Date: Wed, 17 Apr 2019 17:04:41 +0200 From: Mickaël Salaün <mickael.salaun@....gouv.fr> To: Florian Weimer <fweimer@...hat.com>, Steve Grubb <sgrubb@...hat.com> CC: Jan Kara <jack@...e.cz>, Mickaël Salaün <mic@...ikod.net>, <linux-kernel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>, James Morris <jmorris@...ei.org>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Michael Kerrisk <mtk.manpages@...il.com>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe Trébuchet <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>, <kernel-hardening@...ts.openwall.com>, <linux-api@...r.kernel.org>, <linux-security-module@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>, Matthew Bobrowski <mbobrowski@...browski.org> Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() On 17/04/2019 12:01, Florian Weimer wrote: > * Steve Grubb: > >> On Tuesday, April 16, 2019 7:49:39 AM EDT Florian Weimer wrote: >>> * Steve Grubb: >>>> This flag that is being proposed means that you would have to patch all >>>> interpreters to use it. If you are sure that upstreams will accept that, >>>> why not just change the policy to interpreters shouldn't execute >>>> anything unless the execute bit is set? That is simpler and doesn't need >>>> a kernel change. And setting the execute bit is an auditable event. >>> >>> I think we need something like O_MAYEXEC so that security policies can >>> be enforced and noexec mounts can be detected. >> >> Application whitelisting can already today stop unknown software without >> needing O_MAYEXEC. Whitelisting may be a lot of thing (path/TPE, signed binaries…), but being able to handle this with a global system configuration (instead of app-specific hardcoded configuration) is a good idea. ;) > > I'm somewhat interested in using this to add a proper check for > executability to explicit dynamic loader invocations. In other words, > this > > /lib64/ld-linux-x86-64.so.2 /path/to/noexec/fs/program > > should refuse to run the program if the program is located on a file > system mounted with the noexec attribute. What if a sysadmin need to do this on an executable mount point? Being able to enforce a security policy according to a configuration may fit to much more use cases. > >> The problem is that passing O_MAYEXEC is opt-in. You can use ptrace/seccomp/ >> bpf/LD_PRELOAD/LD_AUDIT to remove that bit from an otherwise normal program. >> This does not require privs to do so. > > That doesn't really help with the above. Right, ptrace/LD_PRELOAD and so on must be addressed by something else than only O_MAYEXEC. > >> But let's consider that this comes to pass and every interpreter is >> updated and IMA can see the O_MAYEXEC flag. Attackers now simply pivot >> to running programs via stdin. It never touches disk and therefore >> nothing enforces security policy. This already is among the most >> common ways that malware runs today to evade detection. As my previous reply, use cases like stdin may be restricted as well. > > Are you referring to Windows malware using Powershell? > > I'm not sure this is applicable to Linux. We do not have much > behavioral monitoring anyway. > > Thanks, > Florian > -- Mickaël Salaün ANSSI/SDE/ST/LAM Les données à caractère personnel recueillies et traitées dans le cadre de cet échange, le sont à seule fin d’exécution d’une relation professionnelle et s’opèrent dans cette seule finalité et pour la durée nécessaire à cette relation. Si vous souhaitez faire usage de vos droits de consultation, de rectification et de suppression de vos données, veuillez contacter contact.rgpd@...sn.gouv.fr. Si vous avez reçu ce message par erreur, nous vous remercions d’en informer l’expéditeur et de détruire le message. The personal data collected and processed during this exchange aims solely at completing a business relationship and is limited to the necessary duration of that relationship. If you wish to use your rights of consultation, rectification and deletion of your data, please contact: contact.rgpd@...sn.gouv.fr. If you have received this message in error, we thank you for informing the sender and destroying the message.
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