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Date: Sat, 24 Dec 2022 18:01:15 +0100
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Peter Xu <peterx@...hat.com>, Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH v1 2/2] mm/mprotect: drop pgprot_t parameter from
change_protection()
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@...hat.com>
> ---
> 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@...hat.com>
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@...hat.com>
---
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
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