Re: [PATCH RFC v10 8/17] uapi|audit|ipe: add ipe auditing support

Paul Moore
2023-07-08T05:38:15Z

On Jun 28, 2023 Fan Wu <wufan@linux.microsoft.com> wrote:
>
> Users of IPE require a way to identify when and why an operation fails,
> allowing them to both respond to violations of policy and be notified
> of potentially malicious actions on their systems with respect to IPE
> itself.
>
> This patch introduces 3 new audit events.
>
> AUDIT_IPE_ACCESS(1420) indicates the result of an IPE policy evaluation
> of a resource.
> AUDIT_IPE_CONFIG_CHANGE(1421) indicates the current active IPE policy
> has been changed to another loaded policy.
> AUDIT_IPE_POLICY_LOAD(1422) indicates a new IPE policy has been loaded
> into the kernel.
>
> This patch also adds support for success auditing, allowing users to
> identify why an allow decision was made for a resource. However, it is
> recommended to use this option with caution, as it is quite noisy.
>
> Here are some examples of the new audit record types:
>
> AUDIT_IPE_ACCESS(1420):
>
> audit: AUDIT1420 path="/root/vol/bin/hello" dev="sda"
> ino=3897 rule="op=EXECUTE boot_verified=TRUE action=ALLOW"

The 'dev' field is already in use by audit, and is used to log the
device major and minor numbers, see audit_log_name() for an example.

I would suggest adopting the existing 'dev' field format, but if you
really want to log the device name as a string you will need to find
another audit field name.

> audit: AUDIT1420 path="/mnt/ipe/bin/hello" dev="dm-0"
> ino=2 rule="DEFAULT action=DENY"
>
> audit: AUDIT1420 path="/tmp/tmpdp2h1lub/deny/bin/hello" dev="tmpfs"
> ino=131 rule="DEFAULT action=DENY"
>
> The above three records were generated when the active IPE policy only
> allows binaries from the initial booted drive(sda) to run. The three
> identical `hello` binary were placed at different locations, only the
> first hello from sda was allowed.
>
> Field path followed by the file's path name.
>
> Field dev followed by the device name as found in /dev where the file is
> from.
> Note that for device mappers it will use the name `dm-X` instead of
> the name in /dev/mapper.
> For a file in a temp file system, which is not from a device, it will use
> `tmpfs` for the field.
> The implementation of this part is following another existing use case
> LSM_AUDIT_DATA_INODE in security/lsm_audit.c
>
> Field ino followed by the file's inode number.
>
> Field rule followed by the IPE rule made the access decision. The whole
> rule must be audited because the decision is based on the combination of
> all property conditions in the rule.
>
> Along with the syscall audit event, user can know why a blocked
> happened. For example:
>
> audit: AUDIT1420 path="/mnt/ipe/bin/hello" dev="dm-0"
> ino=2 rule="DEFAULT action=DENY"
> audit[1956]: SYSCALL arch=c000003e syscall=59
> success=no exit=-13 a0=556790138df0 a1=556790135390 a2=5567901338b0
> a3=ab2a41a67f4f1f4e items=1 ppid=147 pid=1956 auid=4294967295 uid=0
> gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts0
> ses=4294967295 comm="bash" exe="/usr/bin/bash" key=(null)
>
> The above two records showed bash used execve to run "hello" and got
> blocked by IPE. Note that the IPE records are always prior to a SYSCALL
> record.
>
> AUDIT_IPE_CONFIG_CHANGE(1421):
>
> audit: AUDIT1421
> old_active_pol_name="Allow_All" old_active_pol_version=0.0.0
> old_policy_digest=sha256:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649B934CA495991B7852B855
> new_active_pol_name="boot_verified" new_active_pol_version=0.0.0
> new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB
> auid=4294967295 ses=4294967295 lsm=ipe res=1

You can trim hash digest strings so they better fit in terminals, for
example:

old_policy_digest=sha256:E3B0C44....

