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

On 12/10/2018 06:27 PM, Michael Roth wrote:

Quoting Daniel Borkmann (2018-12-10 08:26:31)

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On 12/07/2018 04:36 PM, Michael Roth wrote:

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Quoting Michael Ellerman (2018-12-07 06:31:13)

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Michael Roth [off-list ref] writes:

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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.

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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?

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.

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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).

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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?

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

cheers

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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 <redacted>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Michael Roth <redacted>

Thanks for the reports / fixes and sorry for my late reply (bit too
swamped last week), some more thoughts below.

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 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?

I don't think there are today, but it definitely doesn't hurt to change
the interface to be on safe side which I just did for the sent out one.

Thanks,
Daniel