[PATCH RFC v11 8/19] uapi|audit|ipe: add ipe auditing support
Paul Moore
paul at paul-moore.com
Tue Oct 24 03:52:27 UTC 2023
On Oct 4, 2023 Fan Wu <wufan at 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"
>
> 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:E3B0C44298FC1C149AFBF4C8996FB92427AE41E4649
> new_active_pol_name="boot_verified" new_active_pol_version=0.0.0
> new_policy_digest=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F
> auid=4294967295 ses=4294967295 lsm=ipe res=1
>
> 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=sha256:820EEA5B40CA42B51F68962354BA083122A20BB846F26765076DD
> 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 at linux.microsoft.com>
> Signed-off-by: Fan Wu <wufan at linux.microsoft.com>
> ---
>
> 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, the audit system, the evaluation loop,
> and access control hooks into separate patches.
> + Further split audit system patch into two separate patches; one
> for include/uapi, and the usage of the new defines.
> + Split out the permissive functionality into another separate patch,
> for easier review.
> + Correct misuse of audit_log_n_untrusted string to audit_log_format
> + Use get_task_comm instead of comm directly.
> + Quote certain audit values
> + Remove unnecessary help text on choice options - these were
> previously
> idented at the wrong level
> + Correct a stale string constant (ctx_ns_enforce to ctx_enforce)
>
> v8:
>
> + Change dependency for CONFIG_AUDIT to CONFIG_AUDITSYSCALL
> + Drop ctx_* prefix
> + Reuse, where appropriate, the audit fields from the field
> dictionary. This transforms:
> ctx_pathname -> path
> ctx_ino -> ino
> ctx_dev -> dev
>
> + Add audit records and event examples to commit description.
> + Remove new_audit_ctx, replace with audit_log_start. All data that
> would provided by new_audit_ctx is already present in the syscall
> audit record, that is always emitted on these actions. The audit
> records should be correlated as such.
> + Change audit types:
> + AUDIT_TRUST_RESULT -> AUDIT_IPE_ACCESS
> + This prevents overloading of the AVC type.
> + AUDIT_TRUST_POLICY_ACTIVATE -> AUDIT_MAC_CONFIG_CHANGE
> + AUDIT_TRUST_POLICY_LOAD -> AUDIT_MAC_POLICY_LOAD
> + There were no significant difference in meaning between
> these types.
>
> + Remove enforcing parameter passed from the context structure
> for AUDIT_IPE_ACCESS.
> + This field can be inferred from the SYSCALL audit event,
> based on the success field.
>
> + Remove all fields already captured in the syscall record. "hook",
> an IPE specific field, can be determined via the syscall field in
> the syscall record itself, so it has been removed.
> + ino, path, and dev in IPE's record refer to the subject of the
> syscall, while the syscall record refers to the calling process.
>
> + remove IPE prefix from policy load/policy activation events
> + fix a bug wherein a policy change audit record was not fired when
> updating a policy
>
> v9:
> + Merge the AUDIT_IPE_ACCESS definition with the audit support commit
> + Change the audit format of policy load and switch
> + Remove the ipe audit kernel switch
>
> v10:
> + Create AUDIT_IPE_CONFIG_CHANGE and AUDIT_IPE_POLICY_LOAD
> + Change field names per upstream feedback
>
> v11:
> + Fix style issues
> ---
> include/uapi/linux/audit.h | 3 +
> security/ipe/Kconfig | 2 +-
> security/ipe/Makefile | 1 +
> security/ipe/audit.c | 195 +++++++++++++++++++++++++++++++++++++
> security/ipe/audit.h | 18 ++++
> security/ipe/eval.c | 32 ++++--
> security/ipe/eval.h | 8 ++
> security/ipe/fs.c | 70 +++++++++++++
> security/ipe/policy.c | 5 +
> 9 files changed, 327 insertions(+), 7 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..e123701d5e3b
> --- /dev/null
> +++ b/security/ipe/audit.c
> @@ -0,0 +1,195 @@
> +// 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",
> +};
I would suggest taking the same approach for both @audit_op_names and
@audit_prop_names; either include the field name in the string array
for both or leave it out of both.
> +/**
> + * 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, "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.
> + */
Does it make sense to move @match_type into the ipe_eval_ctx struct?
> +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);
> + 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);
> +}
--
paul-moore.com
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