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([2001:df0:0:200c:a550:e6ef:8525:d5e0]) by smtp.gmail.com with ESMTPSA id s11sm3369427pjz.42.2021.06.15.13.56.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Jun 2021 13:56:44 -0700 (PDT) Subject: Re: Kernel stack read with PTRACE_EVENT_EXIT and io_uring threads To: "Eric W. Biederman" Cc: Linus Torvalds , linux-arch , Jens Axboe , Oleg Nesterov , Al Viro , Linux Kernel Mailing List , Richard Henderson , Ivan Kokshaysky , Matt Turner , alpha , Geert Uytterhoeven , linux-m68k , Arnd Bergmann , Ley Foon Tan , Tejun Heo , Kees Cook , John Paul Adrian Glaubitz References: <87sg1p30a1.fsf@disp2133> <87pmwsytb3.fsf@disp2133> <87sg1lwhvm.fsf@disp2133> <6e47eff8-d0a4-8390-1222-e975bfbf3a65@gmail.com> <924ec53c-2fd9-2e1c-bbb1-3fda49809be4@gmail.com> <87eed4v2dc.fsf@disp2133> <5929e116-fa61-b211-342a-c706dcb834ca@gmail.com> <87fsxjorgs.fsf@disp2133> From: Michael Schmitz Message-ID: <7277aa4b-5d3f-6bb3-379a-470df4addd15@gmail.com> Date: Wed, 16 Jun 2021 08:56:35 +1200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <87fsxjorgs.fsf@disp2133> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Eric, On 16/06/21 7:30 am, Eric W. Biederman wrote: > >>> The io_uring tasks are special in that they are user process >>> threads that never run in userspace. So as long as everything >>> ptrace can read is accessible on that process all is well. >> OK, I'm testing a patch that would save extra context in sys_io_uring_setup, >> which ought to ensure that for m68k. > I had to update ret_from_kernel_thread to pop that state to get Linus's > change to boot. Apparently kernel_threads exiting needs to be handled. Hadn't yet got to that stage, sorry. Still stress testing stage 1 of my fix (push complete context). I would have thought that this should be sufficient (gives us a complete stack frame for ptrace code to work on)? But it makes sense that when you push an extra stack frame, you'd need to pop that on exit. > >>> Having stared a bit longer at the code I think the short term >>> fix for both of PTRACE_EVENT_EXIT and io_uring is to guard >>> them both with CONFIG_HAVE_ARCH_TRACEHOOK. > Which does not work because nios2 which looks susceptible > sets CONFIG_HAVE_ARCH_TRACEHOOK. > > A further look shows that there is also PTRACE_EVENT_EXEC that > needs to be handled so execve and execveat need to be wrapped > as well. > > Do you happen to know if there is userspace that will run > in qemu-system-m68k that can be used for testing? I surmise so. I don't use qemu myself - either ARAnyM, or actual hardware. Hardware is limited to 14 MB RAM, which has prevented me from using more than simple regression testing. In particular, I can't test sys_io_uring_setup there. Adrian uses qemu a lot, and has supplied disk images to work from on occasion. Maybe he's got something recent enough to support sys_io_uring_setup ... I've CC:ed him in, as I'd love to do some more testing as well. Cheers,     Michael > > Eric >