Re: [RFC PATCH v9 03/16] ipe: add evaluation loop and introduce 'boot_verified' as a trust provider

[Roberto Sassu]

On Mon, 2023-01-30 at 14:57 -0800, Fan Wu wrote:
> From: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>
> 
> IPE must have a centralized function to evaluate incoming callers
> against IPE's policy. This iteration of the policy against the rules
> for that specific caller is known as the evaluation loop.

Not sure if you check the properties at every access.

>From my previous comments (also for previous versions of the patches)
you could evaluate the property once, by calling the respective
functions in the other subsystems.

Then, you reserve space in the security blob for inodes and superblocks
to cache the decision. The format could be a policy sequence number, to
ensure that the cache is valid only for the current policy, and a bit
for every hook you enforce.

Also, currently you rely on the fact that the properties you defined
are immutable and the immutability is guaranteed by the other
subsystems, so no write can occur.

But if you remove this limitation, the immutability is not guaranteed
anymore by the other subsystems (for example if a file is in an ext4
filesystem), the LSM needs to take extra care to ensure that the
properties are still verified. This would be required for example if
IPE is used in conjuction with DIGLIM.

In my opinion, IPE value would increase if the generic enforcement
mechanism is property-agnostic.

Roberto

> In addition, IPE is designed to provide system level trust guarantees,
> this usually implies that trust starts from bootup with a hardware root
> of trust, which validates the bootloader. After this, the bootloader
> verifies the kernel and the initramfs.
> 
> As there's no currently supported integrity method for initramfs, and
> it's typically already verified by the bootloader, introduce a property
> that causes the first superblock to have an execution to be "pinned",
> which is typically initramfs.
> 
> Signed-off-by: Deven Bowers <deven.desai@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Fan Wu <wufan@xxxxxxxxxxxxxxxxxxx>
> 
> ---
> v2:
>   + Split evaluation loop, access control hooks,
>     and evaluation loop from policy parser and userspace
>     interface to pass mailing list character limit
> 
> v3:
>   + Move ipe_load_properties to patch 04.
>   + Remove useless 0-initializations
>   + Prefix extern variables with ipe_
>   + Remove kernel module parameters, as these are
>     exposed through sysctls.
>   + Add more prose to the IPE base config option
>     help text.
>   + Use GFP_KERNEL for audit_log_start.
>   + Remove unnecessary caching system.
>   + Remove comments from headers
>   + Use rcu_access_pointer for rcu-pointer null check
>   + Remove usage of reqprot; use prot only.
>   + Move policy load and activation audit event to 03/12
> 
> v4:
>   + Remove sysctls in favor of securityfs nodes
>   + Re-add kernel module parameters, as these are now
>     exposed through securityfs.
>   + Refactor property audit loop to a separate function.
> 
> v5:
>   + fix minor grammatical errors
>   + do not group rule by curly-brace in audit record,
>     reconstruct the exact rule.
> 
> v6:
>   + No changes
> 
> v7:
>   + Further split lsm creation into a separate commit from the
>     evaluation loop and audit system, for easier review.
> 
>   + Propogating changes to support the new ipe_context structure in the
>     evaluation loop.
> 
> v8:
>   + Remove ipe_hook enumeration; hooks can be correlated via syscall
