[BUG] arm64: an infinite loop in generic_perform_write()

[Matthew Wilcox]

On Thu, Jun 24, 2021 at 08:04:07AM +0100, Christoph Hellwig wrote:
> On Thu, Jun 24, 2021 at 04:24:46AM +0100, Matthew Wilcox wrote:
> > On Thu, Jun 24, 2021 at 11:10:41AM +0800, Chen Huang wrote:
> > > In userspace, I perform such operation:
> > > 
> > >  	fd = open("/tmp/test", O_RDWR | O_SYNC);
> > >         access_address = (char *)mmap(NULL, uio_size, PROT_READ, MAP_SHARED, uio_fd, 0);
> > >         ret = write(fd, access_address + 2, sizeof(long));
> > 
> > ... you know that accessing this at unaligned offsets isn't going to
> > work.  It's completely meaningless.  Why are you trying to do it?
> 
> We still should not cause an infinite loop in kernel space due to a
> a userspace programmer error.

They're running as root and they've mapped some device memory.  We can't
save them from themself.  Imagine if they'd done this to the NVMe BAR.