Re: [PATCH ghak90 V9 06/13] audit: add contid support for signalling the audit daemon

From: Paul Moore
Date: Sun Jul 05 2020 - 11:11:03 EST

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On Sat, Jun 27, 2020 at 9:22 AM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
>
> Add audit container identifier support to the action of signalling the
> audit daemon.
>
> Since this would need to add an element to the audit_sig_info struct,
> a new record type AUDIT_SIGNAL_INFO2 was created with a new
> audit_sig_info2 struct. Corresponding support is required in the
> userspace code to reflect the new record request and reply type.
> An older userspace won't break since it won't know to request this
> record type.
>
> Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> ---
> include/linux/audit.h | 8 ++++
> include/uapi/linux/audit.h | 1 +
> kernel/audit.c | 95 ++++++++++++++++++++++++++++++++++++++++++++-
> security/selinux/nlmsgtab.c | 1 +
> 4 files changed, 104 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/audit.h b/include/linux/audit.h
> index 5eeba0efffc2..89cf7c66abe6 100644
> --- a/include/linux/audit.h
> +++ b/include/linux/audit.h
> @@ -22,6 +22,13 @@ struct audit_sig_info {
> char ctx[];
> };
>
> +struct audit_sig_info2 {
> + uid_t uid;
> + pid_t pid;
> + u32 cid_len;
> + char data[];
> +};
> +
> struct audit_buffer;
> struct audit_context;
> struct inode;
> @@ -105,6 +112,7 @@ struct audit_contobj {
> u64 id;
> struct task_struct *owner;
> refcount_t refcount;
> + refcount_t sigflag;
> struct rcu_head rcu;
> };

It seems like we need some protection in audit_set_contid() so that we
don't allow reuse of an audit container ID when "refcount == 0 &&
sigflag != 0", yes?

> diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> index fd98460c983f..a56ad77069b9 100644
> --- a/include/uapi/linux/audit.h
> +++ b/include/uapi/linux/audit.h
> @@ -72,6 +72,7 @@
> #define AUDIT_SET_FEATURE 1018 /* Turn an audit feature on or off */
> #define AUDIT_GET_FEATURE 1019 /* Get which features are enabled */
> #define AUDIT_CONTAINER_OP 1020 /* Define the container id and info */
> +#define AUDIT_SIGNAL_INFO2 1021 /* Get info auditd signal sender */
>
> #define AUDIT_FIRST_USER_MSG 1100 /* Userspace messages mostly uninteresting to kernel */
> #define AUDIT_USER_AVC 1107 /* We filter this differently */
> diff --git a/kernel/audit.c b/kernel/audit.c
> index a09f8f661234..54dd2cb69402 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -126,6 +126,8 @@ struct auditd_connection {
> kuid_t audit_sig_uid = INVALID_UID;
> pid_t audit_sig_pid = -1;
> u32 audit_sig_sid = 0;
> +static struct audit_contobj *audit_sig_cid;
> +static struct task_struct *audit_sig_atsk;

This looks like a typo, or did you mean "atsk" for some reason?

> /* Records can be lost in several ways:
> 0) [suppressed in audit_alloc]
> @@ -239,7 +241,33 @@ void _audit_contobj_put(struct audit_contobj *cont)
> {
> if (!cont)
> return;
> - if (refcount_dec_and_test(&cont->refcount)) {
> + if (refcount_dec_and_test(&cont->refcount) && !refcount_read(&cont->sigflag)) {
> + put_task_struct(cont->owner);
> + list_del_rcu(&cont->list);
> + kfree_rcu(cont, rcu);
> + }
> +}

It seems like it might be a good idea to modify the corresponding
_get() to WARN on the reuse of audit container objects where refcount
is zero, similar to the comment I made above. What do you think?

> +/* rcu_read_lock must be held by caller unless new */
> +static struct audit_contobj *_audit_contobj_get_sig(struct task_struct *tsk)
> +{
> + struct audit_contobj *cont;
> +
> + if (!tsk->audit)
> + return NULL;
> + cont = tsk->audit->cont;
> + if (cont)
> + refcount_set(&cont->sigflag, 1);
> + return cont;
> +}

If you are going to use a refcount and call this a "get" function you
might as well make it do an increment and not just a set(1). It a bit
silly with just one auditd per system, but I suppose it will make more
sense when we have multiple audit daemons. In a related comment, you
probably want to rename "sigflag" to "sigcount" or similar.

In summary, it's either a reference that supports multiple gets/puts
or it's a flag with just an on/off; it shouldn't attempt to straddle
both, that's both confusing and fragile.

