Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas

From: Brendan Shanks <bshanks@codeweavers.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Paul Gofman <gofmanp@gmail.com>,
	Gabriel Krisman Bertazi <krisman@collabora.com>,
	Linux-MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel@collabora.com, Thomas Gleixner <tglx@linutronix.de>,
	Kees Cook <keescook@chromium.org>, Will Drewry <wad@chromium.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Zebediah Figura <zfigura@codeweavers.com>
Subject: Re: [PATCH RFC] seccomp: Implement syscall isolation based on memory areas
Date: Sun, 31 May 2020 16:33:54 -0700	[thread overview]
Message-ID: <8DF2868F-E756-4B33-A7AE-C61F4AB9ABB9@codeweavers.com> (raw)
In-Reply-To: <CALCETrV+rYnUnve09=n+Zb8BR8mDBq6txX9LmEw7r8tAA7d+2Q@mail.gmail.com>

> On May 31, 2020, at 11:57 AM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> Using SECCOMP_RET_USER_NOTIF is likely to be considerably more
> expensive than my scheme.  On a non-PTI system, my approach will add a
> few tens of ns to each syscall.  On a PTI system, it will be worse.
> But using any kind of notifier for all syscalls will cause a context
> switch to a different user program for each syscall, and that will be
> much slower.

There’s also no way (at least to my understanding) to modify register state from SECCOMP_RET_USER_NOTIF, which is how the existing -staging SIGSYS handler works:

<https://github.com/wine-staging/wine-staging/blob/master/patches/ntdll-Syscall_Emulation/0001-ntdll-Support-x86_64-syscall-emulation.patch#L62>

> I think that the implementation may well want to live in seccomp, but
> doing this as a seccomp filter isn't quite right.  It's not a security
> thing -- it's an emulation thing.  Seccomp is all about making
> inescapable sandboxes, but that's not what you're doing at all, and
> the fact that seccomp filters are preserved across execve() sounds
> like it'll be annoying for you.

Definitely. Regardless of what approach is taken, we don’t want it to persist across execve.

> What if there was a special filter type that ran a BPF program on each
> syscall, and the program was allowed to access user memory to make its
> decisions, e.g. to look at some list of memory addresses.  But this
> would explicitly *not* be a security feature -- execve() would remove
> the filter, and the filter's outcome would be one of redirecting
> execution or allowing the syscall.  If the "allow" outcome occurs,
> then regular seccomp filters run.  Obviously the exact semantics here
> would need some care.

Although if that’s running a BPF filter on every syscall, wouldn’t it also incur the ~10% overhead that Paul and Gabriel have seen with existing seccomp?

Brendan Shanks
CodeWeavers