Re: introduce get_nth_filter()

Oleg Nesterov
2017-09-20T16:14:29Z

On 09/20, Oleg Nesterov wrote:
>
> On 09/20, Tycho Andersen wrote:
> >
> > Thanks for cleaning this up, I'll be happy to test whatever final
> > patch we come up with.
>
> Well, I just noticed you sent another "[PATCH] ptrace, seccomp: add support
> for retrieving seccomp flags" today...
>
> So if we need get_nth() helper please consider the UNTESTED change below
> (on top of this fix). If you agree with this code, feel free to incorporate
> it into your patch.

and probably we should shift the CAP_SYS_ADMIN/SECCOMP_MODE_DISABLED into
get_nth() too, see v2 below.

Perhaps it makes sense to add a comment to explain that spin_lock_irq(siglock)
is only correct because the caller is the tracer, and thus the TASK_TRACED
"task" can't exit. Otherwise we would need lock_task_sighand().

Oleg.

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -855,48 +855,53 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
}

#if defined(CONFIG_SECCOMP_FILTER) && defined(CONFIG_CHECKPOINT_RESTORE)
-long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
- void __user *data)
+static struct seccomp_filter *
+get_nth_filter(struct task_struct *task, unsigned long filter_off)
{
- struct seccomp_filter *filter;
- struct sock_fprog_kern *fprog;
- long ret;
- unsigned long count = 0;
+ struct seccomp_filter *orig, *filter;
+ unsigned long count;

if (!capable(CAP_SYS_ADMIN) ||
current->seccomp.mode != SECCOMP_MODE_DISABLED) {
- return -EACCES;
+ return ERR_PTR(-EACCES);
}

+ if (task->seccomp.mode != SECCOMP_MODE_FILTER)
+ return ERR_PTR(-EINVAL);
+
spin_lock_irq(&task->sighand->siglock);
- if (task->seccomp.mode != SECCOMP_MODE_FILTER) {
- ret = -EINVAL;
- goto out;
- }
+ get_seccomp_filter(task);
+ orig = task->seccomp.filter;
+ spin_unlock_irq(&task->sighand->siglock);

- filter = task->seccomp.filter;
- while (filter) {
- filter = filter->prev;
+ count = 0;
+ for (filter = orig; filter; filter = filter->prev)
count++;
- }

- if (filter_off >= count) {
- ret = -ENOENT;
+ filter = ERR_PTR(-ENOENT);
+ if (filter_off >= count)
goto out;
- }
- count -= filter_off;

- filter = task->seccomp.filter;
- while (filter && count > 1) {
- filter = filter->prev;
+ count -= filter_off;
+ for (filter = orig; count > 1; filter = filter->prev)
count--;
- }

- if (WARN_ON(count != 1 || !filter)) {
- /* The filter tree shouldn't shrink while we're using it. */
- ret = -ENOENT;
- goto out;
- }
+ refcount_inc(&filter->usage);
+out:
+ __put_seccomp_filter(orig);
+ return filter;
+}
+
+long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
+ void __user *data)
+{
+ struct seccomp_filter *filter;
+ struct sock_fprog_kern *fprog;
+ long ret;
+
+ filter = get_nth_filter(task, filter_off);
+ if (IS_ERR(filter))
+ return PTR_ERR(filter);

fprog = filter->prog->orig_prog;
if (!fprog) {
@@ -912,17 +917,10 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
if (!data)
goto out;

- refcount_inc(&filter->usage);
- spin_unlock_irq(&task->sighand->siglock);
-
if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
ret = -EFAULT;
-
- __put_seccomp_filter(filter);
- return ret;
-
out:
- spin_unlock_irq(&task->sighand->siglock);
+ __put_seccomp_filter(filter);
return ret;
}
#endif