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Message-ID: <CALmYWFvAFfXmHgo6Ca+FsKhAapJ_C1VXhqT7LdFy3ZnU4Vu3Hw@mail.gmail.com> Date: Thu, 18 Jul 2024 15:54:00 -0700 From: Jeff Xu <jeffxu@...gle.com> To: Mickaël Salaün <mic@...ikod.net> Cc: Steve Dower <steve.dower@...hon.org>, Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Paul Moore <paul@...l-moore.com>, "Theodore Ts'o" <tytso@....edu>, Alejandro Colomar <alx@...nel.org>, Aleksa Sarai <cyphar@...har.com>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>, Christian Heimes <christian@...hon.org>, Dmitry Vyukov <dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>, Eric Chiang <ericchiang@...gle.com>, Fan Wu <wufan@...ux.microsoft.com>, Florian Weimer <fweimer@...hat.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, James Morris <jamorris@...ux.microsoft.com>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Jordan R Abrahams <ajordanr@...gle.com>, Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>, Luca Boccassi <bluca@...ian.org>, Luis Chamberlain <mcgrof@...nel.org>, "Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>, Matt Bobrowski <mattbobrowski@...gle.com>, Matthew Garrett <mjg59@...f.ucam.org>, Matthew Wilcox <willy@...radead.org>, Miklos Szeredi <mszeredi@...hat.com>, Mimi Zohar <zohar@...ux.ibm.com>, Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>, Steve Grubb <sgrubb@...hat.com>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Xiaoming Ni <nixiaoming@...wei.com>, Yin Fengwei <fengwei.yin@...el.com>, kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, Elliott Hughes <enh@...gle.com> Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2) On Thu, Jul 18, 2024 at 5:23 AM Mickaël Salaün <mic@...ikod.net> wrote: > > On Wed, Jul 17, 2024 at 06:51:11PM -0700, Jeff Xu wrote: > > On Wed, Jul 17, 2024 at 3:00 AM Mickaël Salaün <mic@...ikod.net> wrote: > > > > > > On Wed, Jul 17, 2024 at 09:26:22AM +0100, Steve Dower wrote: > > > > On 17/07/2024 07:33, Jeff Xu wrote: > > > > > Consider those cases: I think: > > > > > a> relying purely on userspace for enforcement does't seem to be > > > > > effective, e.g. it is trivial to call open(), then mmap() it into > > > > > executable memory. > > > > > > > > If there's a way to do this without running executable code that had to pass > > > > a previous execveat() check, then yeah, it's not effective (e.g. a Python > > > > interpreter that *doesn't* enforce execveat() is a trivial way to do it). > > > > > > > > Once arbitrary code is running, all bets are off. So long as all arbitrary > > > > code is being checked itself, it's allowed to do things that would bypass > > > > later checks (and it's up to whoever audited it in the first place to > > > > prevent this by not giving it the special mark that allows it to pass the > > > > check). > > > > > We will want to define what is considered as "arbitrary code is running" > > > > Using an example of ROP, attackers change the return address in stack, > > e.g. direct the execution flow to a gauge to call "ld.so /tmp/a.out", > > do you consider "arbitrary code is running" when stack is overwritten > > ? or after execve() is called. > > Yes, ROP is arbitrary code execution (which can be mitigated with CFI). > ROP could be enough to interpret custom commands and create a small > interpreter/VM. > > > If it is later, this patch can prevent "ld.so /tmp/a.out". > > > > > Exactly. As explained in the patches, one crucial prerequisite is that > > > the executable code is trusted, and the system must provide integrity > > > guarantees. We cannot do anything without that. This patches series is > > > a building block to fix a blind spot on Linux systems to be able to > > > fully control executability. > > > > Even trusted executable can have a bug. > > Definitely, but this patch series is dedicated to script execution > control. > > > > > I'm thinking in the context of ChromeOS, where all its system services > > are from trusted partitions, and legit code won't load .so from a > > non-exec mount. But we want to sandbox those services, so even under > > some kind of ROP attack, the service still won't be able to load .so > > from /tmp. Of course, if an attacker can already write arbitrary > > length of data into the stack, it is probably already a game over. > > > > OK, you want to tie executable file permission to mmap. That makes > sense if you have a consistent execution model. This can be enforced by > LSMs. Contrary to script interpretation which is a full user space > implementation (and then controlled by user space), mmap restrictions > should indeed be enforced by the kernel. Ya, that is what I meant. it can be out of scope for this patch. Indeed, as you point out, this patch is dedicated to script execution control, and fixing ld.so /tmp/a.out is an extra bonus in addition to script. Thanks -Jeff
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