Re: [PATCH v12 11/31] mm: protect mremap() against SPF hanlder

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

Le 22/04/2019 à 21:51, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:45:02PM +0200, Laurent Dufour wrote:
>> If a thread is remapping an area while another one is faulting on the
>> destination area, the SPF handler may fetch the vma from the RB tree before
>> the pte has been moved by the other thread. This means that the moved ptes
>> will overwrite those create by the page fault handler leading to page
>> leaked.
>>
>> 	CPU 1				CPU2
>> 	enter mremap()
>> 	unmap the dest area
>> 	copy_vma()			Enter speculative page fault handler
>> 	   >> at this time the dest area is present in the RB tree
>> 					fetch the vma matching dest area
>> 					create a pte as the VMA matched
>> 					Exit the SPF handler
>> 					<data written in the new page>
>> 	move_ptes()
>> 	  > it is assumed that the dest area is empty,
>>   	  > the move ptes overwrite the page mapped by the CPU2.
>>
>> To prevent that, when the VMA matching the dest area is extended or created
>> by copy_vma(), it should be marked as non available to the SPF handler.
>> The usual way to so is to rely on vm_write_begin()/end().
>> This is already in __vma_adjust() called by copy_vma() (through
>> vma_merge()). But __vma_adjust() is calling vm_write_end() before returning
>> which create a window for another thread.
>> This patch adds a new parameter to vma_merge() which is passed down to
>> vma_adjust().
>> The assumption is that copy_vma() is returning a vma which should be
>> released by calling vm_raw_write_end() by the callee once the ptes have
>> been moved.
>>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> 
> Small comment about a comment below but can be fix as a fixup
> patch nothing earth shattering.
> 
>> ---
>>   include/linux/mm.h | 24 ++++++++++++++++-----
>>   mm/mmap.c          | 53 +++++++++++++++++++++++++++++++++++-----------
>>   mm/mremap.c        | 13 ++++++++++++
>>   3 files changed, 73 insertions(+), 17 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 906b9e06f18e..5d45b7d8718d 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2343,18 +2343,32 @@ void anon_vma_interval_tree_verify(struct anon_vma_chain *node);
>>   
>>   /* mmap.c */
>>   extern int __vm_enough_memory(struct mm_struct *mm, long pages, int cap_sys_admin);
>> +
>>   extern int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   	unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
>> -	struct vm_area_struct *expand);
>> +	struct vm_area_struct *expand, bool keep_locked);
>> +
>>   static inline int vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   	unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
>>   {
>> -	return __vma_adjust(vma, start, end, pgoff, insert, NULL);
>> +	return __vma_adjust(vma, start, end, pgoff, insert, NULL, false);
>>   }
>> -extern struct vm_area_struct *vma_merge(struct mm_struct *,
>> +
>> +extern struct vm_area_struct *__vma_merge(struct mm_struct *mm,
>> +	struct vm_area_struct *prev, unsigned long addr, unsigned long end,
>> +	unsigned long vm_flags, struct anon_vma *anon, struct file *file,
>> +	pgoff_t pgoff, struct mempolicy *mpol,
>> +	struct vm_userfaultfd_ctx uff, bool keep_locked);
>> +
>> +static inline struct vm_area_struct *vma_merge(struct mm_struct *mm,
>>   	struct vm_area_struct *prev, unsigned long addr, unsigned long end,
>> -	unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
>> -	struct mempolicy *, struct vm_userfaultfd_ctx);
>> +	unsigned long vm_flags, struct anon_vma *anon, struct file *file,
>> +	pgoff_t off, struct mempolicy *pol, struct vm_userfaultfd_ctx uff)
>> +{
>> +	return __vma_merge(mm, prev, addr, end, vm_flags, anon, file, off,
>> +			   pol, uff, false);
>> +}
>> +
>>   extern struct anon_vma *find_mergeable_anon_vma(struct vm_area_struct *);
>>   extern int __split_vma(struct mm_struct *, struct vm_area_struct *,
>>   	unsigned long addr, int new_below);
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index b77ec0149249..13460b38b0fb 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -714,7 +714,7 @@ static inline void __vma_unlink_prev(struct mm_struct *mm,
>>    */
>>   int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   	unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert,
>> -	struct vm_area_struct *expand)
>> +	struct vm_area_struct *expand, bool keep_locked)
>>   {
>>   	struct mm_struct *mm = vma->vm_mm;
>>   	struct vm_area_struct *next = vma->vm_next, *orig_vma = vma;
>> @@ -830,8 +830,12 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   
>>   			importer->anon_vma = exporter->anon_vma;
