Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

[Jann Horn]

On Mon, Oct 5, 2020 at 6:01 PM Alexander Potapenko <glider@xxxxxxxxxx> wrote:
>
> On Tue, Sep 29, 2020 at 5:06 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >
> > On Tue, Sep 29, 2020 at 04:51:29PM +0200, Marco Elver wrote:
> > > On Tue, 29 Sep 2020 at 16:24, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > [...]
> > > >
> > > > From other sub-threads it sounds like these addresses are not part of
> > > > the linear/direct map. Having kmalloc return addresses outside of the
> > > > linear map is going to break anything that relies on virt<->phys
> > > > conversions, and is liable to make DMA corrupt memory. There were
> > > > problems of that sort with VMAP_STACK, and this is why kvmalloc() is
> > > > separate from kmalloc().
> > > >
> > > > Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream.
> > > >
> > > > I strongly suspect this isn't going to be safe unless you always use an
> > > > in-place carevout from the linear map (which could be the linear alias
> > > > of a static carevout).
> > >
> > > That's an excellent point, thank you! Indeed, on arm64, a version with
> > > naive static-pool screams with CONFIG_DEBUG_VIRTUAL.
> > >
> > > We'll try to put together an arm64 version using a carveout as you suggest.
> >
> > Great, thanks!
> >
> > Just to be clear, the concerns for DMA and virt<->phys conversions also
> > apply to x86 (the x86 virt<->phys conversion behaviour is more forgiving
> > in the common case, but still has cases that can go wrong).
>
> To clarify, shouldn't kmalloc/kmem_cache allocations used with DMA be
> allocated with explicit GFP_DMA?
> If so, how practical would it be to just skip such allocations in
> KFENCE allocator?

AFAIK GFP_DMA doesn't really mean "I will use this allocation for
DMA"; it means "I will use this allocation for DMA using some ancient
hardware (e.g. stuff on the ISA bus?) that only supports 16-bit
physical addresses (or maybe different limits on other
architectures)".
There's also GFP_DMA32, which means the same thing, except with 32-bit
physical addresses.

You can see in e.g. __dma_direct_alloc_pages() that the GFP_DMA32 and
GFP_DMA flags are only used if the hardware can't address the full
physical address space supported by the CPU.