Re: [PATCH bpf v2] bpf: fix bpf_jit_limit knob for PAGE_SIZE >= 64K

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Quoting Daniel Borkmann (2018-12-11 05:14:12)
> Michael and Sandipan report:
> 
>   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
>   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.
> 
> Given this and also given that in near future some architectures like
> arm64 will have a custom area for BPF JIT image allocations we should
> get rid of the BPF_JIT_LIMIT_DEFAULT fallback / default entirely. For
> 4.21, we have an overridable bpf_jit_alloc_exec(), bpf_jit_free_exec()
> so therefore add another overridable bpf_jit_alloc_exec_limit() helper
> function which returns the possible size of the memory area for deriving
> the default heuristic in bpf_jit_charge_init().
> 
> Like bpf_jit_alloc_exec() and bpf_jit_free_exec(), the new
> bpf_jit_alloc_exec_limit() assumes that module_alloc() is the default
> JIT memory provider, and therefore in case archs implement their custom
> module_alloc() we use MODULES_{END,_VADDR} for limits and otherwise for
> vmalloc_exec() cases like on ppc64 we use VMALLOC_{END,_START}.
> 
> Additionally, for archs supporting large page sizes, we should change
> the sysctl to be handled as long to not run into sysctl restrictions
> in future.
> 
> Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
> Reported-by: Sandipan Das <sandipan@linux.ibm.com>
> Reported-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Tested-by: Michael Roth <mdroth@linux.vnet.ibm.com>

Thanks!

> ---
>  v1 -> v2:
>    - added missing __maybe_unused when JIT not compiled in
> 
>  include/linux/filter.h     |  2 +-
>  kernel/bpf/core.c          | 21 +++++++++++++++------
>  net/core/sysctl_net_core.c | 20 +++++++++++++++++---
>  3 files changed, 33 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 795ff0b..a8b9d90 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -861,7 +861,7 @@ bpf_run_sk_reuseport(struct sock_reuseport *reuse, struct sock *sk,
>  extern int bpf_jit_enable;
>  extern int bpf_jit_harden;
>  extern int bpf_jit_kallsyms;
> -extern int bpf_jit_limit;
> +extern long bpf_jit_limit;
> 
>  typedef void (*bpf_jit_fill_hole_t)(void *area, unsigned int size);
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index b1a3545..b2890c2 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -365,13 +365,11 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
>  }
> 
>  #ifdef CONFIG_BPF_JIT
> -# define BPF_JIT_LIMIT_DEFAULT (PAGE_SIZE * 40000)
> -
>  /* All BPF JIT sysctl knobs here. */
>  int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_ALWAYS_ON);
>  int bpf_jit_harden   __read_mostly;
>  int bpf_jit_kallsyms __read_mostly;
> -int bpf_jit_limit    __read_mostly = BPF_JIT_LIMIT_DEFAULT;
> +long bpf_jit_limit   __read_mostly;
> 
>  static __always_inline void
>  bpf_get_prog_addr_region(const struct bpf_prog *prog,
> @@ -580,16 +578,27 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
> 
>  static atomic_long_t bpf_jit_current;
> 
> +/* Can be overridden by an arch's JIT compiler if it has a custom,
> + * dedicated BPF backend memory area, or if neither of the two
> + * below apply.
> + */
> +u64 __weak bpf_jit_alloc_exec_limit(void)
> +{
>  #if defined(MODULES_VADDR)
> +       return MODULES_END - MODULES_VADDR;
> +#else
> +       return VMALLOC_END - VMALLOC_START;
> +#endif
> +}
> +
>  static int __init bpf_jit_charge_init(void)
>  {
>         /* Only used as heuristic here to derive limit. */
> -       bpf_jit_limit = min_t(u64, round_up((MODULES_END - MODULES_VADDR) >> 2,
> -                                           PAGE_SIZE), INT_MAX);
> +       bpf_jit_limit = min_t(u64, round_up(bpf_jit_alloc_exec_limit() >> 2,
> +                                           PAGE_SIZE), LONG_MAX);
>         return 0;
>  }
>  pure_initcall(bpf_jit_charge_init);
> -#endif
> 
>  static int bpf_jit_charge_modmem(u32 pages)
>  {
> diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> index 37b4667..d67ec17 100644
> --- a/net/core/sysctl_net_core.c
> +++ b/net/core/sysctl_net_core.c
> @@ -28,6 +28,8 @@ static int two __maybe_unused = 2;
>  static int min_sndbuf = SOCK_MIN_SNDBUF;
>  static int min_rcvbuf = SOCK_MIN_RCVBUF;
>  static int max_skb_frags = MAX_SKB_FRAGS;
> +static long long_one __maybe_unused = 1;
> +static long long_max __maybe_unused = LONG_MAX;
> 
>  static int net_msg_warn;       /* Unused, but still a sysctl */
> 
> @@ -289,6 +291,17 @@ proc_dointvec_minmax_bpf_restricted(struct ctl_table *table, int write,
> 
>         return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
>  }
> +
> +static int
> +proc_dolongvec_minmax_bpf_restricted(struct ctl_table *table, int write,
> +                                    void __user *buffer, size_t *lenp,
> +                                    loff_t *ppos)
> +{
> +       if (!capable(CAP_SYS_ADMIN))
> +               return -EPERM;
> +
> +       return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
> +}
>  #endif
> 
>  static struct ctl_table net_core_table[] = {
> @@ -398,10 +411,11 @@ static struct ctl_table net_core_table[] = {
>         {
>                 .procname       = "bpf_jit_limit",
>                 .data           = &bpf_jit_limit,
> -               .maxlen         = sizeof(int),
> +               .maxlen         = sizeof(long),
>                 .mode           = 0600,
> -               .proc_handler   = proc_dointvec_minmax_bpf_restricted,
> -               .extra1         = &one,
> +               .proc_handler   = proc_dolongvec_minmax_bpf_restricted,
> +               .extra1         = &long_one,
> +               .extra2         = &long_max,
>         },
>  #endif
>         {
> -- 
> 2.9.5
>