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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 18 Jan 2018 08:49:31 -0800
Message-ID: <CA+55aFxw2CtC_28-7Renuzxfu0zegfCHv+QX-ng4NxUrBRhyXA@mail.gmail.com>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Alan Cox <alan@linux.intel.com>,
        Eric Dumazet <edumazet@google.com>,
        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 Thu, Jan 18, 2018 at 8:38 AM, Christoph Hellwig <hch@infradead.org> 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.

Not going to happen. Really. We have how many tens of thousands of
drivers again, all doing "copy_to_user()".

And the fact is, set_fs() really isn't even a problem for this. Never
really has been.   From a security standpoint, it would actually be
*much* worse if we made those ten thousand places do "if (kernel_flag)
memcpy() else copy_to_user()".

We've had some issues with set_fs() being abused in interesting ways.
But "kernel_read()" and friends is not it.

               Linus