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

[Paul Moore]

On Fri, Feb 7, 2020 at 1:08 PM Amol Grover <frextrite@xxxxxxxxx> 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@xxxxxxxxxx>
> Co-developed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Amol Grover <frextrite@xxxxxxxxx>
> ---
>  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?

Acked-by: Paul Moore <paul@xxxxxxxxxxxxxx>

> 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