Hi Jann, On 12/16/20 12:07 AM, Jann Horn wrote: > Am Tue, Dec 15, 2020 at 06:01:25PM +0100 schrieb Alejandro Colomar (man-pages): >> Hi, >> >> There's a bug report: https://bugzilla.kernel.org/show_bug.cgi?id=210655 >> >> [[ >> Under "Ptrace access mode checking", the documentation states: >> "1. If the calling thread and the target thread are in the same thread >> group, access is always allowed." >> >> This is incorrect. A thread may never attach to another in the same group. > > No, that is correct. ptrace-mode access checks do always short-circuit for > tasks in the same thread group: > > /* Returns 0 on success, -errno on denial. */ > static int __ptrace_may_access(struct task_struct *task, unsigned int mode) > { > [...] > /* May we inspect the given task? > * This check is used both for attaching with ptrace > * and for allowing access to sensitive information in /proc. > * > * ptrace_attach denies several cases that /proc allows > * because setting up the necessary parent/child relationship > * or halting the specified task is impossible. > */ > > /* Don't let security modules deny introspection */ > if (same_thread_group(task, current)) > return 0; > [...] > } AFAICS, that code always returns non-zero, at least when called from ptrace_attach(). As you can see below, __ptrace_may_access() is called some lines after the code pointed to by the bug report. static int ptrace_attach(struct task_struct *task, long request, unsigned long addr, unsigned long flags) { [...] if (same_thread_group(task, current)) goto out; /* * Protect exec's credential calculations against our interference; * SUID, SGID and LSM creds get determined differently * under ptrace. */ retval = -ERESTARTNOINTR; if (mutex_lock_interruptible(&task->signal->cred_guard_mutex)) goto out; task_lock(task); retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS); [...] } Thanks, Alex > > As the comment explains, you can't actually *attach* > to another task in the same thread group; but that's > not because of the ptrace-style access check rules, > but because specifically *attaching* to another task > in the same thread group doesn't work. >
