On 12/16/20 12:23 AM, Alejandro Colomar (man-pages) wrote: > 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, Sorry, I should have said "that code never returns 0". > 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. >>
