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Tue, 5 May 2020 13:31:13 +0000 (UTC) Date: Tue, 5 May 2020 14:31:11 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Subject: Re: [PATCH 0/4] Add support for SafeStack Message-ID: <20200505133111.GM764268@redhat.com> References: <20200429194420.21147-1-dbuono@linux.vnet.ibm.com> <51f28cf2-0f34-508f-96f8-02c02b3c8a85@redhat.com> MIME-Version: 1.0 In-Reply-To: <51f28cf2-0f34-508f-96f8-02c02b3c8a85@redhat.com> User-Agent: Mutt/1.13.4 (2020-02-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/05 00:37:19 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -10 X-Spam_score: -1.1 X-Spam_bar: - X-Spam_report: (-1.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FROM_EXCESS_BASE64=0.979, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Cc: Kevin Wolf , Tobin Feldman-Fitzthum , qemu-devel@nongnu.org, Stefan Hajnoczi , Paolo Bonzini , Alex =?utf-8?Q?Benn=C3=A9e?= , Daniele Buono Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, May 05, 2020 at 03:15:18PM +0200, Philippe Mathieu-Daud=C3=A9 wrote= : > +Alex & Daniel who keep track on CI stuff. >=20 > On 4/29/20 9:44 PM, Daniele Buono wrote: > > LLVM supports SafeStack instrumentation to protect against stack buffer > > overflows, since version 3.7 > >=20 > > From https://clang.llvm.org/docs/SafeStack.html: > > "It works by separating the program stack into two distinct regions: th= e > > safe stack and the unsafe stack. The safe stack stores return addresses= , > > register spills, and local variables that are always accessed in a safe > > way, while the unsafe stack stores everything else. This separation > > ensures that buffer overflows on the unsafe stack cannot be used to > > overwrite anything on the safe stack." > >=20 > > Unfortunately, the use of two stack regions does not cope well with > > QEMU's coroutines. The second stack region is not properly set up with > > both ucontext and sigaltstack, so multiple coroutines end up sharing th= e > > same memory area for the unsafe stack, causing undefined behaviors at > > runtime (and most iochecks to fail). > >=20 > > This patch series fixes the implementation of the ucontext backend and > > make sure that sigaltstack is never used if the compiler is applying > > the SafeStack instrumentation. It also adds a configure flag to enable > > SafeStack, and enables iotests when SafeStack is used. > >=20 > > This is an RFC mainly because of the low-level use of the SafeStack > > runtime. > > When running swapcontext(), we have to manually set the unsafe stack > > pointer to the new area allocated for the coroutine. LLVM does not allo= w > > this by using builtin, so we have to use implementation details that ma= y > > change in the future. > > This patch has been tested briefly ( make check on an x86 system ) with > > clang v3.9, v4.0, v5.0, v6.0 > > Heavier testing, with make check-acceptance has been performed with > > clang v7.0 >=20 > I noticed building using SafeStack is slower, and running with it is even > sloooower. It makes sense to have this integrated if we use it regularly.= Do > you have plan for this? Using public CI doesn't seem reasonable. The runtime behaviour is rather odd, given the docs they provide: "The performance overhead of the SafeStack instrumentation is less than 0.1% on average across a variety of benchmarks=20 This is mainly because most small functions do not have any variables that require the unsafe stack and, hence, do not=20 need unsafe stack frames to be created. The cost of creating=20 unsafe stack frames for large functions is amortized by the=20 cost of executing the function. In some cases, SafeStack actually improves the performance" Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberrange= :| |: https://libvirt.org -o- https://fstop138.berrange.com= :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange= :|