Re: WARNING in memtype_reserve

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, May 09, 2020 at 12:00:57PM +0200, Thomas Gleixner wrote:
> Greg KH <gregkh@linuxfoundation.org> writes:
> > On Sat, May 09, 2020 at 12:20:14AM -0700, syzbot wrote:
> >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
> >> WARNING: CPU: 1 PID: 7025 at arch/x86/mm/pat/memtype.c:589 memtype_reserve+0x69f/0x820 arch/x86/mm/pat/memtype.c:589
> >
> > So should memtype_reserve() not do a WARN if given invalid parameters as
> > it can be triggered by userspace requests?
> >
> > A normal "invalid request" debug line is probably all that is needed,
> > right?
> 
> I disagree. The callsite espcially if user space triggerable should not
> attempt to ask for a reservation where start > end:
> 
>   >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
> 
> The real question is which part of the call chain is responsible for
> this. That needs to be fixed.

This is caused by 2bef9aed6f0e ("usb: usbfs: correct kernel->user page
attribute mismatch") which changed a call to remap_pfn_range() to
dma_mmap_coherent().  Looks like the error checking in remap_pfn_range()
handled the invalid options better than dma_mma_coherent() when odd
values are passed in.

We can add the check to dma_mmap_coherent(), again, but really, this
type of check should probably only be needed in one place to ensure we
always get it correct, right?

thanks,

greg k-h