[PATCH v2] usb: mon: make mmapped memory read only

[Tadeusz Struk]

Syzbot found an issue in usbmon module, where the user space client
can corrupt the monitor's internal memory, causing the usbmon module
to crash the kernel with segfault, UAF, etc.
The reproducer mmaps the /dev/usbmon memory to user space, and
overwrites it with arbitrary data, which causes all kinds of issues.
Return an -EPERM error from mon_bin_mmap() if the flag VM_WRTIE is set.
Also clear VM_MAYWRITE to make it impossible to change it to writable
later.

Cc: "Dmitry Vyukov" <dvyukov@xxxxxxxxxx>
Cc: <linux-usb@xxxxxxxxxxxxxxx>
Cc: <linux-kernel@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Fixes: 6f23ee1fefdc ("USB: add binary API to usbmon")

For the VM_MAYWRITE part:
Suggested-by: PaX Team <pageexec@xxxxxxxxxxx>

Link: https://syzkaller.appspot.com/bug?id=2eb1f35d6525fa4a74d75b4244971e5b1411c95a
Reported-by: syzbot+23f57c5ae902429285d7@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
---
v2:
   Return an error instead of quietly clearing the flag,
   when VM_WRTIE is set. Also clear VM_MAYWRITE.
---
 drivers/usb/mon/mon_bin.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/usb/mon/mon_bin.c b/drivers/usb/mon/mon_bin.c
index f48a23adbc35..094e812e9e69 100644
--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -1268,6 +1268,11 @@ static int mon_bin_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	/* don't do anything here: "fault" will set up page table entries */
 	vma->vm_ops = &mon_bin_vm_ops;
+
+	if (vma->vm_flags & VM_WRITE)
+		return -EPERM;
+
+	vma->vm_flags &= ~VM_MAYWRITE;
 	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
 	vma->vm_private_data = filp->private_data;
 	mon_bin_vma_open(vma);
-- 
2.37.3