Re: [PATCH] Yama: remove locking from delete path

[Kees Cook]

On Mon, Nov 19, 2012 at 7:45 PM, Serge Hallyn
<serge.hallyn@xxxxxxxxxxxxx> wrote:
> Quoting Kees Cook (keescook@xxxxxxxxxxxx):
>> 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@xxxxxxxxxx>
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>>  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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