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

[Shi, Yang]

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@xxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/