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

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

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

quoted

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.

Thanks,
Daniel