Subject: [PATCH] arm64: KVM: fix 2-level page tables unmapping

From: [email-masked] (Christoffer Dall)
Date: Tue, 6 Aug 2013 13:49:43 -0700

On Tue, Aug 06, 2013 at 01:05:48PM +0100, Marc Zyngier wrote:
> When using 64kB pages, we only have two levels of page tables,
> meaning that PGD, PUD and PMD are fused. In this case, trying
> to refcount PUDs and PMDs independantly is a a complete disaster,

independently

> as they are the same.
> 
> We manage to get it right for the allocation (stage2_set_pte uses
> {pmd,pud}_none), but the unmapping path clears both pud and pmd
> refcounts, which fails spectacularly with 2-level page tables.
> 
> The fix is to avoid calling clear_pud_entry when both the pmd and
> pud pages are empty. For this, and instead of introducing another
> pud_empty function, consolidate both pte_empty and pmd_empty into
> page_empty (the code is actually identical) and use that to also
> test the validity of the pud.
> 
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm/kvm/mmu.c | 22 ++++++++--------------
>  1 file changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index ca6bea4..7e1d899 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -85,6 +85,12 @@ static void *mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc)
>  	return p;
>  }
>  
> +static bool page_empty(void *ptr)
> +{
> +	struct page *ptr_page = virt_to_page(ptr);
> +	return page_count(ptr_page) == 1;
> +}
> +
>  static void clear_pud_entry(struct kvm *kvm, pud_t *pud, phys_addr_t addr)
>  {
>  	pmd_t *pmd_table = pmd_offset(pud, 0);
> @@ -103,12 +109,6 @@ static void clear_pmd_entry(struct kvm *kvm, pmd_t *pmd, phys_addr_t addr)
>  	put_page(virt_to_page(pmd));
>  }
>  
> -static bool pmd_empty(pmd_t *pmd)
> -{
> -	struct page *pmd_page = virt_to_page(pmd);
> -	return page_count(pmd_page) == 1;
> -}
> -
>  static void clear_pte_entry(struct kvm *kvm, pte_t *pte, phys_addr_t addr)
>  {
>  	if (pte_present(*pte)) {
> @@ -118,12 +118,6 @@ static void clear_pte_entry(struct kvm *kvm, pte_t *pte, phys_addr_t addr)
>  	}
>  }
>  
> -static bool pte_empty(pte_t *pte)
> -{
> -	struct page *pte_page = virt_to_page(pte);
> -	return page_count(pte_page) == 1;
> -}
> -
>  static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
>  			unsigned long long start, u64 size)
>  {
> @@ -153,10 +147,10 @@ static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
>  		range = PAGE_SIZE;
>  
>  		/* If we emptied the pte, walk back up the ladder */
> -		if (pte_empty(pte)) {
> +		if (page_empty(pte)) {
>  			clear_pmd_entry(kvm, pmd, addr);
>  			range = PMD_SIZE;
> -			if (pmd_empty(pmd)) {
> +			if (page_empty(pmd) && !page_empty(pud)) {
>  				clear_pud_entry(kvm, pud, addr);
>  				range = PUD_SIZE;
>  			}

looks right, an alternative would be to check in clear_pud_entry if the
entry actually had a value, but I don't think it's really clearer.

However, this got me thinking a bit.  What happens if we pass a non-pmd
aligned address to unmap_range, and let's assume the size of the range
is more than 2MB, won't we be leaking memory by incrementing with
PMD_SIZE?  (same argument goes for PUD_SIZE).  See the patch below:

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index ca6bea4..80a83ec 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -132,37 +132,37 @@ static void unmap_range(struct kvm *kvm, pgd_t *pgdp,
 	pmd_t *pmd;
 	pte_t *pte;
 	unsigned long long addr = start, end = start + size;
-	u64 range;
+	u64 next;
 
 	while (addr < end) {
 		pgd = pgdp + pgd_index(addr);
 		pud = pud_offset(pgd, addr);
 		if (pud_none(*pud)) {
-			addr += PUD_SIZE;
+			addr = pud_addr_end(addr, end);
 			continue;
 		}
 
 		pmd = pmd_offset(pud, addr);
 		if (pmd_none(*pmd)) {
-			addr += PMD_SIZE;
+			addr = pmd_addr_end(addr, end);
 			continue;
 		}
 
 		pte = pte_offset_kernel(pmd, addr);
 		clear_pte_entry(kvm, pte, addr);
-		range = PAGE_SIZE;
+		next = addr + PAGE_SIZE;
 
 		/* If we emptied the pte, walk back up the ladder */
 		if (pte_empty(pte)) {
 			clear_pmd_entry(kvm, pmd, addr);
-			range = PMD_SIZE;
+			next = pmd_addr_end(addr, end);
 			if (pmd_empty(pmd)) {
 				clear_pud_entry(kvm, pud, addr);
-				range = PUD_SIZE;
+				next = pud_addr_end(addr, end);
 			}
 		}
 
-		addr += range;
+		addr = next;
 	}
 }
 
--
Christoffer