[PATCH] bpf: fix overflow of bpf_jit_limit when PAGE_SIZE >= 64K
Michael Roth
mdroth at linux.vnet.ibm.com
Tue Dec 11 04:27:44 AEDT 2018
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 at 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 at 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?
> >
> > 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 at ozlabs.org
> >>> Cc: Daniel Borkmann <daniel at iogearbox.net>
> >>> Cc: Sandipan Das <sandipan at linux.ibm.com>
> >>> Cc: Alexei Starovoitov <ast at kernel.org>
> >>> Signed-off-by: Michael Roth <mdroth at linux.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 at iogearbox.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 at 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}.
>
> Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
> Reported-by: Sandipan Das <sandipan at linux.ibm.com>
> Reported-by: Michael Roth <mdroth at linux.vnet.ibm.com>
> Signed-off-by: Daniel Borkmann <daniel at iogearbox.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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