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Date:   Tue, 19 May 2020 22:17:03 -0700
From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
To:     Aleksa Sarai <asarai@suse.de>
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Kees Cook <keescook@chromium.org>,
        Chris Palmer <palmer@google.com>, Jann Horn <jannh@google.com>,
        Jeffrey Vander Stoep <jeffv@google.com>,
        Linux Containers <containers@lists.linux-foundation.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Matt Denton <mpdenton@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        bpf <bpf@vger.kernel.org>
Subject: Re: seccomp feature development
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On Wed, May 20, 2020 at 11:20:45AM +1000, Aleksa Sarai wrote:
> 
> No it won't become copy_from_user(), nor will there be a TOCTOU race.
> 
> The idea is that seccomp will proactively copy the struct (and
> recursively any of the struct pointers inside) before the syscall runs
> -- as this is done by seccomp it doesn't require any copy_from_user()
> primitives in cBPF. We then run the cBPF filter on the copied struct,
> just like how cBPF programs currently operate on seccomp_data (how this
> would be exposed to the cBPF program as part of the seccomp ABI is the
> topic of discussion here).
> 
> Then, when the actual syscall code runs, the struct will have already
> been copied and the syscall won't copy it again.

Let's take bpf syscall as an example.
Are you suggesting that all of syscall logic of conditionally parsing
the arguments will be copy-pasted into seccomp-syscall infra, then
it will do copy_from_user() all the data and replace all aligned_u64
in "union bpf_attr" with kernel copied pointers instead of user pointers
and make all of bpf syscall's copy_from_user() actions to be conditional ?
If seccomp is on, use kernel pointers... if seccomp is off, do copy_from_user ?
And the same idea will be replicated for all syscalls?