Message-ID: <154446286490.4685.7141471484530234403@sif>
Date:   Mon, 10 Dec 2018 11:27:44 -0600
From:   Michael Roth <mdroth@...ux.vnet.ibm.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Michael Ellerman <mpe@...erman.id.au>, netdev@...r.kernel.org
Cc:     linuxppc-dev@...abs.org, Sandipan Das <sandipan@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] bpf: fix overflow of bpf_jit_limit when PAGE_SIZE >= 64K

Quoting Daniel Borkmann (2018-12-10 08:26:31)
> On 12/07/2018 04:36 PM, Michael Roth wrote:
> > Quoting Michael Ellerman (2018-12-07 06:31:13)
> >> Michael Roth <mdroth@...ux.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@...ntu:/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?
> > 
> > That seems to be the case, at least, the max number of minimum-sized
> > allocations would be less on ppc64 since the allocations are always at
> > least PAGE_SIZE in size. The init-time default also limits to INT_MAX,
> > so it seemed consistent to do that here too.
> > 
> >>
> >> 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?
> > 
> > I'm not too sure, I would've thought things like the dhclient case in
> > the commit log would fail every time, but sometimes I need to reboot the
> > guest before I start seeing the behavior. Maybe there's something special
> > about when JIT allocations are actually done that can affect
> > reproducibility?
> > 
> > In my case at least the virtio-net networking timeout was consistent
> > enough for a bisect, but maybe it depends on the specific network
> > configuration (single NIC, basic DHCP through netplan/systemd in my case).
> > 
> >>
> >> 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?
> > 
> > I wasn't sure if it was safe here or not, so I tried looking at other
> > users and came across:
> > 
> > mm/vmalloc.c:777:#define VMAP_MIN(x, y)               ((x) < (y) ? (x) : (y)) /* can't use min() */
> > 
> > I'm not sure what the reasoning was (or whether it still applies), but I
> > figured it was safer to do the same here. Maybe Nick still recalls?
> > 
> >>
> >> 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@...abs.org
> >>> Cc: Daniel Borkmann <daniel@...earbox.net>
> >>> Cc: Sandipan Das <sandipan@...ux.ibm.com>
> >>> Cc: Alexei Starovoitov <ast@...nel.org>
> >>> Signed-off-by: Michael Roth <mdroth@...ux.vnet.ibm.com>
> 
> Thanks for the reports / fixes and sorry for my late reply (bit too
> swamped last week), some more thoughts below.
> 
> >>>  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);
> 
> I would actually just like to get rid of the BPF_JIT_LIMIT_DEFAULT
> define also given for 4.21 arm64 will have its own dedicated area for
> JIT allocations where neither the above limit nor the MODULES_END/
> MODULES_VADDR one would fit and I don't want to make this even more
> ugly with adding further cases into the core. Would the below variant
> work for you?

Looks good to me. My one concern (which is probably a separate
issue) is that the INT_MAX limit is a bit more punishing for larger
page sizes since the minimum allocations seem to be 1 page. Are there
reasonable workloads that could actually push this (INT_MAX >>
64K_PAGE_SHIFT) limit, or is that pretty generous in practice?

> 
> Thanks,
> Daniel
> 
> From da9daf462d41ce5506c6b6318a9fa3d6d8a64f6c Mon Sep 17 00:00:00 2001
> From: Daniel Borkmann <daniel@...earbox.net>
> Date: Mon, 10 Dec 2018 14:30:27 +0100
> Subject: [PATCH bpf] bpf: fix bpf_jit_limit knob for PAGE_SIZE >= 64K
> 
> 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@...ntu:/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}.
> 
> Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
> Reported-by: Sandipan Das <sandipan@...ux.ibm.com>
> Reported-by: Michael Roth <mdroth@...ux.vnet.ibm.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  kernel/bpf/core.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index b1a3545..6c2332e 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;
> +int 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,
> +       bpf_jit_limit = min_t(u64, round_up(bpf_jit_alloc_exec_limit() >> 2,
>                                             PAGE_SIZE), INT_MAX);
>         return 0;
>  }
>  pure_initcall(bpf_jit_charge_init);
> -#endif
> 
>  static int bpf_jit_charge_modmem(u32 pages)
>  {
> -- 
> 2.9.5
> 

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