>>   			error = anon_vma_clone(importer, exporter);
>> -			if (error)
>> +			if (error) {
>> +				if (next && next != vma)
>> +					vm_raw_write_end(next);
>> +				vm_raw_write_end(vma);
>>   				return error;
>> +			}
>>   		}
>>   	}
>>   again:
>> @@ -1025,7 +1029,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>   
>>   	if (next && next != vma)
>>   		vm_raw_write_end(next);
>> -	vm_raw_write_end(vma);
>> +	if (!keep_locked)
>> +		vm_raw_write_end(vma);
>>   
>>   	validate_mm(mm);
>>   
>> @@ -1161,12 +1166,13 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
>>    * parameter) may establish ptes with the wrong permissions of NNNN
>>    * instead of the right permissions of XXXX.
>>    */
>> -struct vm_area_struct *vma_merge(struct mm_struct *mm,
>> +struct vm_area_struct *__vma_merge(struct mm_struct *mm,
>>   			struct vm_area_struct *prev, unsigned long addr,
>>   			unsigned long end, unsigned long vm_flags,
>>   			struct anon_vma *anon_vma, struct file *file,
>>   			pgoff_t pgoff, struct mempolicy *policy,
>> -			struct vm_userfaultfd_ctx vm_userfaultfd_ctx)
>> +			struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
>> +			bool keep_locked)
>>   {
>>   	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
>>   	struct vm_area_struct *area, *next;
>> @@ -1214,10 +1220,11 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
>>   							/* cases 1, 6 */
>>   			err = __vma_adjust(prev, prev->vm_start,
>>   					 next->vm_end, prev->vm_pgoff, NULL,
>> -					 prev);
>> +					 prev, keep_locked);
>>   		} else					/* cases 2, 5, 7 */
>>   			err = __vma_adjust(prev, prev->vm_start,
>> -					 end, prev->vm_pgoff, NULL, prev);
>> +					   end, prev->vm_pgoff, NULL, prev,
>> +					   keep_locked);
>>   		if (err)
>>   			return NULL;
>>   		khugepaged_enter_vma_merge(prev, vm_flags);
>> @@ -1234,10 +1241,12 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
>>   					     vm_userfaultfd_ctx)) {
>>   		if (prev && addr < prev->vm_end)	/* case 4 */
>>   			err = __vma_adjust(prev, prev->vm_start,
>> -					 addr, prev->vm_pgoff, NULL, next);
>> +					 addr, prev->vm_pgoff, NULL, next,
>> +					 keep_locked);
>>   		else {					/* cases 3, 8 */
>>   			err = __vma_adjust(area, addr, next->vm_end,
>> -					 next->vm_pgoff - pglen, NULL, next);
>> +					 next->vm_pgoff - pglen, NULL, next,
>> +					 keep_locked);
>>   			/*
>>   			 * In case 3 area is already equal to next and
>>   			 * this is a noop, but in case 8 "area" has
>> @@ -3259,9 +3268,20 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>>   
>>   	if (find_vma_links(mm, addr, addr + len, &prev, &rb_link, &rb_parent))
>>   		return NULL;	/* should never get here */
>> -	new_vma = vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
>> -			    vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
>> -			    vma->vm_userfaultfd_ctx);
>> +
>> +	/* There is 3 cases to manage here in
>> +	 *     AAAA            AAAA              AAAA              AAAA
>> +	 * PPPP....      PPPP......NNNN      PPPP....NNNN      PP........NN
>> +	 * PPPPPPPP(A)   PPPP..NNNNNNNN(B)   PPPPPPPPPPPP(1)       NULL
>> +	 *                                   PPPPPPPPNNNN(2)
>> +	 *                                   PPPPNNNNNNNN(3)
>> +	 *
>> +	 * new_vma == prev in case A,1,2
>> +	 * new_vma == next in case B,3
>> +	 */
>> +	new_vma = __vma_merge(mm, prev, addr, addr + len, vma->vm_flags,
>> +			      vma->anon_vma, vma->vm_file, pgoff,
>> +			      vma_policy(vma), vma->vm_userfaultfd_ctx, true);
>>   	if (new_vma) {
>>   		/*
>>   		 * Source vma may have been merged into new_vma
>> @@ -3299,6 +3319,15 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>>   			get_file(new_vma->vm_file);
>>   		if (new_vma->vm_ops && new_vma->vm_ops->open)
>>   			new_vma->vm_ops->open(new_vma);
>> +		/*
>> +		 * As the VMA is linked right now, it may be hit by the
>> +		 * speculative page fault handler. But we don't want it to
>> +		 * to start mapping page in this area until the caller has
>> +		 * potentially move the pte from the moved VMA. To prevent
>> +		 * that we protect it right now, and let the caller unprotect
>> +		 * it once the move is done.
>> +		 */
> 
> It would be better to say:
> 		/*
> 		 * Block speculative page fault on the new VMA before "linking" it as
> 		 * as once it is linked then it may be hit by speculative page fault.
> 		 * But we don't want it to start mapping page in this area until the
> 		 * caller has potentially move the pte from the moved VMA. To prevent
> 		 * that we protect it before linking and let the caller unprotect it
> 		 * once the move is done.
> 		 */
>    