>     record.
> 
> v9:
>   + Remove ipe_context related code and simplify the evaluation loop.
>   + Merge the evaluation loop commit with the boot_verified commit.
> ---
>  security/ipe/Makefile        |   1 +
>  security/ipe/eval.c          | 180 +++++++++++++++++++++++++++++++++++
>  security/ipe/eval.h          |  28 ++++++
>  security/ipe/hooks.c         |  25 +++++
>  security/ipe/hooks.h         |  14 +++
>  security/ipe/ipe.c           |   1 +
>  security/ipe/policy.c        |  20 ++++
>  security/ipe/policy.h        |   3 +
>  security/ipe/policy_parser.c |   8 +-
>  9 files changed, 279 insertions(+), 1 deletion(-)
>  create mode 100644 security/ipe/eval.c
>  create mode 100644 security/ipe/eval.h
>  create mode 100644 security/ipe/hooks.c
>  create mode 100644 security/ipe/hooks.h
> 
> diff --git a/security/ipe/Makefile b/security/ipe/Makefile
> index 16bbe80991f1..d7f2870d7c09 100644
> --- a/security/ipe/Makefile
> +++ b/security/ipe/Makefile
> @@ -6,6 +6,7 @@
>  #
>  
>  obj-$(CONFIG_SECURITY_IPE) += \
> +	eval.o \
>  	hooks.o \
>  	ipe.o \
>  	policy.o \
> diff --git a/security/ipe/eval.c b/security/ipe/eval.c
> new file mode 100644
> index 000000000000..48b5104a3463
> --- /dev/null
> +++ b/security/ipe/eval.c
> @@ -0,0 +1,180 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +
> +#include "ipe.h"
> +#include "eval.h"
> +#include "hooks.h"
> +#include "policy.h"
> +
> +#include <linux/fs.h>
> +#include <linux/types.h>
> +#include <linux/slab.h>
> +#include <linux/file.h>
> +#include <linux/sched.h>
> +#include <linux/rcupdate.h>
> +#include <linux/spinlock.h>
> +
> +struct ipe_policy __rcu *ipe_active_policy;
> +
> +static struct super_block *pinned_sb;
> +static DEFINE_SPINLOCK(pin_lock);
> +#define FILE_SUPERBLOCK(f) ((f)->f_path.mnt->mnt_sb)
> +
> +/**
> + * pin_sb - Pin the underlying superblock of @f, marking it as trusted.
> + * @f: Supplies a file structure to source the super_block from.
> + */
> +static void pin_sb(const struct file *f)
> +{
> +	if (!f)
> +		return;
> +	spin_lock(&pin_lock);
> +	if (pinned_sb)
> +		goto out;
> +	pinned_sb = FILE_SUPERBLOCK(f);
> +out:
> +	spin_unlock(&pin_lock);
> +}
> +
> +/**
> + * from_pinned - Determine whether @f is source from the pinned super_block.
> + * @f: Supplies a file structure to check against the pinned super_block.
> + *
> + * Return:
> + * * true	- @f is sourced from the pinned super_block
> + * * false	- @f is not sourced from the pinned super_block
> + */
> +static bool from_pinned(const struct file *f)
> +{
> +	bool rv;
> +
> +	if (!f)
> +		return false;
> +	spin_lock(&pin_lock);
> +	rv = !IS_ERR_OR_NULL(pinned_sb) && pinned_sb == FILE_SUPERBLOCK(f);
> +	spin_unlock(&pin_lock);
> +	return rv;
> +}
> +
> +/**
> + * build_eval_ctx - Build an evaluation context.
> + * @ctx: Supplies a pointer to the context to be populdated.
> + * @file: Supplies a pointer to the file to associated with the evaluation.
> + * @op: Supplies the IPE policy operation associated with the evaluation.
> + */
> +void build_eval_ctx(struct ipe_eval_ctx *ctx,
> +		    const struct file *file,
> +		    enum ipe_op_type op)
> +{
> +	ctx->file = file;
> +	ctx->op = op;
> +	ctx->from_init_sb = from_pinned(file);
> +}
> +
> +/**
> + * evaluate_property - Analyze @ctx against a property.
> + * @ctx: Supplies a pointer to the context to be evaluated.
> + * @p: Supplies a pointer to the property to be evaluated.
> + *
> + * Return:
> + * * true	- The current @ctx match the @p
> + * * false	- The current @ctx doesn't match the @p
> + */
