Re: [PATCH v1 2/2] mm/mprotect: drop pgprot_t parameter from change_protection()

[David Hildenbrand]

On 23.12.22 16:56, David Hildenbrand wrote:
> Being able to provide a custom protection opens the door for
> inconsistencies and BUGs: for example, accidentally allowing for more
> permissions than desired by other mechanisms (e.g., softdirty tracking).
> vma->vm_page_prot should be the single source of truth.
>
> Only PROT_NUMA is special: there is no way we can erroneously allow
> for more permissions when removing all permissions. Special-case using
> the MM_CP_PROT_NUMA flag.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   include/linux/mm.h |  3 +--
>   mm/mempolicy.c     |  3 +--
>   mm/mprotect.c      | 14 +++++++++++---
>   mm/userfaultfd.c   |  3 +--
>   4 files changed, 14 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f3f196e4d66d..b8be8c33ca20 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2148,8 +2148,7 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>   			     pte_t pte);
>   extern unsigned long change_protection(struct mmu_gather *tlb,
>   			      struct vm_area_struct *vma, unsigned long start,
> -			      unsigned long end, pgprot_t newprot,
> -			      unsigned long cp_flags);
> +			      unsigned long end, unsigned long cp_flags);
>   extern int mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   			  struct vm_area_struct **pprev, unsigned long start,
>   			  unsigned long end, unsigned long newflags);
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 61aa9aedb728..c3f02703a710 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -635,8 +635,7 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
>   
>   	tlb_gather_mmu(&tlb, vma->vm_mm);
>   
> -	nr_updated = change_protection(&tlb, vma, addr, end, PAGE_NONE,
> -				       MM_CP_PROT_NUMA);
> +	nr_updated = change_protection(&tlb, vma, addr, end, MM_CP_PROT_NUMA);
>   	if (nr_updated)
>   		count_vm_numa_events(NUMA_PTE_UPDATES, nr_updated);
>   
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 908df12caa26..569cefa668a6 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -503,13 +503,21 @@ static unsigned long change_protection_range(struct mmu_gather *tlb,
>   
>   unsigned long change_protection(struct mmu_gather *tlb,
>   		       struct vm_area_struct *vma, unsigned long start,
> -		       unsigned long end, pgprot_t newprot,
> -		       unsigned long cp_flags)
> +		       unsigned long end, unsigned long cp_flags)
>   {
> +	pgprot_t newprot = vma->vm_page_prot;
>   	unsigned long pages;
>   
>   	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
>   
> +	/*
> +	 * Ordinary protection updates (mprotect, uffd-wp, softdirty tracking)
> +	 * are expected to reflect their requirements via VMA flags such that
> +	 * vma_set_page_prot() will adjust vma->vm_page_prot accordingly.
> +	 */
> +	if (cp_flags & MM_CP_PROT_NUMA)
> +		newprot = PAGE_NONE;
> +
>   	if (is_vm_hugetlb_page(vma))
>   		pages = hugetlb_change_protection(vma, start, end, newprot,
>   						  cp_flags);
> @@ -638,7 +646,7 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
>   	vma_set_page_prot(vma);
>   
> -	change_protection(tlb, vma, start, end, vma->vm_page_prot, mm_cp_flags);
> +	change_protection(tlb, vma, start, end, mm_cp_flags);
>   
>   	/*
>   	 * Private VM_LOCKED VMA becoming writable: trigger COW to avoid major
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 351e8d6b398b..be7ee9d82e72 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -744,8 +744,7 @@ void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
>   	if (vma_wants_manual_pte_write_upgrade(dst_vma))
>   		mm_cp_flags |= MM_CP_TRY_CHANGE_WRITABLE;
>   	tlb_gather_mmu(&tlb, dst_mm);
> -	change_protection(&tlb, dst_vma, start, start + len, vma->vm_page_prot,
> -			  mm_cp_flags);
> +	change_protection(&tlb, dst_vma, start, start + len, mm_cp_flags);
>   	tlb_finish_mmu(&tlb);
>   }
>   

The following fix for compilation errors without PAGE_NONE:


From a164d6cf728e353294aa9e65b8ead5241c800421 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@xxxxxxxxxx>
Date: Sat, 24 Dec 2022 15:01:18 +0100
Subject: [PATCH] fixup: mm/mprotect: drop pgprot_t parameter from
 change_protection()

PAGE_NONE might not be defined without CONFIG_NUMA_BALANCING

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 mm/mprotect.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/mprotect.c b/mm/mprotect.c
index 569cefa668a6..809832954898 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -510,6 +510,7 @@ unsigned long change_protection(struct mmu_gather *tlb,
 
 	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
 
+#ifdef CONFIG_NUMA_BALANCING
 	/*
 	 * Ordinary protection updates (mprotect, uffd-wp, softdirty tracking)
 	 * are expected to reflect their requirements via VMA flags such that
@@ -517,6 +518,9 @@ unsigned long change_protection(struct mmu_gather *tlb,
 	 */
 	if (cp_flags & MM_CP_PROT_NUMA)
 		newprot = PAGE_NONE;
+#else
+	WARN_ON_ONCE(cp_flags & MM_CP_PROT_NUMA);
+#endif
 
 	if (is_vm_hugetlb_page(vma))
 		pages = hugetlb_change_protection(vma, start, end, newprot,
--
2.38.1



--
Thanks,

David / dhildenb