Re: [PATCH] bpf: fix overflow of bpf_jit_limit when PAGE_SIZE >= 64K

[Michael Ellerman <mpe@ellerman.id.au>]

Michael Roth <mdroth@linux.vnet.ibm.com> writes:

> Commit ede95a63b5 introduced a bpf_jit_limit tuneable to limit BPF
> JIT allocations. At compile time it defaults to PAGE_SIZE * 40000,
> and is adjusted again at init time if MODULES_VADDR is defined.
>
> For ppc64 kernels, MODULES_VADDR isn't defined, so we're stuck with

But maybe it should be, I don't know why we don't define it.

> the compile-time default at boot-time, which is 0x9c400000 when
> using 64K page size. This overflows the signed 32-bit bpf_jit_limit
> value:
>
>   root@ubuntu:/tmp# cat /proc/sys/net/core/bpf_jit_limit
>   -1673527296
>
> and can cause various unexpected failures throughout the network
> stack. In one case `strace dhclient eth0` reported:
>
>   setsockopt(5, SOL_SOCKET, SO_ATTACH_FILTER, {len=11, filter=0x105dd27f8}, 16) = -1 ENOTSUPP (Unknown error 524)
>
> and similar failures can be seen with tools like tcpdump. This doesn't
> always reproduce however, and I'm not sure why. The more consistent
> failure I've seen is an Ubuntu 18.04 KVM guest booted on a POWER9 host
> would time out on systemd/netplan configuring a virtio-net NIC with no
> noticeable errors in the logs.
>
> Fix this by limiting the compile-time default for bpf_jit_limit to
> INT_MAX.

INT_MAX is a lot more than (4k * 40000), so I guess I'm not clear on
whether we should be using PAGE_SIZE here at all. I guess each BPF
program uses at least one page is the thinking?

Thanks for tracking this down. For some reason none of my ~10 test boxes
have hit this, perhaps I don't have new enough userspace?

You don't mention why you needed to add BPF_MIN(), I assume because the
kernel version of min() has gotten too complicated to work here?

Daniel I assume you'll merge this via your tree?

cheers

> Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
> Cc: linuxppc-dev@ozlabs.org
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Sandipan Das <sandipan@linux.ibm.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> ---
>  kernel/bpf/core.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index b1a3545d0ec8..55de4746cdfd 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -365,7 +365,8 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
>  }
>  
>  #ifdef CONFIG_BPF_JIT
> -# define BPF_JIT_LIMIT_DEFAULT	(PAGE_SIZE * 40000)
> +# define BPF_MIN(x, y) ((x) < (y) ? (x) : (y))
> +# define BPF_JIT_LIMIT_DEFAULT	BPF_MIN((PAGE_SIZE * 40000), INT_MAX)
>  
>  /* All BPF JIT sysctl knobs here. */
>  int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_ALWAYS_ON);
> -- 
> 2.17.1