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Date: Thu, 18 Jan 2018 08:49:31 -0800
From: Linus Torvalds <>
To: Christoph Hellwig <>
Cc: Alan Cox <>, Eric Dumazet <>, 
	Dan Williams <>, 
	Linux Kernel Mailing List <>,, 
	Andi Kleen <>, Kees Cook <>,, 
	Greg Kroah-Hartman <>, "the arch/x86 maintainers" <>, 
	Ingo Molnar <>, Al Viro <>, 
	"H. Peter Anvin" <>, Thomas Gleixner <>, 
	Andrew Morton <>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Thu, Jan 18, 2018 at 8:38 AM, Christoph Hellwig <> 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.


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