linux-kernel - Re: [PATCH 1/1] userfaultfd: clear the vma->vm_userfaultfd_ctx if UFFD_EVENT

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Date:   Mon, 25 Dec 2017 11:07:27 +0200
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Eric Biggers <ebiggers3@...il.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        viro@...iv.linux.org.uk, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH 1/1] userfaultfd: clear the vma->vm_userfaultfd_ctx if
 UFFD_EVENT_FORK fails

On Sat, Dec 23, 2017 at 01:25:05AM +0100, Andrea Arcangeli wrote:
> The previous fix 384632e67e0829deb8015ee6ad916b180049d252 corrected
> the refcounting in case of UFFD_EVENT_FORK failure for the fork
> userfault paths. That still didn't clear the vma->vm_userfaultfd_ctx
> of the vmas that were set to point to the aborted new uffd ctx earlier
> in dup_userfaultfd.
> 
> Cc: stable@...r.kernel.org
> Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>

Reviewed-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>

> ---
>  fs/userfaultfd.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 896f810b6a06..1a88916455bd 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -591,11 +591,14 @@ int handle_userfault(struct vm_fault *vmf, unsigned long reason)
>  static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>  					      struct userfaultfd_wait_queue *ewq)
>  {
> +	struct userfaultfd_ctx *release_new_ctx;

Nit: we could have set release_new_ctx to NULL here...
> +
>  	if (WARN_ON_ONCE(current->flags & PF_EXITING))
>  		goto out;
> 
>  	ewq->ctx = ctx;
>  	init_waitqueue_entry(&ewq->wq, current);
> +	release_new_ctx = NULL;
> 
>  	spin_lock(&ctx->event_wqh.lock);
>  	/*
> @@ -622,8 +625,7 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>  				new = (struct userfaultfd_ctx *)
>  					(unsigned long)
>  					ewq->msg.arg.reserved.reserved1;
> -
> -				userfaultfd_ctx_put(new);
> +				release_new_ctx = new;
>  			}
>  			break;
>  		}
> @@ -638,6 +640,20 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
>  	__set_current_state(TASK_RUNNING);
>  	spin_unlock(&ctx->event_wqh.lock);
> 
> +	if (release_new_ctx) {
> +		struct vm_area_struct *vma;
> +		struct mm_struct *mm = release_new_ctx->mm;
> +
> +		/* the various vma->vm_userfaultfd_ctx still points to it */
> +		down_write(&mm->mmap_sem);
> +		for (vma = mm->mmap; vma; vma = vma->vm_next)
> +			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx)
> +				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> +		up_write(&mm->mmap_sem);
> +
> +		userfaultfd_ctx_put(release_new_ctx);
> +	}
> +
>  	/*
>  	 * ctx may go away after this if the userfault pseudo fd is
>  	 * already released.
> 

-- 
Sincerely yours,
Mike.