Re: [PATCH 3.16 226/328] x86/mm: Use WRITE_ONCE() when setting PTEs

[Ben Hutchings]

On Sun, 2018-12-09 at 21:57 +0000, Nadav Amit wrote:
> This patch causes some sparse warnings. If you want to wait, Iâll send a
> patch to fix it. (No expected functional impact though).

Thanks for the note.  I don't think that's enough of a reason to delay
this fix.

Ben.

> 
> > On Dec 9, 2018, at 1:50 PM, Ben Hutchings <ben@xxxxxxxxxxxxxxx> wrote:
> > 
> > 3.16.62-rc1 review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Nadav Amit <namit@xxxxxxxxxx>
> > 
> > commit 9bc4f28af75a91aea0ae383f50b0a430c4509303 upstream.
> > 
> > When page-table entries are set, the compiler might optimize their
> > assignment by using multiple instructions to set the PTE. This might
> > turn into a security hazard if the user somehow manages to use the
> > interim PTE. L1TF does not make our lives easier, making even an interim
> > non-present PTE a security hazard.
> > 
> > Using WRITE_ONCE() to set PTEs and friends should prevent this potential
> > security hazard.
> > 
> > I skimmed the differences in the binary with and without this patch. The
> > differences are (obviously) greater when CONFIG_PARAVIRT=n as more
> > code optimizations are possible. For better and worse, the impact on the
> > binary with this patch is pretty small. Skimming the code did not cause
> > anything to jump out as a security hazard, but it seems that at least
> > move_soft_dirty_pte() caused set_pte_at() to use multiple writes.
> > 
> > Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> > Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > Link: https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20180902181451.80520-1-namit%40vmware.com&amp;data=02%7C01%7Cnamit%40vmware.com%7C714a85e56274491706a408d65e210edd%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636799893436192539&amp;sdata=QNA9jX%2FSAai7zpZeNn%2FosXL%2BrjkG2lYfDVVUN9Etm0A%3D&amp;reserved=0
> > [bwh: Backported to 3.16:
> > - Use ACCESS_ONCE() instead of WRITE_ONCE()
> > - Drop changes in pmdp_establish(), native_set_p4d(), pudp_set_access_flags()]
> > Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> > ---
> > --- a/arch/x86/include/asm/pgtable_64.h
> > +++ b/arch/x86/include/asm/pgtable_64.h
> > @@ -44,15 +44,15 @@ struct mm_struct;
> > void set_pte_vaddr_pud(pud_t *pud_page, unsigned long vaddr, pte_t new_pte);
> > 
> > 
> > -static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr,
> > -				    pte_t *ptep)
> > +static inline void native_set_pte(pte_t *ptep, pte_t pte)
> > {
> > -	*ptep = native_make_pte(0);
> > +	ACCESS_ONCE(*ptep) = pte;
> > }
> > 
> > -static inline void native_set_pte(pte_t *ptep, pte_t pte)
> > +static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr,
> > +				    pte_t *ptep)
> > {
> > -	*ptep = pte;
> > +	native_set_pte(ptep, native_make_pte(0));
> > }
> > 
> > static inline void native_set_pte_atomic(pte_t *ptep, pte_t pte)
> > @@ -62,7 +62,7 @@ static inline void native_set_pte_atomic
> > 
> > static inline void native_set_pmd(pmd_t *pmdp, pmd_t pmd)
> > {
> > -	*pmdp = pmd;
> > +	ACCESS_ONCE(*pmdp) = pmd;
> > }
> > 
> > static inline void native_pmd_clear(pmd_t *pmd)
> > @@ -98,7 +98,7 @@ static inline pmd_t native_pmdp_get_and_
> > 
> > static inline void native_set_pud(pud_t *pudp, pud_t pud)
> > {
> > -	*pudp = pud;
> > +	ACCESS_ONCE(*pudp) = pud;
> > }
> > 
> > static inline void native_pud_clear(pud_t *pud)
> > @@ -131,7 +131,7 @@ static inline pgd_t *native_get_shadow_p
> > 
> > static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
> > {
> > -	*pgdp = kaiser_set_shadow_pgd(pgdp, pgd);
> > +	ACCESS_ONCE(*pgdp) = kaiser_set_shadow_pgd(pgdp, pgd);
> > }
> > 
> > static inline void native_pgd_clear(pgd_t *pgd)
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -242,7 +242,7 @@ static void pgd_mop_up_pmds(struct mm_st
> > 		if (pgd_val(pgd) != 0) {
> > 			pmd_t *pmd = (pmd_t *)pgd_page_vaddr(pgd);
> > 
> > -			pgdp[i] = native_make_pgd(0);
> > +			pgd_clear(&pgdp[i]);
> > 
> > 			paravirt_release_pmd(pgd_val(pgd) >> PAGE_SHIFT);
> > 			pmd_free(mm, pmd);
> > @@ -352,7 +352,7 @@ int ptep_set_access_flags(struct vm_area
> > 	int changed = !pte_same(*ptep, entry);
> > 
> > 	if (changed && dirty) {
> > -		*ptep = entry;
> > +		set_pte(ptep, entry);
> > 		pte_update_defer(vma->vm_mm, address, ptep);
> > 	}
> > 
> > @@ -369,7 +369,7 @@ int pmdp_set_access_flags(struct vm_area
> > 	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> > 
> > 	if (changed && dirty) {
> > -		*pmdp = entry;
> > +		set_pmd(pmdp, entry);
> > 		pmd_update_defer(vma->vm_mm, address, pmdp);
> > 		/*
> > 		 * We had a write-protection fault here and changed the pmd
> 
> 
-- 
Ben Hutchings
If you seem to know what you are doing, you'll be given more to do.

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