From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads Date: Thu, 10 Jun 2021 15:57:58 -0500 Message-ID: <87sg1p30a1.fsf@disp2133> Mime-Version: 1.0 Return-path: List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-arch@vger.kernel.org Cc: Jens Axboe , Oleg Nesterov , Al Viro , Linus Torvalds , linux-kernel@vger.kernel.org, Richard Henderson , Ivan Kokshaysky , Matt Turner , linux-alpha@vger.kernel.org, Geert Uytterhoeven , linux-m68k@lists.linux-m68k.org, Arnd Bergmann , Ley Foon Tan , Tejun Heo , Daniel Jacobowitz , Kees Cook Folks, Digging through the guts of exit I found something I am not quite certain what to do with. On some architectures such as alpha, m68k, and nios2 the kernel calls into system calls with a subset of the registers saved on the kernel stack, and the kernel calls into signal handling and a few other contexts with all of the registers saved on the kernel stack. The problem is sometimes we read all of the registers from a context where they are not all saved. When this was initially observed it looked just like a coredump problem and it could be solved by tweaking the coredump code. That change was 77f6ab8b7768 ("don't dump the threads that had been already exiting when zapped.") However I have looked farther and we have the location where get_signal is called from io_uring, and we have the ptrace_stop in PTRACE_EVENT_EXIT. In PTRACE_EVENT_EXIT we could be called from exit(2) which is a syscall and we definitely won't have everything saved on the kernel stack. I have not doubled checked create_io_thread but I don't think create_io_threads saves all of the registers on the kernel stack. I think at this point we need to say that the architectures that have a do this need to be fixed to at least call do_exit and the kernel function in create_io_thread with the deeper stack. Is that reasonable of me to ask? Is there some other way to deal with this issue that I am not seeing? Am I missing some critical detail that makes PTRACE_EVENT_EXIT in do_exit not a problem if someone reads the register with ptrace? Eric