Re: [PATCH v4 2/2] arm64/bpf: don't allocate BPF JIT programs in module memory

[Ard Biesheuvel]

On Mon, 3 Dec 2018 at 13:49, Will Deacon <will.deacon@xxxxxxx> wrote:
>
> On Fri, Nov 30, 2018 at 08:20:06PM +0100, Ard Biesheuvel wrote:
> > On Fri, 30 Nov 2018 at 19:26, Will Deacon <will.deacon@xxxxxxx> wrote:
> > >
> > > On Fri, Nov 23, 2018 at 11:18:04PM +0100, Ard Biesheuvel wrote:
> > > > The arm64 module region is a 128 MB region that is kept close to
> > > > the core kernel, in order to ensure that relative branches are
> > > > always in range. So using the same region for programs that do
> > > > not have this restriction is wasteful, and preferably avoided.
> > > >
> > > > Now that the core BPF JIT code permits the alloc/free routines to
> > > > be overridden, implement them by vmalloc()/vfree() calls from a
> > > > dedicated 128 MB region set aside for BPF programs. This ensures
> > > > that BPF programs are still in branching range of each other, which
> > > > is something the JIT currently depends upon (and is not guaranteed
> > > > when using module_alloc() on KASLR kernels like we do currently).
> > > > It also ensures that placement of BPF programs does not correlate
> > > > with the placement of the core kernel or modules, making it less
> > > > likely that leaking the former will reveal the latter.
> > > >
> > > > This also solves an issue under KASAN, where shadow memory is
> > > > needlessly allocated for all BPF programs (which don't require KASAN
> > > > shadow pages since they are not KASAN instrumented)
> > > >
> > > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > > ---
> > > >  arch/arm64/include/asm/memory.h |  5 ++++-
> > > >  arch/arm64/net/bpf_jit_comp.c   | 13 +++++++++++++
> > > >  2 files changed, 17 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> > > > index b96442960aea..ee20fc63899c 100644
> > > > --- a/arch/arm64/include/asm/memory.h
> > > > +++ b/arch/arm64/include/asm/memory.h
> > > > @@ -62,8 +62,11 @@
> > > >  #define PAGE_OFFSET          (UL(0xffffffffffffffff) - \
> > > >       (UL(1) << (VA_BITS - 1)) + 1)
> > > >  #define KIMAGE_VADDR         (MODULES_END)
> > > > +#define BPF_JIT_REGION_START (VA_START + KASAN_SHADOW_SIZE)
> > > > +#define BPF_JIT_REGION_SIZE  (SZ_128M)
> > > > +#define BPF_JIT_REGION_END   (BPF_JIT_REGION_START + BPF_JIT_REGION_SIZE)
> > > >  #define MODULES_END          (MODULES_VADDR + MODULES_VSIZE)
> > > > -#define MODULES_VADDR                (VA_START + KASAN_SHADOW_SIZE)
> > > > +#define MODULES_VADDR                (BPF_JIT_REGION_END)
> > > >  #define MODULES_VSIZE                (SZ_128M)
> > > >  #define VMEMMAP_START                (PAGE_OFFSET - VMEMMAP_SIZE)
> > > >  #define PCI_IO_END           (VMEMMAP_START - SZ_2M)
> > > > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > > > index a6fdaea07c63..76c2ab40c02d 100644
> > > > --- a/arch/arm64/net/bpf_jit_comp.c
> > > > +++ b/arch/arm64/net/bpf_jit_comp.c
> > > > @@ -940,3 +940,16 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > > >                                          tmp : orig_prog);
> > > >       return prog;
> > > >  }
> > > > +
> > > > +void *bpf_jit_alloc_exec(unsigned long size)
> > > > +{
> > > > +     return __vmalloc_node_range(size, PAGE_SIZE, BPF_JIT_REGION_START,
> > > > +                                 BPF_JIT_REGION_END, GFP_KERNEL,
> > > > +                                 PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
> > > > +                                 __builtin_return_address(0));
> > >
> > > I guess we'll want VM_IMMEDIATE_UNMAP here if Rich gets that merged.
> >
> > I think akpm already queued up that patch.
> >
> > > In the
> > > meantime, I wonder if it's worth zeroing the region in bpf_jit_free_exec()?
> > > (although we'd need the size information...).
> > >
> >
> > Not sure. What exactly would that achieve?
>
> I think the zero encoding is guaranteed to be undefined, so it would limit
> the usefulness of any stale, executable TLB entries. However, we'd also need
> cache maintenance to make that stuff visible to the I side, so it's probably
> not worth it, especially if akpm has queued the stuff from Rich.
>
> Maybe just add an:
>
> /* FIXME: Remove this when VM_IMMEDIATE_UNMAP is supported */
> #ifndef VM_IMMEDIATE_UNMAP
> #define VM_IMMEDIATE_UNMAP      0
> #endif
>
> so we remember to come back and sort this out? Up to you.
>

I'll just make a note to send out that patch once the definition lands via -akpm