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SubjectRe: possible deadlock in mon_bin_vma_fault
On Sun, 24 Nov 2019, syzbot wrote:

> Hello,
>
> syzbot tried to test the proposed patch but build/boot failed:
>
> failed to apply patch:
> checking file drivers/usb/mon/mon_bin.c
> patch: **** unexpected end of file in patch

One more try...

Alan Stern

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v5.3

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;
}

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