> The above record showed the current IPE active policy switch from
> `Allow_All` to `boot_verified` along with the version and the hash
> digest of the two policies. Note IPE can only have one policy active
> at a time, all access decision evaluation is based on the current active
> policy.
> The normal procedure to deploy a policy is loading the policy to deploy
> into the kernel first, then switch the active policy to it.
>
> AUDIT_IPE_POLICY_LOAD(1422):
>
> audit: AUDIT1422 policy_name="boot_verified" policy_version=0.0.0
> policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD8EED7B8F4DB
> auid=4294967295 ses=4294967295 lsm=ipe res=1
>
> The above record showed a new policy has been loaded into the kernel
> with the policy name, policy version and policy hash.
>
> Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
> Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
> ---
> include/uapi/linux/audit.h | 3 +
> security/ipe/Kconfig | 2 +-
> security/ipe/Makefile | 1 +
> security/ipe/audit.c | 197 +++++++++++++++++++++++++++++++++++++
> security/ipe/audit.h | 18 ++++
> security/ipe/eval.c | 26 ++++-
> security/ipe/eval.h | 8 ++
> security/ipe/fs.c | 71 +++++++++++++
> security/ipe/policy.c | 5 +
> 9 files changed, 327 insertions(+), 4 deletions(-)
> create mode 100644 security/ipe/audit.c
> create mode 100644 security/ipe/audit.h

...

> diff --git a/security/ipe/audit.c b/security/ipe/audit.c
> new file mode 100644
> index 000000000000..d3f78a7fc93f
> --- /dev/null
> +++ b/security/ipe/audit.c
> @@ -0,0 +1,197 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) Microsoft Corporation. All rights reserved.
> + */
> +
> +#include <linux/slab.h>
> +#include <linux/audit.h>
> +#include <linux/types.h>
> +#include <crypto/hash.h>
> +
> +#include "ipe.h"
> +#include "eval.h"
> +#include "hooks.h"
> +#include "policy.h"
> +#include "audit.h"
> +
> +#define ACTSTR(x) ((x) == __IPE_ACTION_ALLOW ? "ALLOW" : "DENY")
> +
> +#define IPE_AUDIT_HASH_ALG "sha256"
> +
> +#define AUDIT_POLICY_LOAD_FMT "policy_name=\"%s\" policy_version=%hu.%hu.%hu "\
> + "policy_digest=" IPE_AUDIT_HASH_ALG ":"
> +#define AUDIT_OLD_ACTIVE_POLICY_FMT "old_active_pol_name=\"%s\" "\
> + "old_active_pol_version=%hu.%hu.%hu "\
> + "old_policy_digest=" IPE_AUDIT_HASH_ALG ":"
> +#define AUDIT_NEW_ACTIVE_POLICY_FMT "new_active_pol_name=\"%s\" "\
> + "new_active_pol_version=%hu.%hu.%hu "\
> + "new_policy_digest=" IPE_AUDIT_HASH_ALG ":"
> +
> +static const char *const audit_op_names[__IPE_OP_MAX] = {
> + "EXECUTE",
> + "FIRMWARE",
> + "KMODULE",
> + "KEXEC_IMAGE",
> + "KEXEC_INITRAMFS",
> + "IMA_POLICY",
> + "IMA_X509_CERT",
> +};
> +
> +static const char *const audit_prop_names[__IPE_PROP_MAX] = {
> + "boot_verified=FALSE",
> + "boot_verified=TRUE",
> +};
> +
> +/**
> + * audit_rule - audit an IPE policy rule approximation.
> + * @ab: Supplies a pointer to the audit_buffer to append to.
> + * @r: Supplies a pointer to the ipe_rule to approximate a string form for.
> + */
> +static void audit_rule(struct audit_buffer *ab, const struct ipe_rule *r)
> +{
> + const struct ipe_prop *ptr;
> +
> + audit_log_format(ab, "rule=\"op=%s ", audit_op_names[r->op]);
> +
> + list_for_each_entry(ptr, &r->props, next) {
> + audit_log_format(ab, "%s", audit_prop_names[ptr->type]);
> + audit_log_format(ab, " ");

Why make two calls to audit_log_format()? You can just use "%s " as
the format string.

