Re: possible deadlock in mon_bin_vma_fault

[Dmitry Vyukov]

On Fri, Nov 22, 2019 at 12:38 AM Pete Zaitcev <zaitcev@xxxxxxxxxx> wrote:
>
> On Thu, 21 Nov 2019 11:20:20 -0500 (EST)
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> > On Thu, 21 Nov 2019, Pete Zaitcev wrote:
> >
> > > Anyway... If you are looking at it too, what do you think about not using
> > > any locks in mon_bin_vma_fault() at all? Isn't it valid? I think I tried
> > > to be "safe", but it only uses things that are constants unless we're
> > > opening and closing; a process cannot make page faults unless it has
> > > some thing mapped; and that is only possible if device is open and stays
> > > open. Can you find a hole in this reasoning?
> >
> > I think you're right. [...]
>
> How about the appended patch, then? You like?
>
> Do you happen to know how to refer to a syzbot report in a commit message?
>
> -- Pete
>
> commit 628f3bbf37eee21cce4cfbfaa6a796b129d7736d
> Author: Pete Zaitcev <zaitcev@xxxxxxxxxxxxxxxxx>
> Date:   Thu Nov 21 17:24:00 2019 -0600
>
>     usb: Fix a deadlock in usbmon between mmap and read
>
>     Signed-off-by: Pete Zaitcev <zaitcev@xxxxxxxxxx>

/\/\/\/\/\/\/\/\/\/\

Please don't forget the Reported-by

> diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
> index ac2b4fcc265f..fb7df9810bad 100644
> --- a/drivers/usb/mon/mon_bin.c
> +++ b/drivers/usb/mon/mon_bin.c
> @@ -1039,12 +1039,18 @@ static long mon_bin_ioctl(struct file *file, unsigned int cmd, unsigned long arg
>
>                 mutex_lock(&rp->fetch_lock);
>                 spin_lock_irqsave(&rp->b_lock, flags);
> -               mon_free_buff(rp->b_vec, rp->b_size/CHUNK_SIZE);
> -               kfree(rp->b_vec);
> -               rp->b_vec  = vec;
> -               rp->b_size = size;
> -               rp->b_read = rp->b_in = rp->b_out = rp->b_cnt = 0;
> -               rp->cnt_lost = 0;
> +               if (rp->mmap_active) {
> +                       mon_free_buff(vec, size/CHUNK_SIZE);
> +                       kfree(vec);
> +                       ret = -EBUSY;
> +               } else {
> +                       mon_free_buff(rp->b_vec, rp->b_size/CHUNK_SIZE);
> +                       kfree(rp->b_vec);
> +                       rp->b_vec  = vec;
> +                       rp->b_size = size;
> +                       rp->b_read = rp->b_in = rp->b_out = rp->b_cnt = 0;
> +                       rp->cnt_lost = 0;
> +               }
>                 spin_unlock_irqrestore(&rp->b_lock, flags);
>                 mutex_unlock(&rp->fetch_lock);
>                 }
> @@ -1093,11 +1099,11 @@ static long mon_bin_ioctl(struct file *file, unsigned int cmd, unsigned long arg
>                         return ret;
>                 if (put_user(ret, &uptr->nfetch))
>                         return -EFAULT;
> -               ret = 0;
>                 }
>                 break;
>
> -       case MON_IOCG_STATS: {
> +       case MON_IOCG_STATS:
> +               {
>                 struct mon_bin_stats __user *sp;
>                 unsigned int nevents;
>                 unsigned int ndropped;
> @@ -1113,7 +1119,6 @@ static long mon_bin_ioctl(struct file *file, unsigned int cmd, unsigned long arg
>                         return -EFAULT;
>                 if (put_user(nevents, &sp->queued))
>                         return -EFAULT;
> -
>                 }
>                 break;
>
> @@ -1216,13 +1221,21 @@ mon_bin_poll(struct file *file, struct poll_table_struct *wait)
>  static void mon_bin_vma_open(struct vm_area_struct *vma)
>  {
>         struct mon_reader_bin *rp = vma->vm_private_data;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(&rp->b_lock, flags);
>         rp->mmap_active++;
> +       spin_unlock_irqrestore(&rp->b_lock, flags);
>  }
>
>  static void mon_bin_vma_close(struct vm_area_struct *vma)
>  {
> +       unsigned long flags;
> +
>         struct mon_reader_bin *rp = vma->vm_private_data;
> +       spin_lock_irqsave(&rp->b_lock, flags);
>         rp->mmap_active--;
> +       spin_unlock_irqrestore(&rp->b_lock, flags);
>  }
>
>  /*
> @@ -1234,16 +1247,12 @@ static vm_fault_t mon_bin_vma_fault(struct vm_fault *vmf)
>         unsigned long offset, chunk_idx;
>         struct page *pageptr;
>
> -       mutex_lock(&rp->fetch_lock);
>         offset = vmf->pgoff << PAGE_SHIFT;
> -       if (offset >= rp->b_size) {
> -               mutex_unlock(&rp->fetch_lock);
> +       if (offset >= rp->b_size)
>                 return VM_FAULT_SIGBUS;
> -       }
>         chunk_idx = offset / CHUNK_SIZE;
>         pageptr = rp->b_vec[chunk_idx].pg;
>         get_page(pageptr);
> -       mutex_unlock(&rp->fetch_lock);
>         vmf->page = pageptr;
>         return 0;
>  }
>
> --
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