Subject: Re: [RFC][PATCHES] iov_iter.c rewrite

From: Al Viro <[email-masked]>
Date: Mon, 8 Dec 2014 18:20:16 +0000

On Mon, Dec 08, 2014 at 10:14:13AM -0800, Linus Torvalds wrote:

> For a vmalloc() address, you'd have to actually walk the page tables.
> Which is a f*cking horrible idea. Don't do it. We do have a
> "vmalloc_to_page()" that does it, but the basic issue is that you damn
> well shouldn't do IO on vmalloc'ed addresses.  vmalloc'ed addresses
> only exist in the first place to give a linear *virtual* mapping, if
> you want physical pages you shouldn't have mixed it up with vmalloc in
> the first place!
> 
> Where the hell does this crop up, and who does this insane thing
> anyway? It's wrong. How did it ever work before?

finit_module() with O_DIRECT descriptor.  And I suspect that "not well"
is the answer - it used to call get_user_pages_fast() in that case.

I certainly had missed that insanity during the analysis - we don't do
a lot of O_DIRECT IO to/from kernel addresses of any sort...  This
codepath allows it ;-/  Ability to trigger it is equivalent to ability
to run any code in kernel mode, so it's not an additional security hole,
but...