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

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Message-ID: <20151203203809.GA15235@ravnborg.org>
Date:	Thu, 3 Dec 2015 21:38:10 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Yang Shi <yang.shi@...aro.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

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.


>  
>  	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
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