linux-kernel - Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get

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Date:   Wed, 17 Jan 2018 11:26:08 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alan Cox <alan@...ux.intel.com>, Eric Dumazet <edumazet@...gle.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Wed, Jan 17, 2018 at 6:17 AM, Alan Cox <alan@...ux.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