Re: possible deadlock in mon_bin_vma_fault

[Pete Zaitcev <zaitcev@redhat.com>]

On Thu, 21 Nov 2019 11:20:20 -0500 (EST)
Alan Stern <stern@rowland.harvard.edu> 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@kotori.zaitcev.us>
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@redhat.com>

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;
 }