Re: [PATCH v10 09/25] mm: protect VMA modifications using VMA sequence count

[Laurent Dufour]

On 23/04/2018 09:19, Minchan Kim wrote:
> On Tue, Apr 17, 2018 at 04:33:15PM +0200, Laurent Dufour wrote:
>> The VMA sequence count has been introduced to allow fast detection of
>> VMA modification when running a page fault handler without holding
>> the mmap_sem.
>>
>> This patch provides protection against the VMA modification done in :
>> 	- madvise()
>> 	- mpol_rebind_policy()
>> 	- vma_replace_policy()
>> 	- change_prot_numa()
>> 	- mlock(), munlock()
>> 	- mprotect()
>> 	- mmap_region()
>> 	- collapse_huge_page()
>> 	- userfaultd registering services
>>
>> In addition, VMA fields which will be read during the speculative fault
>> path needs to be written using WRITE_ONCE to prevent write to be split
>> and intermediate values to be pushed to other CPUs.
>>
>> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx>
>> ---
>>  fs/proc/task_mmu.c |  5 ++++-
>>  fs/userfaultfd.c   | 17 +++++++++++++----
>>  mm/khugepaged.c    |  3 +++
>>  mm/madvise.c       |  6 +++++-
>>  mm/mempolicy.c     | 51 ++++++++++++++++++++++++++++++++++-----------------
>>  mm/mlock.c         | 13 ++++++++-----
>>  mm/mmap.c          | 22 +++++++++++++---------
>>  mm/mprotect.c      |  4 +++-
>>  mm/swap_state.c    |  8 ++++++--
>>  9 files changed, 89 insertions(+), 40 deletions(-)
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index c486ad4b43f0..aeb417f28839 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);
> 
> trivial:
> 
> I think It's tricky to maintain that VMA fields to be read during SPF should be
> (READ|WRITE_ONCE). I think we need some accessor to read/write them rather than
> raw accessing like like vma_set_page_prot. Maybe spf prefix would be helpful. 
> 
> 	vma_spf_set_value(vma, vm_flags, val);
> 
> We also add some markers in vm_area_struct's fileds to indicate that
> people shouldn't access those fields directly.
> 
> Just a thought.

At the beginning I was liking that idea but...

I'm not sure this will change a lot the code, most of the time the
vm_write_begin()/end() are surrounding part of code larger than one VMA
structure's field change. For this particular case and few others this will be
applicable but that's not the majority.

Thanks,
Laurent.

> 
> 
>>  				}
>>  				downgrade_write(&mm->mmap_sem);
> 
> 
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index fe079756bb18..8a8a402ed59f 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -575,6 +575,10 @@ static unsigned long swapin_nr_pages(unsigned long offset)
>>   * the readahead.
>>   *
>>   * Caller must hold down_read on the vma->vm_mm if vmf->vma is not NULL.
>> + * This is needed to ensure the VMA will not be freed in our back. In the case
>> + * of the speculative page fault handler, this cannot happen, even if we don't
>> + * hold the mmap_sem. Callees are assumed to take care of reading VMA's fields
> 
> I guess reader would be curious on *why* is safe with SPF.
> Comment about the why could be helpful for reviewer.
> 
>> + * using READ_ONCE() to read consistent values.
>>   */
>>  struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>>  				struct vm_fault *vmf)
>> @@ -668,9 +672,9 @@ static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
>>  				     unsigned long *start,
>>  				     unsigned long *end)
>>  {
>> -	*start = max3(lpfn, PFN_DOWN(vma->vm_start),
>> +	*start = max3(lpfn, PFN_DOWN(READ_ONCE(vma->vm_start)),
>>  		      PFN_DOWN(faddr & PMD_MASK));
>> -	*end = min3(rpfn, PFN_DOWN(vma->vm_end),
>> +	*end = min3(rpfn, PFN_DOWN(READ_ONCE(vma->vm_end)),
>>  		    PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
>>  }
>>  
>> -- 
>> 2.7.4
>>
>