Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as usercopy caches

[Matthew Wilcox <willy@infradead.org>]

On Sat, Feb 01, 2020 at 08:27:49PM +0100, Jann Horn wrote:
> FWIW, as far as I understand, usercopy doesn't actually have any
> effect on drivers that use the modern, proper APIs, since those don't
> use the slab allocator at all - as I pointed out in my last mail, the
> dma-kmalloc* slabs are used very rarely. (Which is good, because
> putting objects from less-than-page-size slabs into iommu entries is a
> terrible idea from a security and reliability perspective because it
> gives the hardware access to completely unrelated memory.) Instead,
> they get pages from the page allocator, and these pages may e.g. be
> allocated from the DMA, DMA32 or NORMAL zones depending on the
> restrictions imposed by hardware. So I think the usercopy restriction
> only affects a few oddball drivers (like this s390 stuff), which is
> why you're not seeing more bug reports caused by this.

Getting pages from the page allocator is true for dma_alloc_coherent()
and friends.  But it's not true for streaming DMA mappings (dma_map_*)
for which the memory usually comes from kmalloc().  If this is something
we want to fix (and I have an awful feeling we're going to regret it
if we say "no, we trust the hardware"), we're going to have to come up
with a new memory allocation API for these cases.  Or bounce bugger the
memory for devices we don't trust.

The problem with the dma_map_* API is that memory might end up being
allocated once and then used multiple times by different drivers.  eg if
I allocate an NFS packet, it might get sent first to eth0, then (when the
route fails) sent to eth1.  Similarly in storage, a RAID-5 driver might
map the same memory several times to send to different disk controllers.