linux-kernel - Re: [PATCH 3/3] auditsc: Do not use RCU primitive to read from cred pointer

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Date:   Thu, 13 Feb 2020 13:41:39 +0530
From:   Amol Grover <frextrite@...il.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        James Morris <jamorris@...ux.microsoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Eric Paris <eparis@...hat.com>,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, linux-audit@...hat.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH 3/3] auditsc: Do not use RCU primitive to read from cred
 pointer

On Tue, Feb 11, 2020 at 10:19:04AM -0500, Paul Moore wrote:
> On Fri, Feb 7, 2020 at 1:08 PM Amol Grover <frextrite@...il.com> wrote:
> >
> > task_struct::cred is only used task-synchronously and does
> > not require any RCU locks, hence, rcu_dereference_check is
> > not required to read from it.
> >
> > Suggested-by: Jann Horn <jannh@...gle.com>
> > Co-developed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Amol Grover <frextrite@...il.com>
> > ---
> >  kernel/auditsc.c | 15 +++++----------
> >  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> Considering the other changes in this patchset this change seems
> reasonable to me.  I'm assuming you were intending this patchset to go
> in via some tree other than audit?
> 

Yes, that's correct. Thank you for the ack!

> Acked-by: Paul Moore <paul@...l-moore.com>
> 
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index 4effe01ebbe2..d3510513cdd1 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -430,24 +430,19 @@ static int audit_field_compare(struct task_struct *tsk,
> >  /* Determine if any context name data matches a rule's watch data */
> >  /* Compare a task_struct with an audit_rule.  Return 1 on match, 0
> >   * otherwise.
> > - *
> > - * If task_creation is true, this is an explicit indication that we are
> > - * filtering a task rule at task creation time.  This and tsk == current are
> > - * the only situations where tsk->cred may be accessed without an rcu read lock.
> >   */
> >  static int audit_filter_rules(struct task_struct *tsk,
> >                               struct audit_krule *rule,
> >                               struct audit_context *ctx,
> >                               struct audit_names *name,
> > -                             enum audit_state *state,
> > -                             bool task_creation)
> > +                             enum audit_state *state)
> >  {
> >         const struct cred *cred;
> >         int i, need_sid = 1;
> >         u32 sid;
> >         unsigned int sessionid;
> >
> > -       cred = rcu_dereference_check(tsk->cred, tsk == current || task_creation);
> > +       cred = tsk->cred;
> >
> >         for (i = 0; i < rule->field_count; i++) {
> >                 struct audit_field *f = &rule->fields[i];
> > @@ -745,7 +740,7 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
> >         rcu_read_lock();
> >         list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_TASK], list) {
> >                 if (audit_filter_rules(tsk, &e->rule, NULL, NULL,
> > -                                      &state, true)) {
> > +                                      &state)) {
> >                         if (state == AUDIT_RECORD_CONTEXT)
> >                                 *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
> >                         rcu_read_unlock();
> > @@ -791,7 +786,7 @@ static enum audit_state audit_filter_syscall(struct task_struct *tsk,
> >         list_for_each_entry_rcu(e, list, list) {
> >                 if (audit_in_mask(&e->rule, ctx->major) &&
> >                     audit_filter_rules(tsk, &e->rule, ctx, NULL,
> > -                                      &state, false)) {
> > +                                      &state)) {
> >                         rcu_read_unlock();
> >                         ctx->current_state = state;
> >                         return state;
> > @@ -815,7 +810,7 @@ static int audit_filter_inode_name(struct task_struct *tsk,
> >
> >         list_for_each_entry_rcu(e, list, list) {
> >                 if (audit_in_mask(&e->rule, ctx->major) &&
> > -                   audit_filter_rules(tsk, &e->rule, ctx, n, &state, false)) {
> > +                   audit_filter_rules(tsk, &e->rule, ctx, n, &state)) {
> >                         ctx->current_state = state;
> >                         return 1;
> >                 }
> > --
> > 2.24.1
> >
> 
> 
> -- 
> paul moore
> www.paul-moore.com