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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 17 Jan 2018 11:26:08 -0800
Message-ID: <CA+55aFzCgqjRR=x37fzUecK5ERMBUmBOag-EpM1zQQxFeH2+bw@mail.gmail.com>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths
To:     Alan Cox <alan@linux.intel.com>, Eric Dumazet <edumazet@google.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        kernel-hardening@lists.openwall.com,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>
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On Wed, Jan 17, 2018 at 6:17 AM, Alan Cox <alan@linux.intel.com> wrote:
>
> Can we kill off the remaining users of set_fs() ?

I would love to, but it's not going to happen short-term. If ever.

Some could be removed today: the code in arch/x86/net/bpf_jit_comp.c
seems to be literally the ramblings of a diseased mind. There's no
reason for the set_fs(), there's no reason for the
flush_icache_range() (it's a no-op on x86 anyway), and the smp_wmb()
looks bogus too.

I have no idea how that braindamage happened, but I assume it got
copied from some broken source.

But there are about ~100 set_fs() calls in generic code, and some of
those really are pretty fundamental. Doing things like "kernel_read()"
without set_fs() is basically impossible.

We've had set_fs() since the beginning. The naming is obviously very
historical. We have it for a very good reason, and I don't really see
that reason going away.

So realistically, we want to _minimize_ set_fs(), and we might want to
make sure that it's done only in limited settings (it might, for
example, be a good idea and a realistic goal to make sure that drivers
and modules can't do it, and use proper helper functions like that
"read_kernel()").

But getting rid of the concept entirely? Doesn't seem likely.

             Linus