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SubjectRe: [PATCH v9 08/24] mm: Protect VMA modifications using VMA sequence count
On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 65ae54659833..a2d9c87b7b0b 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1136,8 +1136,11 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf,
> goto out_mm;
> }
> for (vma = mm->mmap; vma; vma = vma->vm_next) {
> - vma->vm_flags &= ~VM_SOFTDIRTY;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags,
> + vma->vm_flags & ~VM_SOFTDIRTY);
> vma_set_page_prot(vma);
> + vm_write_end(vma);
> }
> downgrade_write(&mm->mmap_sem);
> break;
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index cec550c8468f..b8212ba17695 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -659,8 +659,11 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
>
> octx = vma->vm_userfaultfd_ctx.ctx;
> if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> + vm_write_begin(vma);
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
> + WRITE_ONCE(vma->vm_flags,
> + vma->vm_flags & ~(VM_UFFD_WP | VM_UFFD_MISSING));
> + vm_write_end(vma);
> return 0;
> }
>

In several locations in this patch vm_write_begin(vma) ->
vm_write_end(vma) is nesting things other than vma->vm_flags,
vma->vm_policy, etc. I think it's better to do vm_write_end(vma) as soon
as the members that the seqcount protects are modified. In other words,
this isn't offering protection for vma->vm_userfaultfd_ctx. There are
several examples of this in the patch.

> @@ -885,8 +888,10 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
> vma = prev;
> else
> prev = vma;
> - vma->vm_flags = new_flags;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags, new_flags);
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> + vm_write_end(vma);
> }
> up_write(&mm->mmap_sem);
> mmput(mm);
> @@ -1434,8 +1439,10 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> * the next vma was merged into the current one and
> * the current one has not been updated yet.
> */
> - vma->vm_flags = new_flags;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags, new_flags);
> vma->vm_userfaultfd_ctx.ctx = ctx;
> + vm_write_end(vma);
>
> skip:
> prev = vma;
> @@ -1592,8 +1599,10 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> * the next vma was merged into the current one and
> * the current one has not been updated yet.
> */
> - vma->vm_flags = new_flags;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags, new_flags);
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> + vm_write_end(vma);
>
> skip:
> prev = vma;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b7e2268dfc9a..32314e9e48dd 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1006,6 +1006,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> if (mm_find_pmd(mm, address) != pmd)
> goto out;
>
> + vm_write_begin(vma);
> anon_vma_lock_write(vma->anon_vma);
>
> pte = pte_offset_map(pmd, address);
> @@ -1041,6 +1042,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> pmd_populate(mm, pmd, pmd_pgtable(_pmd));
> spin_unlock(pmd_ptl);
> anon_vma_unlock_write(vma->anon_vma);
> + vm_write_end(vma);
> result = SCAN_FAIL;
> goto out;
> }
> @@ -1075,6 +1077,7 @@ static void collapse_huge_page(struct mm_struct *mm,
> set_pmd_at(mm, address, pmd, _pmd);
> update_mmu_cache_pmd(vma, address, pmd);
> spin_unlock(pmd_ptl);
> + vm_write_end(vma);
>
> *hpage = NULL;
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 4d3c922ea1a1..e328f7ab5942 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -184,7 +184,9 @@ static long madvise_behavior(struct vm_area_struct *vma,
> /*
> * vm_flags is protected by the mmap_sem held in write mode.
> */
> - vma->vm_flags = new_flags;
> + vm_write_begin(vma);
> + WRITE_ONCE(vma->vm_flags, new_flags);
> + vm_write_end(vma);
> out:
> return error;
> }
> @@ -450,9 +452,11 @@ static void madvise_free_page_range(struct mmu_gather *tlb,
> .private = tlb,
> };
>
> + vm_write_begin(vma);
> tlb_start_vma(tlb, vma);
> walk_page_range(addr, end, &free_walk);
> tlb_end_vma(tlb, vma);
> + vm_write_end(vma);
> }
>
> static int madvise_free_single_vma(struct vm_area_struct *vma,
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index e0e706f0b34e..2632c6f93b63 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -380,8 +380,11 @@ void mpol_rebind_mm(struct mm_struct *mm, nodemask_t *new)
> struct vm_area_struct *vma;
>
> down_write(&mm->mmap_sem);
> - for (vma = mm->mmap; vma; vma = vma->vm_next)
> + for (vma = mm->mmap; vma; vma = vma->vm_next) {
> + vm_write_begin(vma);
> mpol_rebind_policy(vma->vm_policy, new);
> + vm_write_end(vma);
> + }
> up_write(&mm->mmap_sem);
> }
>
> @@ -554,9 +557,11 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
> {
> int nr_updated;
>
> + vm_write_begin(vma);
> nr_updated = change_protection(vma, addr, end, PAGE_NONE, 0, 1);
> if (nr_updated)
> count_vm_numa_events(NUMA_PTE_UPDATES, nr_updated);
> + vm_write_end(vma);
>
> return nr_updated;
> }
> @@ -657,6 +662,7 @@ static int vma_replace_policy(struct vm_area_struct *vma,
> if (IS_ERR(new))
> return PTR_ERR(new);
>
> + vm_write_begin(vma);
> if (vma->vm_ops && vma->vm_ops->set_policy) {
> err = vma->vm_ops->set_policy(vma, new);
> if (err)
> @@ -664,11 +670,17 @@ static int vma_replace_policy(struct vm_area_struct *vma,
> }
>
> old = vma->vm_policy;
> - vma->vm_policy = new; /* protected by mmap_sem */
> + /*
> + * The speculative page fault handler access this field without
> + * hodling the mmap_sem.
> + */

"The speculative page fault handler accesses this field without holding
vma->vm_mm->mmap_sem"

> + WRITE_ONCE(vma->vm_policy, new);
> + vm_write_end(vma);
> mpol_put(old);
>
> return 0;
> err_out:
> + vm_write_end(vma);
> mpol_put(new);
> return err;
> }

Wait, doesn't vma_dup_policy() also need to protect dst->vm_policy?

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2121,7 +2121,9 @@ int vma_dup_policy(struct vm_area_struct *src, struct vm_area_struct *dst)

if (IS_ERR(pol))
return PTR_ERR(pol);
- dst->vm_policy = pol;
+ vm_write_begin(dst);
+ WRITE_ONCE(dst->vm_policy, pol);
+ vm_write_end(dst);
return 0;
}

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