Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

Christophe Leroy
2020-09-02T14:46:42Z

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