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

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Date:   Thu, 18 Jan 2018 08:38:18 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alan Cox <alan@...ux.intel.com>,
        Eric Dumazet <edumazet@...gle.com>,
        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 11:26:08AM -0800, Linus Torvalds wrote:
> 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.

Not if we move to iov_iter or iov_iter-like behavior for all reads
and writes.  There is an issue with how vectored writes are handles
in plain read/write vs read_iter/write_iter inherited from readv/writev,
but that's nothing a flag, or a second set of methods with the
same signature.

But there are more annoying things, most notable in-kernel ioctls
calls.  We have quite a few of them, and while many are just utterly
stupid and can be replaced with direct function calls or new methods
(I've done quite a few conversions of those) some might be left.
Something like iov_iter might be the answer again.

Then we have things like probe_kernel_read/probe_kernel_write which
abuse the exception handling in get/put user.  But with a little
arch helper we don't strictly need get_fs/set_fs for that.