linux-kernel - Re: [V2 PATCH] sparc64/gup: check address scope legitimacy

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Message-ID: <5660C0D4.8050800@linaro.org>
Date:	Thu, 03 Dec 2015 14:23:16 -0800
From:	"Shi, Yang" <yang.shi@...aro.org>
To:	Sam Ravnborg <sam@...nborg.org>
CC:	davem@...emloft.net, linux-kernel@...r.kernel.org,
	sparclinux@...r.kernel.org, linaro-kernel@...ts.linaro.org
Subject: Re: [V2 PATCH] sparc64/gup: check address scope legitimacy

On 12/3/2015 12:38 PM, Sam Ravnborg wrote:
> Hi Yang.
>
> On Wed, Nov 25, 2015 at 02:45:43PM -0800, Yang Shi wrote:
>> Check if user address is accessible in atomic version __get_user_pages_fast()
>> before walking the page table.
>> And, check if end > start in get_user_pages_fast(), otherwise fallback to slow
>> path.
>
> Two different but related things in one patch is often a bad thing.
> It would have been better to split it up.
>
>
>>
>> Signed-off-by: Yang Shi <yang.shi@...aro.org>
>> ---
>> Just found slow_irqon label is not defined, added it to avoid compile error.
>>
>>   arch/sparc/mm/gup.c | 7 ++++++-
>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/sparc/mm/gup.c b/arch/sparc/mm/gup.c
>> index 2e5c4fc..cf4fb47 100644
>> --- a/arch/sparc/mm/gup.c
>> +++ b/arch/sparc/mm/gup.c
>> @@ -173,6 +173,9 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>   	addr = start;
>>   	len = (unsigned long) nr_pages << PAGE_SHIFT;
>>   	end = start + len;
>> +	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
>> +					(void __user *)start, len)))
>> +		return 0;
> This change is not justified.
> Why would we take the time to first do the access_ok() stuff.
> If this had been an expensive operation then we had made this function
> slower in the normal case ( assuming there were no access violations in the
> normal case).
> When I look at the implementation of access_ok() I get the impression that
> this is not really a check we need.
>
> access_ok() always returns 1.

Thanks for pointing it out. And, I didn't notice that gup is just built 
for SPARC64. I though it is built by both 64 bit and 32 bit.

A follow-up question, is there any reason to just have sparc specific 
fast gup for 64 bit not for 32 bit?

>
>
>>
>>   	local_irq_save(flags);
>>   	pgdp = pgd_offset(mm, addr);
>> @@ -203,6 +206,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
>>   	addr = start;
>>   	len = (unsigned long) nr_pages << PAGE_SHIFT;
>>   	end = start + len;
>> +	if (end < start)
>> +		goto slow_irqon;
>
> end can only be smaller than start if there is some overflow.
> See how end is calculated just the line above.
>
> This looks like a highly suspicious change.

I'm supposed this is used to protect the overflow. I copied the code 
from other arch. Actually, every arch has this except sparc.

Thanks,
Yang

>
> 	Sam
>

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