Re: [RFC PATCH 01/11] mm/hmm: Fix use after free with struct hmm in the mmu notifiers

[Ira Weiny <ira.weiny@intel.com>]

On Thu, May 23, 2019 at 12:34:26PM -0300, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@mellanox.com>
> 
> mmu_notifier_unregister_no_release() is not a fence and the mmu_notifier
> system will continue to reference hmm->mn until the srcu grace period
> expires.
> 
> Resulting in use after free races like this:
> 
>          CPU0                                     CPU1
>                                                __mmu_notifier_invalidate_range_start()
>                                                  srcu_read_lock
>                                                  hlist_for_each ()
>                                                    // mn == hmm->mn
> hmm_mirror_unregister()
>   hmm_put()
>     hmm_free()
>       mmu_notifier_unregister_no_release()
>          hlist_del_init_rcu(hmm-mn->list)
> 			                           mn->ops->invalidate_range_start(mn, range);
> 					             mm_get_hmm()
>       mm->hmm = NULL;
>       kfree(hmm)
>                                                      mutex_lock(&hmm->lock);
> 
> Use SRCU to kfree the hmm memory so that the notifiers can rely on hmm
> existing. Get the now-safe hmm struct through container_of and directly
> check kref_get_unless_zero to lock it against free.
> 
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> ---
>  include/linux/hmm.h |  1 +
>  mm/hmm.c            | 25 +++++++++++++++++++------
>  2 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index 51ec27a8466816..8b91c90d3b88cb 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -102,6 +102,7 @@ struct hmm {
>  	struct mmu_notifier	mmu_notifier;
>  	struct rw_semaphore	mirrors_sem;
>  	wait_queue_head_t	wq;
> +	struct rcu_head		rcu;
>  	long			notifiers;
>  	bool			dead;
>  };
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 816c2356f2449f..824e7e160d8167 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -113,6 +113,11 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
>  	return NULL;
>  }
>  
> +static void hmm_fee_rcu(struct rcu_head *rcu)

NIT: "free"

Other than that looks good.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> +{
> +	kfree(container_of(rcu, struct hmm, rcu));
> +}
> +
>  static void hmm_free(struct kref *kref)
>  {
>  	struct hmm *hmm = container_of(kref, struct hmm, kref);
> @@ -125,7 +130,7 @@ static void hmm_free(struct kref *kref)
>  		mm->hmm = NULL;
>  	spin_unlock(&mm->page_table_lock);
>  
> -	kfree(hmm);
> +	mmu_notifier_call_srcu(&hmm->rcu, hmm_fee_rcu);
>  }
>  
>  static inline void hmm_put(struct hmm *hmm)
> @@ -153,10 +158,14 @@ void hmm_mm_destroy(struct mm_struct *mm)
>  
>  static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
>  {
> -	struct hmm *hmm = mm_get_hmm(mm);
> +	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
>  	struct hmm_mirror *mirror;
>  	struct hmm_range *range;
>  
> +	/* hmm is in progress to free */
> +	if (!kref_get_unless_zero(&hmm->kref))
> +		return;
> +
>  	/* Report this HMM as dying. */
>  	hmm->dead = true;
>  
> @@ -194,13 +203,15 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
>  static int hmm_invalidate_range_start(struct mmu_notifier *mn,
>  			const struct mmu_notifier_range *nrange)
>  {
> -	struct hmm *hmm = mm_get_hmm(nrange->mm);
> +	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
>  	struct hmm_mirror *mirror;
>  	struct hmm_update update;
>  	struct hmm_range *range;
>  	int ret = 0;
>  
> -	VM_BUG_ON(!hmm);
> +	/* hmm is in progress to free */
> +	if (!kref_get_unless_zero(&hmm->kref))
> +		return 0;
>  
>  	update.start = nrange->start;
>  	update.end = nrange->end;
> @@ -248,9 +259,11 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
>  static void hmm_invalidate_range_end(struct mmu_notifier *mn,
>  			const struct mmu_notifier_range *nrange)
>  {
> -	struct hmm *hmm = mm_get_hmm(nrange->mm);
> +	struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
>  
> -	VM_BUG_ON(!hmm);
> +	/* hmm is in progress to free */
> +	if (!kref_get_unless_zero(&hmm->kref))
> +		return;
>  
>  	mutex_lock(&hmm->lock);
>  	hmm->notifiers--;
> -- 
> 2.21.0
>