> +static bool evaluate_property(const struct ipe_eval_ctx *const ctx,
> +			      struct ipe_prop *p)
> +{
> +	bool eval = false;
> +
> +	switch (p->type) {
> +	case ipe_prop_boot_verified_false:
> +		eval = !ctx->from_init_sb;
> +		break;
> +	case ipe_prop_boot_verified_true:
> +		eval = ctx->from_init_sb;
> +		break;
> +	default:
> +		eval = false;
> +	}
> +
> +	return eval;
> +}
> +
> +/**
> + * ipe_evaluate_event - Analyze @ctx against the current active policy.
> + * @ctx: Supplies a pointer to the context to be evaluated.
> + *
> + * This is the loop where all policy evaluation happens against IPE policy.
> + *
> + * Return:
> + * * 0		- OK
> + * * -EACCES	- @ctx did not pass evaluation.
> + * * !0		- Error
> + */
> +int ipe_evaluate_event(const struct ipe_eval_ctx *const ctx)
> +{
> +	int rc = 0;
> +	bool match = false;
> +	enum ipe_action_type action;
> +	struct ipe_policy *pol = NULL;
> +	const struct ipe_rule *rule = NULL;
> +	const struct ipe_op_table *rules = NULL;
> +	struct ipe_prop *prop = NULL;
> +
> +	if (ctx->op == ipe_op_exec)
> +		pin_sb(ctx->file);
> +
> +	pol = ipe_get_policy_rcu(ipe_active_policy);
> +	if (!pol)
> +		goto out;
> +
> +	if (ctx->op == ipe_op_max) {
> +		action = pol->parsed->global_default_action;
> +		goto eval;
> +	}
> +
> +	rules = &pol->parsed->rules[ctx->op];
> +
> +	list_for_each_entry(rule, &rules->rules, next) {
> +		match = true;
> +
> +		list_for_each_entry(prop, &rule->props, next)
> +			match = match && evaluate_property(ctx, prop);
> +
> +		if (match)
> +			break;
> +	}
> +
> +	if (match)
> +		action = rule->action;
> +	else if (rules->default_action != ipe_action_max)
> +		action = rules->default_action;
> +	else
> +		action = pol->parsed->global_default_action;
> +
> +eval:
> +	if (action == ipe_action_deny)
> +		rc = -EACCES;
> +
> +out:
> +	return rc;
> +}
> +
> +/**
> + * ipe_invalidate_pinned_sb - invalidte the ipe pinned super_block.
> + * @mnt_sb: super_block to check against the pinned super_block.
> + *
> + * This function is called a super_block like the initramfs's is freed,
> + * if the super_block is currently pinned by ipe it will be invalided,
> + * so ipe won't consider the block device is boot verified afterward.
> + */
> +void ipe_invalidate_pinned_sb(const struct super_block *mnt_sb)
> +{
> +	spin_lock(&pin_lock);
> +
> +	if (!IS_ERR_OR_NULL(pinned_sb) && mnt_sb == pinned_sb)
> +		pinned_sb = ERR_PTR(-EIO);
> +
> +	spin_unlock(&pin_lock);
> +}
> diff --git a/security/ipe/eval.h b/security/ipe/eval.h
> new file mode 100644
> index 000000000000..887797438b9b
> --- /dev/null
> +++ b/security/ipe/eval.h
> @@ -0,0 +1,28 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +
> +#ifndef IPE_EVAL_H
> +#define IPE_EVAL_H
> +
> +#include <linux/file.h>
> +#include <linux/types.h>
> +
> +#include "hooks.h"
> +#include "policy.h"
> +
> +extern struct ipe_policy __rcu *ipe_active_policy;
> +
> +struct ipe_eval_ctx {
> +	enum ipe_op_type op;
> +
> +	const struct file *file;
> +	bool from_init_sb;
> +};
> +
> +void build_eval_ctx(struct ipe_eval_ctx *ctx, const struct file *file, enum ipe_op_type op);
> +int ipe_evaluate_event(const struct ipe_eval_ctx *const ctx);
> +void ipe_invalidate_pinned_sb(const struct super_block *mnt_sb);
> +
> +#endif /* IPE_EVAL_H */
> diff --git a/security/ipe/hooks.c b/security/ipe/hooks.c
> new file mode 100644
> index 000000000000..335b773c7ae1
> --- /dev/null
> +++ b/security/ipe/hooks.c
> @@ -0,0 +1,25 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +
> +#include "ipe.h"
> +#include "hooks.h"
