Re: KASAN: slab-out-of-bounds Read in handle_vmptrld

Greg Kroah-Hartman
2019-09-13T13:06:54Z

On Fri, Sep 13, 2019 at 09:34:32AM +0200, Paolo Bonzini wrote:
> On 13/09/19 06:46, Greg Kroah-Hartman wrote:
> > USB drivers expect kmalloc to return DMA-able memory. I don't know
> > about specific alignment issues, that should only an issue for the host
> > controller being used here, which you do not say in the above list.
>
> I have no idea, this is just the analysis of a syzkaller report. From
> the backtrace, it's one that ends up calling kmalloc; all of them should
> have the same issue with KASAN.
>
> The specific alignment requirement for this bug comes from this call in
> usbdev_mmap:
>
> if (remap_pfn_range(vma, vma->vm_start,
> virt_to_phys(usbm->mem) >> PAGE_SHIFT,
> size, vma->vm_page_prot) < 0) {
>
> > We have had some reports that usbdev_mmap() does not do the "correct
> > thing" for all host controllers, but a lot of the DMA work that is in
> > linux-next for 5.4-rc1 should have helped resolve those issues. What
> > tree are you seeing these bug reports happening from?
>
> It's in master, but the relevant code is the same in linux-next; in fact
> in this case there is no DMA involved at all. hcd_buffer_alloc hits
> the case "some USB hosts just use PIO".
>
> On those host controllers, it should be reproducible with just this:
>
> diff --git a/drivers/usb/core/usb.c b/drivers/usb/core/usb.c
> index 7fcb9f782931..cc0460730bce 100644
> --- a/drivers/usb/core/usb.c
> +++ b/drivers/usb/core/usb.c
> @@ -905,9 +905,12 @@ EXPORT_SYMBOL_GPL(__usb_get_extra_descriptor);
> void *usb_alloc_coherent(struct usb_device *dev, size_t size, gfp_t mem_flags,
> dma_addr_t *dma)
> {
> + void *buf;
> if (!dev || !dev->bus)
> return NULL;
> - return hcd_buffer_alloc(dev->bus, size, mem_flags, dma);
> + buf = hcd_buffer_alloc(dev->bus, size, mem_flags, dma);
> + WARN_ON_ONCE(virt_to_phys(buf) & ~PAGE_MASK);
> + return buf;
> }
> EXPORT_SYMBOL_GPL(usb_alloc_coherent);

Look at linux-next, we "should" have fixed up hcd_buffer_alloc() now to
not need this type of thing. If we got it wrong, please let us know and
then yes, a fix like this would be most appreciated :)

thanks,

greg k-h

>
>
> and CONFIG_KASAN=y or possibly just CONFIG_DEBUG_SLAB=y. mmap-ing /dev/usb
> should warn if my analysis is correct.
>
> If you think the above patch makes sense, I can test it and submit it formally.
>
> Paolo