Re: [PATCH 1/5] x86/pti: check the return value ofpti_user_pagetable_walk_p4d

[Thomas Gleixner]

On Tue, 17 Jul 2018, jiang.biao2@xxxxxxxxxx wrote:
> > On 07/15/2018 09:03 PM, Jiang Biao wrote:
> >> diff --git a/arch/x86/mm/pti.c b/arch/x86/mm/pti.c
> >> index 4d418e7..be9e5bc 100644
> >> --- a/arch/x86/mm/pti.c
> >> +++ b/arch/x86/mm/pti.c
> >> @@ -195,8 +195,10 @@ static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
> >>  static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
> >>  {
> >>      gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
> >> -    p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
> >>      pud_t *pud;
> >> +    p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
> >> +    if (WARN_ON(!p4d))
> >> +        return NULL;
> > 
> > First of all, I don't think we need the (new) warning here.
> > pti_user_pagetable_walk_p4d() only returns NULL if you try it on a
> > userspace _address_ or the page allocation fails.  It already warns on
> > the address case.
> > 
> > If you think the allocation path needs a warning, please do it as close
> > as possible to the _source_ of the warning (the failed allocation), not
> > in the caller.
> Hi,
> Just taking  pti_clone_pmds() as reference, which add warning for NULL 
> *target_pmd*. The warning does not really matter, I will remove the 
> *WARN_ON* here and add a warning close to the failed allocation. 
> Furthermore do you think we need to check  _NULL_ return value?  
> to avoid NULL pointer dereference when the return value is used later, 
> such as,
>   p4d_large(*p4d) //p4d is return value of pti_user_pagetable_walk_p4d
>   *user_p4d = *kernel_p4d;//user_p4d is return value of pti_user_pagetable_walk_p4d

The Null pointer check makes sense.

Thanks,

	tglx