> +#include "eval.h"
> +
> +#include <linux/fs.h>
> +#include <linux/types.h>
> +#include <linux/binfmts.h>
> +#include <linux/mman.h>
> +
> +/**
> + * ipe_sb_free_security - ipe security hook function for super_block.
> + * @mnt_sb: Supplies a pointer to a super_block is about to be freed.
> + *
> + * IPE does not have any structures with mnt_sb, but uses this hook to
> + * invalidate a pinned super_block.
> + */
> +void ipe_sb_free_security(struct super_block *mnt_sb)
> +{
> +	ipe_invalidate_pinned_sb(mnt_sb);
> +}
> diff --git a/security/ipe/hooks.h b/security/ipe/hooks.h
> new file mode 100644
> index 000000000000..30fe455389bf
> --- /dev/null
> +++ b/security/ipe/hooks.h
> @@ -0,0 +1,14 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +#ifndef IPE_HOOKS_H
> +#define IPE_HOOKS_H
> +
> +#include <linux/fs.h>
> +#include <linux/binfmts.h>
> +#include <linux/security.h>
> +
> +void ipe_sb_free_security(struct super_block *mnt_sb);
> +
> +#endif /* IPE_HOOKS_H */
> diff --git a/security/ipe/ipe.c b/security/ipe/ipe.c
> index 9ed3bf4dcc04..551c6d90ac11 100644
> --- a/security/ipe/ipe.c
> +++ b/security/ipe/ipe.c
> @@ -9,6 +9,7 @@ static struct lsm_blob_sizes ipe_blobs __lsm_ro_after_init = {
>  };
>  
>  static struct security_hook_list ipe_hooks[] __lsm_ro_after_init = {
> +	LSM_HOOK_INIT(sb_free_security, ipe_sb_free_security),
>  };
>  
>  /**
> diff --git a/security/ipe/policy.c b/security/ipe/policy.c
> index e446f4b84152..772d876b1087 100644
> --- a/security/ipe/policy.c
> +++ b/security/ipe/policy.c
> @@ -97,3 +97,23 @@ struct ipe_policy *ipe_new_policy(const char *text, size_t textlen,
>  err:
>  	return ERR_PTR(rc);
>  }
> +
> +/**
> + * ipe_get_policy_rcu - Dereference a rcu-protected policy pointer.
> + *
> + * @p: rcu-protected pointer to a policy.
> + *
> + * Not safe to call on IS_ERR.
> + *
> + * Return: the value of @p
> + */
> +struct ipe_policy *ipe_get_policy_rcu(struct ipe_policy __rcu *p)
> +{
> +	struct ipe_policy *rv = NULL;
> +
> +	rcu_read_lock();
> +	rv = rcu_dereference(p);
> +	rcu_read_unlock();
> +
> +	return rv;
> +}
> diff --git a/security/ipe/policy.h b/security/ipe/policy.h
> index 6af2d9a811ec..967d816cd5cd 100644
> --- a/security/ipe/policy.h
> +++ b/security/ipe/policy.h
> @@ -26,6 +26,8 @@ enum ipe_action_type {
>  };
>  
>  enum ipe_prop_type {
> +	ipe_prop_boot_verified_false,
> +	ipe_prop_boot_verified_true,
>  	ipe_prop_max
>  };
>  
> @@ -73,5 +75,6 @@ struct ipe_policy {
>  struct ipe_policy *ipe_new_policy(const char *text, size_t textlen,
>  				  const char *pkcs7, size_t pkcs7len);
>  void ipe_free_policy(struct ipe_policy *pol);
> +struct ipe_policy *ipe_get_policy_rcu(struct ipe_policy __rcu *p);
>  
>  #endif /* IPE_POLICY_H */
> diff --git a/security/ipe/policy_parser.c b/security/ipe/policy_parser.c
> index c7ba0e865366..7efafc482e46 100644
> --- a/security/ipe/policy_parser.c
> +++ b/security/ipe/policy_parser.c
> @@ -265,7 +265,9 @@ static enum ipe_action_type parse_action(char *t)
>  }
>  
>  static const match_table_t property_tokens = {
> -	{ipe_prop_max,					NULL}
> +	{ipe_prop_boot_verified_false,	"boot_verified=FALSE"},
> +	{ipe_prop_boot_verified_true,	"boot_verified=TRUE"},
> +	{ipe_prop_max,			NULL}
>  };
>  
>  /**
> @@ -295,6 +297,10 @@ int parse_property(char *t, struct ipe_rule *r)
>  	token = match_token(t, property_tokens, args);
>  
>  	switch (token) {
> +	case ipe_prop_boot_verified_false:
> +	case ipe_prop_boot_verified_true:
> +		p->type = token;
> +		break;
>  	case ipe_prop_max:
>  	default:
>  		rc = -EBADMSG;