algo_name[alg];
+	info.digest = digest;
+	info.digest_len = hash_digest_size[alg];
+
+	return ipe_digest_eval(p->value, &info);
+}
+
+/**
+ * evaluate_fsv_sig_false - Analyze @ctx against a fsv sig false property.
+ * @ctx: Supplies a pointer to the context being evaluated.
+ *
+ * Return:
+ * * true	- The current @ctx match the property
+ * * false	- The current @ctx doesn't match the property
+ */
+static bool evaluate_fsv_sig_false(const struct ipe_eval_ctx *const ctx)
+{
+	return !ctx->ino ||
+	       !IS_VERITY(ctx->ino) ||
+	       !ctx->ipe_inode ||
+	       !ctx->ipe_inode->fs_verity_signed;
+}
+
+/**
+ * evaluate_fsv_sig_true - Analyze @ctx against a fsv sig true property.
+ * @ctx: Supplies a pointer to the context being evaluated.
+ *
+ * Return:
+ * * true - The current @ctx match the property
+ * * false - The current @ctx doesn't match the property
+ */
+static bool evaluate_fsv_sig_true(const struct ipe_eval_ctx *const ctx)
+{
+	return !evaluate_fsv_sig_false(ctx);
+}
+#else
+static bool evaluate_fsv_digest(const struct ipe_eval_ctx *const ctx,
+				struct ipe_prop *p)
+{
+	return false;
+}
+
+static bool evaluate_fsv_sig_false(const struct ipe_eval_ctx *const ctx)
+{
+	return false;
+}
+
+static bool evaluate_fsv_sig_true(const struct ipe_eval_ctx *const ctx)
+{
+	return false;
+}
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
+
 /**
  * evaluate_property - Analyze @ctx against a property.
  * @ctx: Supplies a pointer to the context to be evaluated.
@@ -171,6 +271,12 @@  static bool evaluate_property(const struct ipe_eval_ctx *const ctx,
 		return evaluate_dmv_sig_false(ctx);
 	case IPE_PROP_DMV_SIG_TRUE:
 		return evaluate_dmv_sig_true(ctx);
+	case IPE_PROP_FSV_DIGEST:
+		return evaluate_fsv_digest(ctx, p);
+	case IPE_PROP_FSV_SIG_FALSE:
+		return evaluate_fsv_sig_false(ctx);
+	case IPE_PROP_FSV_SIG_TRUE:
+		return evaluate_fsv_sig_true(ctx);
 	default:
 		return false;
 	}
diff --git a/security/ipe/eval.h b/security/ipe/eval.h
index 494c0754f512..73ac905dc97d 100644
--- a/security/ipe/eval.h
+++ b/security/ipe/eval.h
@@ -29,6 +29,12 @@  struct ipe_bdev {
 };
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
 
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+struct ipe_inode {
+	bool fs_verity_signed;
+};
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
+
 struct ipe_eval_ctx {
 	enum ipe_op_type op;
 	enum ipe_hook_type hook;
@@ -38,6 +44,10 @@  struct ipe_eval_ctx {
 #ifdef CONFIG_IPE_PROP_DM_VERITY
 	const struct ipe_bdev *ipe_bdev;
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+	const struct inode *ino;
+	const struct ipe_inode *ipe_inode;
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
 };
 
 enum ipe_match {
diff --git a/security/ipe/hooks.c b/security/ipe/hooks.c
index 1abddd6e1e7f..b9dc23047463 100644
--- a/security/ipe/hooks.c
+++ b/security/ipe/hooks.c
@@ -254,3 +254,31 @@  int ipe_bdev_setintegrity(struct block_device *bdev, enum lsm_intgr_type type,
 	return 0;
 }
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
+
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+/**
+ * ipe_inode_setintegrity - save integrity data from a inode to IPE's LSM blob.
+ * @inode: The inode to source the security blob from.
+ * @type: Supplies the integrity type.
+ * @value: The value to be stored.
+ * @size: The size of @value.
+ *
+ * Saves fsverity signature into inode security blob
+ *
+ * Return:
+ * * 0	- OK
+ * * !0	- Error
+ */
+int ipe_inode_setintegrity(struct inode *inode, enum lsm_intgr_type type,
+			   const void *value, size_t size)
+{
+	struct ipe_inode *inode_sec = ipe_inode(inode);
+
+	if (type == LSM_INTGR_FSV_SIG) {
+		inode_sec->fs_verity_signed = size > 0 && value;
+		return 0;
+	}
+
+	return 0;
+}
+#endif /* CONFIG_CONFIG_IPE_PROP_FS_VERITY */
diff --git a/security/ipe/hooks.h b/security/ipe/hooks.h
index 6b735aac4028..cfa9920aa2e5 100644
--- a/security/ipe/hooks.h
+++ b/security/ipe/hooks.h
@@ -9,6 +9,7 @@ 
 #include <linux/binfmts.h>
 #include <linux/security.h>
 #include <linux/blk_types.h>
+#include <linux/fsverity.h>
 
 enum ipe_hook_type {
 	IPE_HOOK_BPRM_CHECK = 0,
@@ -43,4 +44,9 @@  int ipe_bdev_setintegrity(struct block_device *bdev, enum lsm_intgr_type type,
 			  const void *value, size_t len);
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
 
