Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: "Jia He" <justin.he@arm.com>, "Will Deacon" <will@kernel.org>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"James Morse" <james.morse@arm.com>,
	"Marc Zyngier" <maz@kernel.org>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	"Suzuki Poulose" <Suzuki.Poulose@arm.com>,
	"Punit Agrawal" <punitagrawal@gmail.com>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Jun Yao" <yaojun8558363@gmail.com>,
	"Alex Van Brunt" <avanbrunt@nvidia.com>,
	"Robin Murphy" <robin.murphy@arm.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	hejianet@gmail.com, "Kaly Xin" <Kaly.Xin@arm.com>
Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared
Date: Thu, 19 Sep 2019 18:00:07 +0300	[thread overview]
Message-ID: <20190919150007.k7scjplcya53j7r4@box> (raw)
In-Reply-To: <20190918180029.GB20601@iMac.local>

On Wed, Sep 18, 2019 at 07:00:30PM +0100, Catalin Marinas wrote:
> On Wed, Sep 18, 2019 at 05:00:27PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote:
> > > @@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
> > >  	 */
> > >  	if (unlikely(!src)) {
> > >  		void *kaddr = kmap_atomic(dst);
> > > -		void __user *uaddr = (void __user *)(va & PAGE_MASK);
> > > +		void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> > > +		pte_t entry;
> > >  
> > >  		/*
> > >  		 * This really shouldn't fail, because the page is there
> > >  		 * in the page tables. But it might just be unreadable,
> > >  		 * in which case we just give up and fill the result with
> > > -		 * zeroes.
> > > +		 * zeroes. On architectures with software "accessed" bits,
> > > +		 * we would take a double page fault here, so mark it
> > > +		 * accessed here.
> > >  		 */
> > > +		if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
> > > +			spin_lock(vmf->ptl);
> > > +			if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> > > +				entry = pte_mkyoung(vmf->orig_pte);
> > > +				if (ptep_set_access_flags(vma, addr,
> > > +							  vmf->pte, entry, 0))
> > > +					update_mmu_cache(vma, addr, vmf->pte);
> > > +			}
> > 
> > I don't follow.
> > 
> > So if pte has changed under you, you don't set the accessed bit, but never
> > the less copy from the user.
> > 
> > What makes you think it will not trigger the same problem?
> > 
> > I think we need to make cow_user_page() fail in this case and caller --
> > wp_page_copy() -- return zero. If the fault was solved by other thread, we
> > are fine. If not userspace would re-fault on the same address and we will
> > handle the fault from the second attempt.
> 
> It would be nice to clarify the semantics of this function and do as
> you suggest but the current comment is slightly confusing:
> 
> 	/*
> 	 * If the source page was a PFN mapping, we don't have
> 	 * a "struct page" for it. We do a best-effort copy by
> 	 * just copying from the original user address. If that
> 	 * fails, we just zero-fill it. Live with it.
> 	 */
> 
> Would any user-space rely on getting a zero-filled page here instead of
> a recursive fault?

I don't see the point in zero-filled page in this case. SIGBUS sounds like
more appropriate response, no?

-- 
 Kirill A. Shutemov