> +/* rcu_read_lock must be held by caller */
> +static void _audit_contobj_put_sig(struct audit_contobj *cont)
> +{
> + if (!cont)
> + return;
> + refcount_set(&cont->sigflag, 0);
> + if (!refcount_read(&cont->refcount)) {
> put_task_struct(cont->owner);
> list_del_rcu(&cont->list);
> kfree_rcu(cont, rcu);
> @@ -309,6 +337,13 @@ void audit_free(struct task_struct *tsk)
> info = tsk->audit;
> tsk->audit = NULL;
> kmem_cache_free(audit_task_cache, info);
> + rcu_read_lock();
> + if (audit_sig_atsk == tsk) {
> + _audit_contobj_put_sig(audit_sig_cid);
> + audit_sig_cid = NULL;
> + audit_sig_atsk = NULL;
> + }
> + rcu_read_unlock();
> }
>
> /**
> @@ -1132,6 +1167,7 @@ static int audit_netlink_ok(struct sk_buff *skb, u16 msg_type)
> case AUDIT_ADD_RULE:
> case AUDIT_DEL_RULE:
> case AUDIT_SIGNAL_INFO:
> + case AUDIT_SIGNAL_INFO2:
> case AUDIT_TTY_GET:
> case AUDIT_TTY_SET:
> case AUDIT_TRIM:
> @@ -1294,6 +1330,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
> struct audit_buffer *ab;
> u16 msg_type = nlh->nlmsg_type;
> struct audit_sig_info *sig_data;
> + struct audit_sig_info2 *sig_data2;
> char *ctx = NULL;
> u32 len;
>
> @@ -1559,6 +1596,52 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
> sig_data, sizeof(*sig_data) + len);
> kfree(sig_data);
> break;
> + case AUDIT_SIGNAL_INFO2: {
> + unsigned int contidstrlen = 0;
> +
> + len = 0;
> + if (audit_sig_sid) {
> + err = security_secid_to_secctx(audit_sig_sid, &ctx,
> + &len);
> + if (err)
> + return err;
> + }
> + if (audit_sig_cid) {
> + contidstr = kmalloc(21, GFP_KERNEL);
> + if (!contidstr) {
> + if (audit_sig_sid)
> + security_release_secctx(ctx, len);
> + return -ENOMEM;
> + }
> + contidstrlen = scnprintf(contidstr, 20, "%llu", audit_sig_cid->id);
> + }
> + sig_data2 = kmalloc(sizeof(*sig_data2) + contidstrlen + len, GFP_KERNEL);
> + if (!sig_data2) {
> + if (audit_sig_sid)
> + security_release_secctx(ctx, len);
> + kfree(contidstr);
> + return -ENOMEM;
> + }
> + sig_data2->uid = from_kuid(&init_user_ns, audit_sig_uid);
> + sig_data2->pid = audit_sig_pid;
> + if (audit_sig_cid) {
> + memcpy(sig_data2->data, contidstr, contidstrlen);
> + sig_data2->cid_len = contidstrlen;
> + kfree(contidstr);
> + }
> + if (audit_sig_sid) {
> + memcpy(sig_data2->data + contidstrlen, ctx, len);
> + security_release_secctx(ctx, len);
> + }
> + rcu_read_lock();
> + _audit_contobj_put_sig(audit_sig_cid);
> + rcu_read_unlock();

We probably want to drop the reference in the legacy/AUDIT_SIGNAL_INFO
case too, right?

> + audit_sig_cid = NULL;
> + audit_send_reply(skb, seq, AUDIT_SIGNAL_INFO2, 0, 0,
> + sig_data2, sizeof(*sig_data2) + contidstrlen + len);
> + kfree(sig_data2);
> + break;
> + }
> case AUDIT_TTY_GET: {
> struct audit_tty_status s;
> unsigned int t;
> @@ -2470,6 +2553,11 @@ int audit_signal_info(int sig, struct task_struct *t)
> else
> audit_sig_uid = uid;
> security_task_getsecid(current, &audit_sig_sid);
> + rcu_read_lock();
> + _audit_contobj_put_sig(audit_sig_cid);
> + audit_sig_cid = _audit_contobj_get_sig(current);
> + rcu_read_unlock();
> + audit_sig_atsk = t;
> }
>
> return audit_signal_info_syscall(t);
> @@ -2532,6 +2620,11 @@ int audit_set_contid(struct task_struct *task, u64 contid)
> if (cont->id == contid) {
> /* task injection to existing container */
> if (current == cont->owner) {
> + if (!refcount_read(&cont->refcount)) {
> + rc = -ESHUTDOWN;

Reuse -ENOTUNIQ; I'm not overly excited about providing a lot of
detail here as these are global system objects. If you must have a
different errno (and I would prefer you didn't), use something like
-EBUSY.

> + spin_unlock(&audit_contobj_list_lock);
> + goto conterror;
> + }
> _audit_contobj_hold(cont);
> newcont = cont;
> } else {
> diff --git a/security/selinux/nlmsgtab.c b/security/selinux/nlmsgtab.c
> index b69231918686..8303bb7a63d0 100644
> --- a/security/selinux/nlmsgtab.c
> +++ b/security/selinux/nlmsgtab.c
> @@ -137,6 +137,7 @@ struct nlmsg_perm {
> { AUDIT_DEL_RULE, NETLINK_AUDIT_SOCKET__NLMSG_WRITE },
> { AUDIT_USER, NETLINK_AUDIT_SOCKET__NLMSG_RELAY },
> { AUDIT_SIGNAL_INFO, NETLINK_AUDIT_SOCKET__NLMSG_READ },
> + { AUDIT_SIGNAL_INFO2, NETLINK_AUDIT_SOCKET__NLMSG_READ },
> { AUDIT_TRIM, NETLINK_AUDIT_SOCKET__NLMSG_WRITE },
> { AUDIT_MAKE_EQUIV, NETLINK_AUDIT_SOCKET__NLMSG_WRITE },
> { AUDIT_TTY_GET, NETLINK_AUDIT_SOCKET__NLMSG_READ },

--
paul moore
www.paul-moore.com

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