linux-kernel - Re: [PATCH 2/2] kernel/fork: handle put_user errors for CLONE_PARENT

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Date:	Sat, 7 Feb 2015 00:13:24 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	Linux API <linux-api@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roman Gushchin <klamm@...dex-team.ru>,
	Nikita Vetoshkin <nekto0n@...dex-team.ru>,
	Oleg Nesterov <oleg@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH 2/2] kernel/fork: handle put_user errors for CLONE_PARENT_SETTID

On Fri, Feb 6, 2015 at 11:49 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Feb 6, 2015 at 8:23 AM, Konstantin Khlebnikov
> <khlebnikov@...dex-team.ru> wrote:
>> Handling of flag CLONE_PARENT_SETTID has the same problem: error returned
>> from put_user() is ignored. Glibc completely relies on that feature and uses
>> value returned from syscall only for error checking.
>
> I'm not seeing the advantage of the error checking part of the pacth
> patch. It generates extra code, possibly changing existing interfaces,
> and it doesn't actually buy us anything.
>
> What's the upside? If somebody passes in a bad pointer, it's their
> problem. For all we know, people used to pass in NULL, even if they
> had the SETTID bit set. This makes it now return EFAULT.

Currently that works fine only because kernel retries 0-order allocations
endlessly. But pagefault_out_of_memory() is never called for non-user PF.
For kernel PF all oom-kills are triggered by buddy-allocator.
If buddy allocator gave up earlier then page-faults from kernel space
could fail without OOM. And in CoW area user-space will see stale data.
So, either we must handle all put_user/copy_to_user errors (which isn't
that bad idea) or kernel must force all PF to success-or-die policy.

First patch is that ugly because kernel has never checked errors
in that place. So, I've tried to find solution which could fix problem
without breaking backward compatibility.

>
> So I don't mind moving things into copy_process(), but I *do* mind the
> new error return thing.
>
> It's actually better in this patch than in 1/2, because 1/2 was just
> insane with the whole "readable vs writable" thing. That I refuse to
> even look at, for fear of going blind.
>
>                       Linus
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