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Message-ID: <CAGXu5j+B=kx6847EUn6tppNwNk=YHbBwekqr6V22+gh2Lfea0g@mail.gmail.com>
Date: Mon, 19 Nov 2012 22:14:08 -0800
From: Kees Cook <keescook@...omium.org>
To: Serge Hallyn <serge.hallyn@...onical.com>
Cc: linux-kernel@...r.kernel.org,
James Morris <james.l.morris@...cle.com>,
John Johansen <john.johansen@...onical.com>,
"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
Eric Paris <eparis@...hat.com>,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH] Yama: remove locking from delete path
On Mon, Nov 19, 2012 at 7:45 PM, Serge Hallyn
<serge.hallyn@...onical.com> wrote:
> Quoting Kees Cook (keescook@...omium.org):
>> 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 <sasha.levin@...cle.com>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>> 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.
Unless I'm mistaken and my lockdep tests are wrong,
list_for_each_entry_rcu runs under rcu_read_lock().
I could optimize it to only run schedule_work() once all the marking
is done at the end of the loop.
-Kees
>
>> 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
--
Kees Cook
Chrome OS Security
--
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