On 2020/6/19 5:09, Cong Wang wrote: > On Thu, Jun 18, 2020 at 12:36 PM Roman Gushchin <g...@fb.com> wrote: >> >> On Thu, Jun 18, 2020 at 12:19:13PM -0700, Cong Wang wrote: >>> On Wed, Jun 17, 2020 at 6:44 PM Zefan Li <lize...@huawei.com> wrote: >>>> >>>> Cc: Roman Gushchin <g...@fb.com> >>>> >>>> Thanks for fixing this. >>>> >>>> On 2020/6/17 2:03, Cong Wang wrote: >>>>> When we clone a socket in sk_clone_lock(), its sk_cgrp_data is >>>>> copied, so the cgroup refcnt must be taken too. And, unlike the >>>>> sk_alloc() path, sock_update_netprioidx() is not called here. >>>>> Therefore, it is safe and necessary to grab the cgroup refcnt >>>>> even when cgroup_sk_alloc is disabled. >>>>> >>>>> sk_clone_lock() is in BH context anyway, the in_interrupt() >>>>> would terminate this function if called there. And for sk_alloc() >>>>> skcd->val is always zero. So it's safe to factor out the code >>>>> to make it more readable. >>>>> >>>>> Fixes: 090e28b229af92dc5b ("netprio_cgroup: Fix unlimited memory leak of >>>>> v2 cgroups") >>>> >>>> but I don't think the bug was introduced by this commit, because there >>>> are already calls to cgroup_sk_alloc_disable() in write_priomap() and >>>> write_classid(), which can be triggered by writing to ifpriomap or >>>> classid in cgroupfs. This commit just made it much easier to happen >>>> with systemd invovled. >>>> >>>> I think it's 4bfc0bb2c60e2f4c ("bpf: decouple the lifetime of cgroup_bpf >>>> from cgroup itself"), >>>> which added cgroup_bpf_get() in cgroup_sk_alloc(). >>> >>> Good point. >>> >>> I take a deeper look, it looks like commit d979a39d7242e06 >>> is the one to blame, because it is the first commit that began to >>> hold cgroup refcnt in cgroup_sk_alloc(). >> >> I agree, ut seems that the issue is not related to bpf and probably >> can be reproduced without CONFIG_CGROUP_BPF. d979a39d7242e06 indeed >> seems closer to the origin. > > Yeah, I will update the Fixes tag and send V2. >
Commit d979a39d7242e06 looks innocent to me. With this commit when cgroup_sk_alloc is disabled and then a socket is cloned the cgroup refcnt will not be incremented, but this is fine, because when the socket is to be freed: sk_prot_free() cgroup_sk_free() cgroup_put(sock_cgroup_ptr(skcd)) == cgroup_put(&cgrp_dfl_root.cgrp) cgroup_put() does nothing for the default root cgroup, so nothing bad will happen. but cgroup_bpf_put() will decrement the bpf refcnt while this refcnt were not incremented as cgroup_sk_alloc has already been disabled. That's why I think it's 4bfc0bb2c60e2f4c that needs to be fixed. >> >> Btw, based on the number of reported-by tags it seems that there was >> a real issue which the patch is fixing. Maybe you'll a couple of words >> about how it reveals itself in the real life? > > I still have no idea how exactly this is triggered. According to the > people who reported this bug, they just need to wait for some hours > to trigger. So I am not sure what to add here, just the stack trace? > > Thanks. > . >