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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id c206sm480031pfc.220.2021.09.29.11.54.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Sep 2021 11:54:56 -0700 (PDT) Date: Wed, 29 Sep 2021 11:54:55 -0700 From: Kees Cook To: Josh Poimboeuf Cc: Mark Rutland , Vito Caputo , Jann Horn , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Jens Axboe , Peter Zijlstra , Stefan Metzmacher , Andy Lutomirski , Lai Jiangshan , Christian Brauner , Andrew Morton , "Kenta.Tada@sony.com" , Daniel Bristot de Oliveira , Michael =?iso-8859-1?Q?Wei=DF?= , Anand K Mistry , Alexey Gladkov , Michal Hocko , Helge Deller , Dave Hansen , Andrea Righi , Ohhoon Kwon , Kalesh Singh , YiFei Zhu , "Eric W. Biederman" , Qi Zheng , linux-kernel@vger.kernel.org, x86@kernel.org, linux-fsdevel@vger.kernel.org, linux-hardening@vger.kernel.org Subject: Re: [PATCH] proc: Disable /proc/$pid/wchan Message-ID: <202109291152.681444A135@keescook> References: <20210923233105.4045080-1-keescook@chromium.org> <20210923234917.pqrxwoq7yqnvfpwu@shells.gnugeneration.com> <20210924002230.sijoedia65hf5bj7@shells.gnugeneration.com> <202109231814.FD09DBAD3@keescook> <20210924135424.GA33573@C02TD0UTHF1T.local> <202109240716.A0792BE46@keescook> <20210927090337.GB1131@C02TD0UTHF1T.local> <202109271103.4E15FC0@keescook> <20210927205056.jjdlkof5w6fs5wzw@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210927205056.jjdlkof5w6fs5wzw@treble> Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On Mon, Sep 27, 2021 at 01:50:56PM -0700, Josh Poimboeuf wrote: > On Mon, Sep 27, 2021 at 11:07:27AM -0700, Kees Cook wrote: > > On Mon, Sep 27, 2021 at 10:03:51AM +0100, Mark Rutland wrote: > > > On Fri, Sep 24, 2021 at 07:26:22AM -0700, Kees Cook wrote: > > > > On Fri, Sep 24, 2021 at 02:54:24PM +0100, Mark Rutland wrote: > > > > > On Thu, Sep 23, 2021 at 06:16:16PM -0700, Kees Cook wrote: > > > > > > On Thu, Sep 23, 2021 at 05:22:30PM -0700, Vito Caputo wrote: > > > > > > > Instead of unwinding stacks maybe the kernel should be sticking an > > > > > > > entrypoint address in the current task struct for get_wchan() to > > > > > > > access, whenever userspace enters the kernel? > > > > > > > > > > > > wchan is supposed to show where the kernel is at the instant the > > > > > > get_wchan() happens. (i.e. recording it at syscall entry would just > > > > > > always show syscall entry.) > > > > > > > > > > It's supposed to show where a blocked task is blocked; the "wait > > > > > channel". > > > > > > > > > > I'd wanted to remove get_wchan since it requires cross-task stack > > > > > walking, which is generally painful. > > > > > > > > Right -- this is the "fragile" part I'm worried about. > > > > I'd like to clarify this concern first -- is the proposed fix actually > > fragile? Because I think we'd be better off just restoring behavior than > > trying to invent new behavior... > > > > i.e. Josh, Jann, do you see any issues with Qi Zheng's fix here: > > https://lore.kernel.org/all/20210924062006.231699-4-keescook@chromium.org/ > > Even with that patch, it doesn't lock the task's runqueue before reading > the stack, so there's still the possibility of the task running on > another CPU and the unwinder going off the rails a bit, which might be > used by an attacker in creative ways similar to the /proc//stack > vulnerability Jann mentioned earlier. Since I think we're considering get_wchan() to be slow-path, can we just lock the runqueue and use arch_stack_walk_reliable()? -- Kees Cook