[PATCH 4.9 025/159] usb: mon: make mmapped memory read only

[Greg Kroah-Hartman]

From: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>

commit a659daf63d16aa883be42f3f34ff84235c302198 upstream.

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: stable <stable@xxxxxxxxxx>
Fixes: 6f23ee1fefdc ("USB: add binary API to usbmon")
Suggested-by: PaX Team <pageexec@xxxxxxxxxxx>	# for the VM_MAYRITE portion
Link: https://syzkaller.appspot.com/bug?id=2eb1f35d6525fa4a74d75b4244971e5b1411c95a
Reported-by: syzbot+23f57c5ae902429285d7@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220919215957.205681-1-tadeusz.struk@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/usb/mon/mon_bin.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/usb/mon/mon_bin.c
+++ b/drivers/usb/mon/mon_bin.c
@@ -1265,6 +1265,11 @@ static int mon_bin_mmap(struct file *fil
 {
 	/* 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);