linux-kernel - Re: [PATCH RFC v10 3/17] ipe: add evaluation loop

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Date:   Sat, 08 Jul 2023 01:36:56 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Fan Wu <wufan@...ux.microsoft.com>, corbet@....net,
        zohar@...ux.ibm.com, jmorris@...ei.org, serge@...lyn.com,
        tytso@....edu, ebiggers@...nel.org, axboe@...nel.dk,
        agk@...hat.com, snitzer@...nel.org, eparis@...hat.com
Cc:     linux-doc@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, audit@...r.kernel.org,
        roberto.sassu@...wei.com, linux-kernel@...r.kernel.org,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Fan Wu <wufan@...ux.microsoft.com>
Subject: Re: [PATCH RFC v10 3/17] ipe: add evaluation loop

On Jun 28, 2023 Fan Wu <wufan@...ux.microsoft.com> wrote:
> 
> IPE must have a centralized function to evaluate incoming callers
> against IPE's policy. This iteration of the policy for against the rules
> for that specific caller is known as the evaluation loop.

Can you rewrite that second sentence, it reads a bit awkward and I'm
unclear as to the meaning.

> Signed-off-by: Deven Bowers <deven.desai@...ux.microsoft.com>
> Signed-off-by: Fan Wu <wufan@...ux.microsoft.com>
> ---
>  security/ipe/Makefile |  1 +
>  security/ipe/eval.c   | 94 +++++++++++++++++++++++++++++++++++++++++++
>  security/ipe/eval.h   | 25 ++++++++++++
>  3 files changed, 120 insertions(+)
>  create mode 100644 security/ipe/eval.c
>  create mode 100644 security/ipe/eval.h

...

> diff --git a/security/ipe/eval.c b/security/ipe/eval.c
> new file mode 100644
> index 000000000000..59144b2ecdda
> --- /dev/null
> +++ b/security/ipe/eval.c
> @@ -0,0 +1,94 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +
> +#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 "ipe.h"
> +#include "eval.h"
> +#include "hooks.h"

There is no "hooks.h" at this point in the patchset.

In order for 'git bisect' to remain useful (and it can be a very handy
tool), we need to ensure that each point in the patchset compiles
cleanly.

> +#include "policy.h"
> +
> +struct ipe_policy __rcu *ipe_active_policy;
> +
> +/**
> + * 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)
> +{
> +	return false;
> +}
> +
> +/**
> + * 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;
> +
> +	rcu_read_lock();
> +
> +	pol = rcu_dereference(ipe_active_policy);
> +	if (!pol) {
> +		rcu_read_unlock();
> +		return 0;
> +	}
> +
> +	if (ctx->op == __IPE_OP_INVALID) {
> +		action = pol->parsed->global_default_action;
> +		goto eval;

It looks like you are missing a rcu_read_unlock() in this case.

Also, given how simplistic the evaluation is in this case, why not
just do it here, saving the assignment, jump, etc.?

  if (ctx->op == INVALID) {
    rcu_read_unlock()
    if (global_action == DENY)
      return -EACCES;
    return 0;
  }

> +	}
> +
> +	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);

Why not break from this loop once evaluate_property() returns false?

> +
> +		if (match)
> +			break;
> +	}
> +
> +	if (match)
> +		action = rule->action;
> +	else if (rules->default_action != __IPE_ACTION_INVALID)
> +		action = rules->default_action;
> +	else
> +		action = pol->parsed->global_default_action;
> +
> +	rcu_read_unlock();
> +eval:
> +	if (action == __IPE_ACTION_DENY)
> +		rc = -EACCES;
> +
> +	return rc;

This can just be 'return 0;' right?

> +}

--
paul-moore.com