Re: [PATCH v3 2/2] arm64/bpf: don't allocate BPF JIT programs in module memory

[Ard Biesheuvel]

On Fri, 23 Nov 2018 at 10:42, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> 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  |  3 +++
>  arch/arm64/include/asm/pgtable.h |  2 +-
>  arch/arm64/net/bpf_jit_comp.c    | 13 +++++++++++++
>  3 files changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index b96442960aea..506e319da98f 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -69,6 +69,9 @@
>  #define PCI_IO_END             (VMEMMAP_START - SZ_2M)
>  #define PCI_IO_START           (PCI_IO_END - PCI_IO_SIZE)
>  #define FIXADDR_TOP            (PCI_IO_START - SZ_2M)
> +#define BPF_JIT_REGION_BASE    (VMALLOC_END)
> +#define BPF_JIT_REGION_SIZE    (SZ_128M)
> +#define BPF_JIT_REGION_END     (BPF_JIT_REGION_BASE + BPF_JIT_REGION_SIZE)
>

Discussing this off-line with Daniel, it may be better to put the BPF
region before the module space instead.

This will permit the use of adrp/add/b[l]r sequences for long
jumps/calls. When booting with KASLR enabled, we can enhance the logic
there to ensure that the BPF region remains inside the same 4 GB
window as the module region and the core kernel (and randomize it
relatively as well)

>  #define KERNEL_START      _text
>  #define KERNEL_END        _end
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 50b1ef8584c0..9db98a4cd9b4 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -31,7 +31,7 @@
>   *     and fixed mappings
>   */
>  #define VMALLOC_START          (MODULES_END)
> -#define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - SZ_64K)
> +#define VMALLOC_END            (PAGE_OFFSET - PUD_SIZE - VMEMMAP_SIZE - BPF_JIT_REGION_SIZE - SZ_64K)
>
>  #define vmemmap                        ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))
>
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index a6fdaea07c63..298beba29fa5 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_BASE,
> +                                   BPF_JIT_REGION_END, GFP_KERNEL,
> +                                   PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
> +                                   __builtin_return_address(0));
> +}
> +
> +void bpf_jit_free_exec(const void *addr)
> +{
> +       return vfree(addr);
> +}
> --
> 2.17.1
>