I'm fine with your proposal.

Thanks for reviewing this.


>> +		vm_raw_write_begin(new_vma);
>>   		vma_link(mm, new_vma, prev, rb_link, rb_parent);
>>   		*need_rmap_locks = false;
>>   	}
>> diff --git a/mm/mremap.c b/mm/mremap.c
>> index fc241d23cd97..ae5c3379586e 100644
>> --- a/mm/mremap.c
>> +++ b/mm/mremap.c
>> @@ -357,6 +357,14 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>>   	if (!new_vma)
>>   		return -ENOMEM;
>>   
>> +	/* new_vma is returned protected by copy_vma, to prevent speculative
>> +	 * page fault to be done in the destination area before we move the pte.
>> +	 * Now, we must also protect the source VMA since we don't want pages
>> +	 * to be mapped in our back while we are copying the PTEs.
>> +	 */
>> +	if (vma != new_vma)
>> +		vm_raw_write_begin(vma);
>> +
>>   	moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
>>   				     need_rmap_locks);
>>   	if (moved_len < old_len) {
>> @@ -373,6 +381,8 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>>   		 */
>>   		move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
>>   				 true);
>> +		if (vma != new_vma)
>> +			vm_raw_write_end(vma);
>>   		vma = new_vma;
>>   		old_len = new_len;
>>   		old_addr = new_addr;
>> @@ -381,7 +391,10 @@ static unsigned long move_vma(struct vm_area_struct *vma,
>>   		mremap_userfaultfd_prep(new_vma, uf);
>>   		arch_remap(mm, old_addr, old_addr + old_len,
>>   			   new_addr, new_addr + new_len);
>> +		if (vma != new_vma)
>> +			vm_raw_write_end(vma);
>>   	}
>> +	vm_raw_write_end(new_vma);
>>   
>>   	/* Conceal VM_ACCOUNT so old reservation is not undone */
>>   	if (vm_flags & VM_ACCOUNT) {
>> -- 
>> 2.21.0
>>
>