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

[Nadav Amit]

This patch causes some sparse warnings. If you want to wait, Iâll send a
patch to fix it. (No expected functional impact though).


> 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