Re: possible deadlock in mon_bin_vma_fault

[Alan Stern <stern@rowland.harvard.edu>]

On Fri, 22 Nov 2019, Pete Zaitcev wrote:

> > It would be more elegant to do the rp->mmap_active test before calling
> > kcalloc and mon_alloc_buf.  But of course that's a pretty minor thing.
> 
> Indeed it feels wrong that so much work gets discarded. However, memory
> allocations can block, right? In the same time, our main objective here is
> to make sure that when a page fault happens, we fill in the page that VMA
> is intended to refer, and not one that was re-allocated. Therefore, I'm
> trying to avoid a situation where:
> 
> 1. thread A checks mmap_active, finds it at zero and proceeds into the
> reallocation ioctl
> 2. thread A sleeps in get_free_page()
> 3. thread B runs mmap() and succeeds
> 4. thread A obtains its pages and proceeds to substitute the buffer
> 5. thread B (or any other) pagefaults and ends with the new, unexpected page
> 
> The code is not pretty, but I don't see an alternative. Heck, I would
> love you to find more races if you can.

The alternative is to have the routines for mmap() hold fetch_lock
instead of b_lock.  mmap() is allowed to sleep, so that would be okay.  
Then you would also hold fetch_lock while checking mmap_active and
doing the memory allocations.  That would prevent any races -- in your
example above, thread A would acquire fetch_lock in step 1, so thread B
would block in step 3 until step 4 was finished.  Hence B would end up 
mapping the correct pages.

In practice, I don't see this being a routine problem.  How often do 
multiple threads independently try to mmap the same usbmon buffer?

Still, let's see syzbot reacts to your current patch.  The line below 
is how you ask syzbot to test a candidate patch.

Alan Stern

#syz test: linux-4.19.y f6e27dbb1afa

commit 5252eb4c8297fedbf1c5f1e67da44efe00e6ef6b
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>
    Reported-by: syzbot+56f9673bb4cdcbeb0e92@syzkaller.appspotmail.com

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;
 }