Re: [offlist] Re: Crash in netlink/sk_filter_trim_cap on ARMv7 on 4.18rc1

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On 08/17/2018 06:17 PM, Russell King - ARM Linux wrote:

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On Fri, Aug 17, 2018 at 02:40:19PM +0200, Daniel Borkmann wrote:

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I'd have one potential bug suspicion, for the 4.18 one you were trying,
could you run with the below patch to see whether it would help?

I think this is almost certainly the problem - looking at the history,
it seems that the "-4" was assumed to be part of the scratch stuff in
commit 38ca93060163 ("bpf, arm32: save 4 bytes of unneeded stack space")
but it isn't - it's because "off" of zero refers to the top word in the
stack (iow at STACK_SIZE-4).

Yeah agree, my thinking as well (albeit bit late, sigh, sorry about that).
Waiting for Peter to get back with results for definite confirmation. Your
rework in 1c35ba122d4a ("ARM: net: bpf: use negative numbers for stacked
registers") and 96cced4e774a ("ARM: net: bpf: access eBPF scratch space using
ARM FP register") fixes this in mainline, so unless I'm missing something this
would only need a stand-alone fix for 4.18/stable which I can cook up and
submit then.

i was able to reproduce this issue on RPi 3 with Linux 4.18.1 + multi_v7_defconfig and the following  config changes:

 --- a/arch/arm/configs/multi_v7_defconfig

+++ b/arch/arm/configs/multi_v7_defconfig

@@ -2,7 +2,10 @@ CONFIG_SYSVIPC=y
 CONFIG_NO_HZ=y
 CONFIG_HIGH_RES_TIMERS=y
 CONFIG_CGROUPS=y
+CONFIG_CGROUP_BPF=y
 CONFIG_BLK_DEV_INITRD=y
+CONFIG_BPF_SYSCALL=y
+CONFIG_BPF_JIT_ALWAYS_ON=y
 CONFIG_EMBEDDED=y
 CONFIG_PERF_EVENTS=y
 CONFIG_MODULES=y

@@ -153,6 +156,8 @@ CONFIG_IPV6_MIP6=m
 CONFIG_IPV6_TUNNEL=m
 CONFIG_IPV6_MULTIPLE_TABLES=y
 CONFIG_NET_DSA=m
+CONFIG_BPF_JIT=y
+CONFIG_BPF_STREAM_PARSER=y
 CONFIG_CAN=y
 CONFIG_CAN_AT91=m
 CONFIG_CAN_FLEXCAN=m

After applying the "-4" patch the oopses doesn't appear during boot anymore.