[CRIU] [PATCH 0/4] c/r non-dev net syctls

[Pavel Tikhomirov]

Ok I found the problem:

In Linux v4.3 commit df2cf4a78e48 ("IGMP: Inhibit reports for local 
multicast groups") sysctl igmp_link_local_mcast_reports was introduced 
in ipv4_net_table.

And in ipv4_net_table it's data was initialized to point on 
sysctl_igmp_llm_reports variable. That was so before commit 87a8a2ae65b7 
("igmp: Namespaceify igmp_llm_reports sysctl knob").

So next it's data pointer is shifted to the offset of current 
netnamespace relative to init_net in ipv4_sysctl_init_net function. But 
that is completely wrong if variable is not net-namespaced, so we get 
random kernel address and can write/read to/from it one int, that can 
lead to memory corruption and crashes in random places in kernel.

So conclusion is: we can not touch 
/proc/sys/net/ipv4/igmp_link_local_mcast_reports in v4.3-v4.5 between 
those two patches. So we need to drop its c/r and test if kernel is 
pre-v4.6.

to @xemul: I do not know another way to check if the sysctl is alright 
except checking kernel version, is that acceptable?

Simple reproduction:
while :; do unshare -n echo 1999999 > 
/proc/sys/net/ipv4/igmp_link_local_mcast_reports; done

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.