> + }
> +
> + audit_log_format(ab, "action=%s\"", ACTSTR(r->action));
> +}
> +
> +/**
> + * ipe_audit_match - audit a match for IPE policy.
> + * @ctx: Supplies a pointer to the evaluation context that was used in the
> + * evaluation.
> + * @match_type: Supplies the scope of the match: rule, operation default,
> + * global default.
> + * @act: Supplies the IPE's evaluation decision, deny or allow.
> + * @r: Supplies a pointer to the rule that was matched, if possible.
> + * @enforce: Supplies the enforcement/permissive state at the point
> + * the enforcement decision was made.
> + */
> +void ipe_audit_match(const struct ipe_eval_ctx *const ctx,
> + enum ipe_match match_type,
> + enum ipe_action_type act, const struct ipe_rule *const r)
> +{
> + struct inode *inode;
> + struct audit_buffer *ab;
> + const char *op = audit_op_names[ctx->op];
> +
> + if (act != __IPE_ACTION_DENY && !READ_ONCE(success_audit))
> + return;
> +
> + ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_IPE_ACCESS);
> + if (!ab)
> + return;
> +
> + if (ctx->file) {
> + audit_log_d_path(ab, "path=", &ctx->file->f_path);
> + inode = file_inode(ctx->file);
> + if (inode) {
> + audit_log_format(ab, " dev=");
> + audit_log_untrustedstring(ab, inode->i_sb->s_id);

See my comments above about using the 'dev' field name, however, you
shouldn't need to log the device name as an untrusted string as the
string is coming from a trusted source within the kernel (the driver).

> + audit_log_format(ab, " ino=%lu ", inode->i_ino);
> + }
> + }
> +
> + if (match_type == __IPE_MATCH_RULE)
> + audit_rule(ab, r);
> + else if (match_type == __IPE_MATCH_TABLE)
> + audit_log_format(ab, "rule=\"DEFAULT op=%s action=%s\"", op,
> + ACTSTR(act));
> + else
> + audit_log_format(ab, "rule=\"DEFAULT action=%s\"",
> + ACTSTR(act));
> +
> + audit_log_end(ab);
> +}

...

> diff --git a/security/ipe/eval.h b/security/ipe/eval.h
> index 88c625d6af97..8fcdb2ea19f4 100644
> --- a/security/ipe/eval.h
> +++ b/security/ipe/eval.h
> @@ -13,6 +13,7 @@
> #include "policy.h"
>
> extern struct ipe_policy __rcu *ipe_active_policy;
> +extern bool success_audit;
>
> struct ipe_eval_ctx {
> enum ipe_op_type op;
> @@ -21,6 +22,13 @@ struct ipe_eval_ctx {
> bool from_init_sb;
> };
>
> +enum ipe_match {
> + __IPE_MATCH_RULE = 0,
> + __IPE_MATCH_TABLE,
> + __IPE_MATCH_GLOBAL,
> + __IPE_MATCH_MAX
> +};
> +
> 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);
> diff --git a/security/ipe/fs.c b/security/ipe/fs.c
> index 522162af741a..6bd2aa84831b 100644
> --- a/security/ipe/fs.c
> +++ b/security/ipe/fs.c
> @@ -8,11 +8,65 @@
>
> #include "ipe.h"
> #include "fs.h"
> +#include "eval.h"
> #include "policy.h"
> +#include "audit.h"
>
> static struct dentry *np __ro_after_init;
> static struct dentry *root __ro_after_init;
> struct dentry *policy_root __ro_after_init;
> +static struct dentry *audit_node __ro_after_init;
> +
> +/**
> + * setaudit - Write handler for the securityfs node, "ipe/success_audit"
> + * @f: Supplies a file structure representing the securityfs node.
> + * @data: Supplies a buffer passed to the write syscall.
> + * @len: Supplies the length of @data.
> + * @offset: unused.
> + *
> + * Return:
> + * * >0 - Success, Length of buffer written
> + * * <0 - Error
> + */
> +static ssize_t setaudit(struct file *f, const char __user *data,
> + size_t len, loff_t *offset)
> +{
> + int rc = 0;
> + bool value;
> +
> + if (!file_ns_capable(f, &init_user_ns, CAP_MAC_ADMIN))
> + return -EPERM;
> +
> + value = READ_ONCE(success_audit);

I don't understand why you are setting @value here only to change its
value in the line below ... ?

> + rc = kstrtobool_from_user(data, len, &value);
> + if (rc)
> + return rc;
> +
> + WRITE_ONCE(success_audit, value);
> +
> + return len;
> +}

--
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