Re: [PATCH 0/4] Fix type confusion in page_table_check

[Ruihan Li]

On Thu, May 11, 2023 at 07:51:58AM +0900, Greg Kroah-Hartman wrote:
> On Wed, May 10, 2023 at 04:55:23PM +0800, Ruihan Li wrote:
> > Recently, syzbot reported [1] ("kernel BUG in page_table_check_clear").
> > The root cause is that usbdev_mmap calls remap_pfn_range on kmalloc'ed
> > memory, which leads to type confusion between struct page and slab in
> > page_table_check. This series of patches fixes the usb side by avoiding
> > mapping slab pages into userspace, and fixes the mm side by enforcing
> > that all user-accessible pages are not slab pages. A more detailed
> > analysis and some discussion of how to fix the problem can also be found
> > in [1].
> > 
> >  [1] https://lore.kernel.org/lkml/20230507135844.1231056-1-lrh2000@xxxxxxxxxx/T/
> 
> Can you see if you can implement Christoph's proposed change instead:
> 	https://lore.kernel.org/r/ZFuZVDcU81WmqEvJ@xxxxxxxxxxxxx
> 
> As it might not actually be as bad as you think to require this type of
> churn.
> 
> thanks,
> 
> greg k-h

Sorry, but no.

Christoph's patch perfectly fixes _one_ problem: kmalloc'ed memory
cannot be mapped to user space. However, as I detailed in the commit
message, this series of patches fixes _three_ problems.

I have to say that the original code is quite buggy. In the
gen_pool_dma_alloc path, there is no guarantee of page alignment. 

void *hcd_buffer_alloc(...)
{
	// ...
	if (hcd->localmem_pool)
		return gen_pool_dma_alloc(hcd->localmem_pool, size, dma);
	// ...
}

When localmem_pool gets initialized, the alignment bits are set to 4
(instead of PAGE_SHIFT).

int usb_hcd_setup_local_mem(struct usb_hcd *hcd, phys_addr_t phys_addr,
			    dma_addr_t dma, size_t size)
{
	// ...
	hcd->localmem_pool = devm_gen_pool_create(hcd->self.sysdev, 4,
						  dev_to_node(hcd->self.sysdev),
						  dev_name(hcd->self.sysdev));
	// ...
}

It is introduced by commit ff2437befd8f ("usb: host: Fix excessive
alignment restriction for local memory allocations") [1].

 [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ff2437befd8fe52046e0db949347b5bcfab6b097

If you don't believe me, please see my test results. In the following
qemu setup,

	qemu-system-sh4 -M r2d -kernel arch/sh/boot/zImage \
		-usb -device usb-storage,drive=d0 \
		-drive file=test.img,if=none,id=d0,format=raw \
		-append "console=tty0 console=ttySC1,115200 rootwait root=/dev/sda init=/bin/sh" \
		-serial null -serial stdio

together with the following patch,

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index e501a03d6..d7069c986 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -265,6 +265,10 @@ static int usbdev_mmap(struct file *file, struct vm_area_struct *vma)
 	INIT_LIST_HEAD(&usbm->memlist);
 
 	if (hcd->localmem_pool || !hcd_uses_dma(hcd)) {
+		printk("usb_alloc_coherent returns %px\n", usbm->mem);
+		printk("so the mapping starts at %lx\n",
+				virt_to_phys(usbm->mem) >> PAGE_SHIFT);
+
 		if (remap_pfn_range(vma, vma->vm_start,
 				    virt_to_phys(usbm->mem) >> PAGE_SHIFT,
 				    size, vma->vm_page_prot) < 0) {

it quickly leads to the following output when I manage to map a page
from /dev/bus/usb/001/002,

	usb_alloc_coherent returns b07c06c0
	so the mapping starts at 307c0

What's more, in this case, remap_pfn_range should _not_ be used, since
we are going to map a DMA page. However, as you can see, usbdev_mmap
actually goes to the wrong path.

If you say I shouldn't fix all these bugs in _this_ patch series, that's
fine. However, I do think that these bugs should be fixed, perhaps in
another patch series.

Thanks,
Ruihan Li