Re: Re: possible deadlock in mon_bin_vma_fault

[Alan Stern]

On Fri, 22 Nov 2019, syzbot wrote:

> > #syz test: linux-4.19.y f6e27dbb1afa
> 
> "linux-4.19.y" does not look like a valid git repo address.

Let's try again.  The "git tree" value in the original bug report was
"upstream", so I'll use that even though it doesn't look like a valid 
git repo address either.

Alan Stern

#syz test: upstream f6e27dbb1afa

commit 5252eb4c8297fedbf1c5f1e67da44efe00e6ef6b
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>
     Reported-by: syzbot+56f9673bb4cdcbeb0e92@xxxxxxxxxxxxxxxxxxxxxxxxx

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index ac2b4fcc265f..f48a23adbc35 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);
  		}
@@ -1216,13 +1222,21 @@ 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 +1248,12 @@ 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;
  }