Re: [PATCH v9 08/24] mm: Protect VMA modifications using VMA sequence count

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

On 27/03/2018 23:45, David Rientjes wrote:
> 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.

That's true in this particular case, and I could change that to not include the
change to vm_userfaultfd_ctx.
This being said, I don't think this will have a major impact, but I'll make a
close review on this patch to be sure there is too large protected part of code.

>> @@ -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"

Oops :/

> 
>> +	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?

Indeed this is not necessary because vma_dup_policy() is called when dst is not
yet linked in the RB tree, so it can't be found by the speculative page fault
handler. This is not the case of vma_replace_policy, and this is why the
protection are needed here.

> 
> 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;
>  }