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+int ipe_inode_setintegrity(struct inode *inode, enum lsm_intgr_type type,
+			   const void *value, size_t size);
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
+
 #endif /* _IPE_HOOKS_H */
diff --git a/security/ipe/ipe.c b/security/ipe/ipe.c
index 3d63e9672f12..86b87e287b69 100644
--- a/security/ipe/ipe.c
+++ b/security/ipe/ipe.c
@@ -16,6 +16,9 @@  static struct lsm_blob_sizes ipe_blobs __ro_after_init = {
 #ifdef CONFIG_IPE_PROP_DM_VERITY
 	.lbs_bdev = sizeof(struct ipe_bdev),
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+	.lbs_inode = sizeof(struct ipe_inode),
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
 };
 
 static const struct lsm_id ipe_lsmid = {
@@ -35,6 +38,13 @@  struct ipe_bdev *ipe_bdev(struct block_device *b)
 }
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
 
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+struct ipe_inode *ipe_inode(const struct inode *inode)
+{
+	return inode->i_security + ipe_blobs.lbs_inode;
+}
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
+
 static struct security_hook_list ipe_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(bprm_check_security, ipe_bprm_check_security),
 	LSM_HOOK_INIT(mmap_file, ipe_mmap_file),
@@ -46,6 +56,9 @@  static struct security_hook_list ipe_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(bdev_free_security, ipe_bdev_free_security),
 	LSM_HOOK_INIT(bdev_setintegrity, ipe_bdev_setintegrity),
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+	LSM_HOOK_INIT(inode_setintegrity, ipe_inode_setintegrity),
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
 };
 
 /**
diff --git a/security/ipe/ipe.h b/security/ipe/ipe.h
index 64626702b689..57d3fc22fe10 100644
--- a/security/ipe/ipe.h
+++ b/security/ipe/ipe.h
@@ -19,5 +19,8 @@  extern bool ipe_enabled;
 #ifdef CONFIG_IPE_PROP_DM_VERITY
 struct ipe_bdev *ipe_bdev(struct block_device *b);
 #endif /* CONFIG_IPE_PROP_DM_VERITY */
+#ifdef CONFIG_IPE_PROP_FS_VERITY
+struct ipe_inode *ipe_inode(const struct inode *inode);
+#endif /* CONFIG_IPE_PROP_FS_VERITY */
 
 #endif /* _IPE_H */
diff --git a/security/ipe/policy.h b/security/ipe/policy.h
index 35629e9699f7..ae6264874fd3 100644
--- a/security/ipe/policy.h
+++ b/security/ipe/policy.h
@@ -36,6 +36,9 @@  enum ipe_prop_type {
 	IPE_PROP_DMV_ROOTHASH,
 	IPE_PROP_DMV_SIG_FALSE,
 	IPE_PROP_DMV_SIG_TRUE,
+	IPE_PROP_FSV_DIGEST,
+	IPE_PROP_FSV_SIG_FALSE,
+	IPE_PROP_FSV_SIG_TRUE,
 	__IPE_PROP_MAX
 };
 
diff --git a/security/ipe/policy_parser.c b/security/ipe/policy_parser.c
index 802e31f14b22..48dc11739072 100644
--- a/security/ipe/policy_parser.c
+++ b/security/ipe/policy_parser.c
@@ -273,6 +273,9 @@  static const match_table_t property_tokens = {
 	{IPE_PROP_DMV_ROOTHASH,		"dmverity_roothash=%s"},
 	{IPE_PROP_DMV_SIG_FALSE,	"dmverity_signature=FALSE"},
 	{IPE_PROP_DMV_SIG_TRUE,		"dmverity_signature=TRUE"},
+	{IPE_PROP_FSV_DIGEST,		"fsverity_digest=%s"},
+	{IPE_PROP_FSV_SIG_FALSE,	"fsverity_signature=FALSE"},
+	{IPE_PROP_FSV_SIG_TRUE,		"fsverity_signature=TRUE"},
 	{IPE_PROP_INVALID,		NULL}
 };
 
@@ -302,6 +305,7 @@  static int parse_property(char *t, struct ipe_rule *r)
 
 	switch (token) {
 	case IPE_PROP_DMV_ROOTHASH:
+	case IPE_PROP_FSV_DIGEST:
 		dup = match_strdup(&args[0]);
 		if (!dup) {
 			rc = -ENOMEM;
@@ -317,6 +321,8 @@  static int parse_property(char *t, struct ipe_rule *r)
 	case IPE_PROP_BOOT_VERIFIED_TRUE:
 	case IPE_PROP_DMV_SIG_FALSE:
 	case IPE_PROP_DMV_SIG_TRUE:
+	case IPE_PROP_FSV_SIG_FALSE:
+	case IPE_PROP_FSV_SIG_TRUE:
 		p->type = token;
 		break;
 	default: