Re: [PATCH] hugetlb: fix memory leak associated with vma_lock structure

[Mike Kravetz]

On 10/19/22 16:16, Miaohe Lin wrote:
> On 2022/10/19 7:36, Mike Kravetz wrote:
> > The hugetlb vma_lock structure hangs off the vm_private_data pointer
> > of sharable hugetlb vmas.  The structure is vma specific and can not
> > be shared between vmas.  At fork and various other times, vmas are
> > duplicated via vm_area_dup().  When this happens, the pointer in the
> > newly created vma must be cleared and the structure reallocated.  Two
> > hugetlb specific routines deal with this hugetlb_dup_vma_private and
> > hugetlb_vm_op_open.  Both routines are called for newly created vmas.
> > hugetlb_dup_vma_private would always clear the pointer and
> > hugetlb_vm_op_open would allocate the new vms_lock structure.  This did
> > not work in the case of this calling sequence pointed out in [1].
> >   move_vma
> >     copy_vma
> >       new_vma = vm_area_dup(vma);
> >       new_vma->vm_ops->open(new_vma); --> new_vma has its own vma lock.
> >     is_vm_hugetlb_page(vma)
> >       clear_vma_resv_huge_pages
> >         hugetlb_dup_vma_private --> vma->vm_private_data is set to NULL
> > When clearing hugetlb_dup_vma_private we actually leak the associated
> > vma_lock structure.
> > 
> > The vma_lock structure contains a pointer to the associated vma.  This
> > information can be used in hugetlb_dup_vma_private and hugetlb_vm_op_open
> > to ensure we only clear the vm_private_data of newly created (copied)
> > vmas.  In such cases, the vma->vma_lock->vma field will not point to the
> > vma.
> > 
> > Update hugetlb_dup_vma_private and hugetlb_vm_op_open to not clear
> > vm_private_data if vma->vma_lock->vma == vma.  Also, log a warning if
> > hugetlb_vm_op_open ever encounters the case where vma_lock has already
> > been correctly allocated for the vma.
> > 
> > [1] https://lore.kernel.org/linux-mm/5154292a-4c55-28cd-0935-82441e512fc3@xxxxxxxxxx/
> > 
> > Fixes: 131a79b474e9 ("hugetlb: fix vma lock handling during split vma and range unmapping")
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 31 ++++++++++++++++++++++++-------
> >  1 file changed, 24 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 02f781624fce..7f74cbff6619 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1014,15 +1014,23 @@ void hugetlb_dup_vma_private(struct vm_area_struct *vma)
> >  	VM_BUG_ON_VMA(!is_vm_hugetlb_page(vma), vma);
> >  	/*
> >  	 * Clear vm_private_data
> > +	 * - For shared mappings this is a per-vma semaphore that may be
> > +	 *   allocated in a subsequent call to hugetlb_vm_op_open.
> > +	 *   Before clearing, make sure pointer is not associated with vma
> > +	 *   as this will leak the structure.  This is the case when called
> > +	 *   via clear_vma_resv_huge_pages() and hugetlb_vm_op_open has already
> > +	 *   been called to allocate a new structure.
> >  	 * - For MAP_PRIVATE mappings, this is the reserve map which does
> >  	 *   not apply to children.  Faults generated by the children are
> >  	 *   not guaranteed to succeed, even if read-only.
> > -	 * - For shared mappings this is a per-vma semaphore that may be
> > -	 *   allocated in a subsequent call to hugetlb_vm_op_open.
> >  	 */
> > -	vma->vm_private_data = (void *)0;
> > -	if (!(vma->vm_flags & VM_MAYSHARE))
> > -		return;
> > +	if (vma->vm_flags & VM_MAYSHARE) {
> > +		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> > +
> > +		if (vma_lock && vma_lock->vma != vma)
> > +			vma->vm_private_data = NULL;
> > +	} else
> > +		vma->vm_private_data = NULL;
> >  }
> >  
> >  /*
> > @@ -4601,6 +4609,7 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
> >  	struct resv_map *resv = vma_resv_map(vma);
> >  
> >  	/*
> > +	 * HPAGE_RESV_OWNER indicates a private mapping.
> >  	 * This new VMA should share its siblings reservation map if present.
> >  	 * The VMA will only ever have a valid reservation map pointer where
> >  	 * it is being copied for another still existing VMA.  As that VMA
> > @@ -4616,10 +4625,18 @@ static void hugetlb_vm_op_open(struct vm_area_struct *vma)
> >  	/*
> >  	 * vma_lock structure for sharable mappings is vma specific.
> >  	 * Clear old pointer (if copied via vm_area_dup) and create new.
> > +	 * Before clearing, make sure vma_lock is not for this vma.
> >  	 */
> >  	if (vma->vm_flags & VM_MAYSHARE) {
> > -		vma->vm_private_data = NULL;
> > -		hugetlb_vma_lock_alloc(vma);
> > +		struct hugetlb_vma_lock *vma_lock = vma->vm_private_data;
> > +
> > +		if (vma_lock) {
> 
> Thanks Mike. It seems the case of "vma_lock == NULL" is missed, i.e. if vma->vm_private_data == NULL,
> hugetlb_vm_op_open won't allocate a new vma lock?

Thank you so much!  Yes, you are correct.

Your review comments have prevented numerous bugs and led to better code.

I will send v2 shortly.
-- 
Mike Kravetz