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

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Message-ID: <6e88048a-8b30-400e-11c6-8d91ba77cbb0@csgroup.eu>
Date:   Wed, 2 Sep 2020 16:12:50 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Laight <David.Laight@...LAB.COM>,
        '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()



Le 02/09/2020 à 15:51, David Laight a écrit :
> 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 mean on 64 bit or on any platform ?

What about a word write to 0xbffffffe, won't it overwrite 0xc0000000 ?

> You don't even need to in copy_to/from_user() provided
> it always does a forwards copy.

Do you mean due to the gap ?
Is it garantied to be a gap ? Even on a 32 bits having TASK_SIZE set to 
0xc0000000 and PAGE_OFFSET set to the same ?


Christophe