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SubjectRe: [PATCH 2/2] Yama: add PTRACE exception tracking
Quoting Kees Cook (kees.cook@canonical.com):
> Some application suites have external crash handlers that depend on
> being able to use PTRACE to generate crash reports (KDE, Chromium, etc).
> Since the inferior process generally knows the PID of the debugger,
> it can use PR_SET_PTRACER to allow a specific PID and its descendants
> to perform the PTRACE instead of only a direct ancestor.
>
> Signed-off-by: Kees Cook <kees.cook@canonical.com>
> ---

Hi Kees - very nice, overall. One little note though:

> @@ -32,27 +204,20 @@ static int yama_ptrace_access_check(struct task_struct *child,
> {
> int rc;
>
> + /* If standard caps disallows it, so does Yama. We should
> + * should only tighten restrictions further.
> + */
> rc = cap_ptrace_access_check(child, mode);

This means that if capable(CAP_SYS_PTRACE) we'll always shortcut
here, so

> - if (rc != 0)
> + if (rc)
> return rc;
>
> /* require ptrace target be a child of ptracer on attach */
> - if (mode == PTRACE_MODE_ATTACH && ptrace_scope &&
> - !capable(CAP_SYS_PTRACE)) {
> - struct task_struct *walker = child;
> -
> - rcu_read_lock();
> - read_lock(&tasklist_lock);
> - while (walker->pid > 0) {
> - if (walker == current)
> - break;
> - walker = walker->real_parent;
> - }
> - if (walker->pid == 0)
> - rc = -EPERM;
> - read_unlock(&tasklist_lock);
> - rcu_read_unlock();
> - }
> + if (mode == PTRACE_MODE_ATTACH &&
> + ptrace_scope &&
> + !capable(CAP_SYS_PTRACE) &&

You don't need the CAP_SYS_PTRACE check here AFAICS.

> + !task_is_descendant(current, child) &&
> + !ptracer_exception_found(current, child))
> + rc = -EPERM;
>
> if (rc) {
> char name[sizeof(current->comm)];
> @@ -170,6 +335,8 @@ static struct security_operations yama_ops = {
> .ptrace_access_check = yama_ptrace_access_check,
> .inode_follow_link = yama_inode_follow_link,
> .path_link = yama_path_link,
> + .task_prctl = yama_task_prctl,
> + .task_free = yama_task_free,
> };
>
> #ifdef CONFIG_SYSCTL
> @@ -221,6 +388,8 @@ static __init int yama_init(void)
>
> printk(KERN_INFO "Yama: becoming mindful.\n");
>
> + spin_lock_init(&ptracer_relations_lock);
> +
> if (register_security(&yama_ops))
> panic("Yama: kernel registration failed.\n");
>
> --
> 1.7.1
>
>
> --
> Kees Cook
> Ubuntu Security Team
> --
> To unsubscribe from this list: send the line "unsubscribe linux-security-module" in
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