Quoting Kees Cook ([email protected]):
> Instead of locking the list during a delete, mark entries as invalid
> and trigger a workqueue to clean them up. This lets us easily handle
> task_free from interrupt context.
>
> Cc: Sasha Levin <[email protected]>
> Signed-off-by: Kees Cook <[email protected]>
> ---
> security/yama/yama_lsm.c | 43 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 36 insertions(+), 7 deletions(-)
>
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index 17da6ca..1cba901 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -17,6 +17,7 @@
> #include <linux/ptrace.h>
> #include <linux/prctl.h>
> #include <linux/ratelimit.h>
> +#include <linux/workqueue.h>
>
> #define YAMA_SCOPE_DISABLED 0
> #define YAMA_SCOPE_RELATIONAL 1
> @@ -29,6 +30,7 @@ static int ptrace_scope = YAMA_SCOPE_RELATIONAL;
> struct ptrace_relation {
> struct task_struct *tracer;
> struct task_struct *tracee;
> + bool invalid;
> struct list_head node;
> struct rcu_head rcu;
> };
> @@ -36,6 +38,27 @@ struct ptrace_relation {
> static LIST_HEAD(ptracer_relations);
> static DEFINE_SPINLOCK(ptracer_relations_lock);
>
> +static void yama_relation_cleanup(struct work_struct *work);
> +static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);
> +
> +/**
> + * yama_relation_cleanup - remove invalid entries from the relation list
> + *
> + */
> +static void yama_relation_cleanup(struct work_struct *work)
> +{
> + struct ptrace_relation *relation;
> +
> + spin_lock(&ptracer_relations_lock);
> + list_for_each_entry_rcu(relation, &ptracer_relations, node) {
> + if (relation->invalid) {
> + list_del_rcu(&relation->node);
> + kfree_rcu(relation, rcu);
> + }
> + }
> + spin_unlock(&ptracer_relations_lock);
> +}
> +
> /**
> * yama_ptracer_add - add/replace an exception for this tracer/tracee pair
> * @tracer: the task_struct of the process doing the ptrace
> @@ -57,9 +80,12 @@ static int yama_ptracer_add(struct task_struct *tracer,
>
> added->tracee = tracee;
> added->tracer = tracer;
> + added->invalid = false;
>
> - spin_lock_bh(&ptracer_relations_lock);
> + spin_lock(&ptracer_relations_lock);
> list_for_each_entry_rcu(relation, &ptracer_relations, node) {
> + if (relation->invalid)
> + continue;
> if (relation->tracee == tracee) {
> list_replace_rcu(&relation->node, &added->node);
> kfree_rcu(relation, rcu);
> @@ -70,7 +96,7 @@ static int yama_ptracer_add(struct task_struct *tracer,
> list_add_rcu(&added->node, &ptracer_relations);
>
> out:
> - spin_unlock_bh(&ptracer_relations_lock);
> + spin_unlock(&ptracer_relations_lock);
> return 0;
> }
>
> @@ -84,15 +110,15 @@ static void yama_ptracer_del(struct task_struct *tracer,
> {
> struct ptrace_relation *relation;
>
> - spin_lock_bh(&ptracer_relations_lock);
I don't understand - is there a patch I don't have sitting around
which puts the calls to yama_ptracer_del() under rcu_read_lock()?
If not, I don't see how it's safe to walk the list here and risk
racing against another yama_relation_cleanup() run.
I'm probably missing something really cool about the locking,
but it doesn't look right to me. I would think you'd want to
do the loop under rcu_read_lock(), set a boolean if one is
changed, and call schedule_work() once at the end if the boolean
is set.
> list_for_each_entry_rcu(relation, &ptracer_relations, node) {
> + if (relation->invalid)
> + continue;
> if (relation->tracee == tracee ||
> (tracer && relation->tracer == tracer)) {
> - list_del_rcu(&relation->node);
> - kfree_rcu(relation, rcu);
> + relation->invalid = true;
> + schedule_work(&yama_relation_work);
> }
> }
> - spin_unlock_bh(&ptracer_relations_lock);
> }
>
> /**
> @@ -219,12 +245,15 @@ static int ptracer_exception_found(struct task_struct
> *tracer,
> rcu_read_lock();
> if (!thread_group_leader(tracee))
> tracee = rcu_dereference(tracee->group_leader);
> - list_for_each_entry_rcu(relation, &ptracer_relations, node)
> + list_for_each_entry_rcu(relation, &ptracer_relations, node) {
> + if (relation->invalid)
> + continue;
> if (relation->tracee == tracee) {
> parent = relation->tracer;
> found = true;
> break;
> }
> + }
>
> if (found && (parent == NULL || task_is_descendant(parent, tracer)))
> rc = 1;
> --
> 1.7.9.5
>
>
> --
> Kees Cook
> Chrome OS Security
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