Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1283321 > unrolled thread

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

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  Re: [V2 PATCH] sparc64/gup: check address scope legitimacy Sam Ravnborg <sam@ravnborg.org> - 2015-12-03 21:40 +0100
    Re: [V2 PATCH] sparc64/gup: check address scope legitimacy "Shi, Yang" <yang.shi@linaro.org> - 2015-12-03 23:30 +0100
      Re: [V2 PATCH] sparc64/gup: check address scope legitimacy Sam Ravnborg <sam@ravnborg.org> - 2015-12-05 11:10 +0100

#1283321 — Re: [V2 PATCH] sparc64/gup: check address scope legitimacy

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@linaro.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.


>  
>  	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.

	Sam
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [next] | [standalone]

#1283421

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@linaro.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
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [next] | [standalone]

#1284510

> >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?
I do not know - sorry.

> >>@@ -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.
The the other archs are likely confused as well - there is most
likely some history behind that can be found if digging a little.

The code is not present in the generic version.

	Sam
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web