Subject: Re: Coverity: shrink_node_memcgs(): Null pointer dereferences

From: Kees Cook <[email-masked]>
Date: Tue, 12 Nov 2019 13:04:03 -0800

On Tue, Nov 12, 2019 at 10:21:23AM -0500, Johannes Weiner wrote:
> On Mon, Nov 11, 2019 at 05:35:37PM -0800, coverity-bot wrote:
> > Hello!
> > 
> > This is an experimental automated report about issues detected by Coverity
> > from a scan of next-20191108 as part of the linux-next weekly scan project:
> > https://scan.coverity.com/projects/linux-next-weekly-scan
> > 
> > You're getting this email because you were associated with the identified
> > lines of code (noted below) that were touched by recent commits:
> > 
> > c34aa3085f94 ("mm-vmscan-split-shrink_node-into-node-part-and-memcgs-part-fix")
> > 
> > Coverity reported the following:
> > 
> > *** CID 1487844:  Null pointer dereferences  (NULL_RETURNS)
> > /mm/vmscan.c: 2695 in shrink_node_memcgs()
> > 2689     	memcg = mem_cgroup_iter(target_memcg, NULL, NULL);
> > 2690     	do {
> > 2691     		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
> > 2692     		unsigned long reclaimed;
> > 2693     		unsigned long scanned;
> > 2694
> > vvv     CID 1487844:  Null pointer dereferences  (NULL_RETURNS)
> > vvv     Dereferencing a pointer that might be "NULL" "memcg" when calling "mem_cgroup_protected".
> > 2695     		switch (mem_cgroup_protected(target_memcg, memcg)) {
> 
> This appears to be a false alarm.

Okay, thanks!

> All the "culprit" patch did was rename the local variable
> "target_memcg".
> 
> And while it's correct that memcg can be NULL (befor and after this
> patch), it's the case only when mem_cgroup_disabled(), and
> mem_cgroup_protected() checks for this case.

Right, that's certainly the design. I wonder if in the interests of
defensively asserting requirements, it would be worth adding something
like this to mem_cgroup_protected():

if (WARN_ON_ONCE(!memcg))
    return MEMCG_PROT_NONE;

?

-- 
Kees Cook