linux-kernel - RE: [PATCH 10/10] powerpc: remove address space overrides using set

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Message-ID: <0c298e0d972a48bd9ee178225e404b12@AcuMS.aculab.com>
Date:   Wed, 2 Sep 2020 13:51:05 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        'Christoph Hellwig' <hch@....de>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 10/10] powerpc: remove address space overrides using
 set_fs()

From: Christophe Leroy
> Sent: 02 September 2020 14:25
> Le 02/09/2020 à 15:13, David Laight a écrit :
> > From: Christoph Hellwig
> >> Sent: 02 September 2020 13:37
> >>
> >> On Wed, Sep 02, 2020 at 08:15:12AM +0200, Christophe Leroy wrote:
> >>>> -		return 0;
> >>>> -	return (size == 0 || size - 1 <= seg.seg - addr);
> >>>> +	if (addr >= TASK_SIZE_MAX)
> >>>> +		return false;
> >>>> +	if (size == 0)
> >>>> +		return false;
> >>>
> >>> __access_ok() was returning true when size == 0 up to now. Any reason to
> >>> return false now ?
> >>
> >> No, this is accidental and broken.  Can you re-run your benchmark with
> >> this fixed?
> >
> > Is TASK_SIZE_MASK defined such that you can do:
> >
> > 	return (addr | size) < TASK_SIZE_MAX) || !size;
> 
> TASK_SIZE_MAX will usually be 0xc0000000
> 
> With:
> addr = 0x80000000;
> size = 0x80000000;
> 
> I expect it to fail ....
> 
> With the formula you propose it will succeed, won't it ?

Hmmm... Was i getting confused about some comments for 64bit
about there being such a big hole between valid user and kernel
addresses that it was enough to check that 'size < TASK_SIZE_MAX'.

That would be true for 64bit x86 (and probably ppc (& arm??))
if TASK_SIZE_MAX were 0x4 << 60.
IIUC the highest user address is (much) less than 0x0 << 60
and the lowest kernel address (much) greater than 0xf << 60
on all these 64bit platforms.

Actually if doing access_ok() inside get_user() you don't
need to check the size at all.
You don't even need to in copy_to/from_user() provided
it always does a forwards copy.
(Rather that copying the last word first for misaligned lengths.)

	David

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