[PATCH bpf] bpf: fix default unprivileged allocation limit

[Michael Roth]

Quoting Sandipan Das (2018-12-06 03:27:32)
> When using a large page size, the default value of the bpf_jit_limit
> knob becomes invalid and users are not able to run unprivileged bpf
> programs.
> 
> The bpf_jit_limit knob is represented internally as a 32-bit signed
> integer because of which the default value, i.e. PAGE_SIZE * 40000,
> overflows in case of an architecture like powerpc64 which uses 64K
> as the default page size (i.e. CONFIG_PPC_64K_PAGES is set).
> 
> So, instead of depending on the page size, use a constant value.
> 
> Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")

This also consistently caused a virtio-net KVM Ubuntu 18.04 guest to time out
on configuring networking during boot via systemd/netplan. A bisect
pointed to the same commit this patch addresses.

> Signed-off-by: Sandipan Das <sandipan at linux.ibm.com>
> ---
>  kernel/bpf/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index b1a3545d0ec8..a81d097a17fb 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -365,7 +365,7 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
>  }
> 
>  #ifdef CONFIG_BPF_JIT
> -# define BPF_JIT_LIMIT_DEFAULT (PAGE_SIZE * 40000)
> +# define BPF_JIT_LIMIT_DEFAULT (4096 * 40000)

This isn't quite right as we still use (bpf_jit_limit >> PAGE_SHIFT) to check
allocations in bpf_jit_charge_modmem(), so that should be fixed up as well.

Another alternative which is to clamp BPF_JIT_LIMIT_DEFAULT to INT_MAX,
which fixes the issue for me and is similar to what
bpf_jit_charge_init() does for kernels where MODULES_VADDR is defined.
I'll go ahead and send the patch in case that seems preferable.

> 
>  /* All BPF JIT sysctl knobs here. */
>  int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_ALWAYS_ON);
> -- 
> 2.19.2
>