Re: [PATCH] mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty()

[Kefeng Wang]

On 2023/1/30 5:48, Andrew Morton wrote:
> On Sun, 29 Jan 2023 10:44:51 +0800 Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
>
> > As commit 18365225f044 ("hwpoison, memcg: forcibly uncharge LRU pages"),
>
> Merged in 2017.
>
> > hwpoison will forcibly uncharg a LRU hwpoisoned page, the folio_memcg
> > could be NULl, then, mem_cgroup_track_foreign_dirty_slowpath() could
> > occurs a NULL pointer dereference, let's do not record the foreign
> > writebacks for folio memcg is null in mem_cgroup_track_foreign() to
> > fix it.
> >
> > Reported-by: Ma Wupeng <mawupeng1@xxxxxxxxxx>
> > Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
>
> Merged in 2019.
>
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -1688,10 +1688,13 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio,
> >   static inline void mem_cgroup_track_foreign_dirty(struct folio *folio,
> >   						  struct bdi_writeback *wb)
> >   {
> > +	struct mem_cgroup *memcg;
> > +
> >   	if (mem_cgroup_disabled())
> >   		return;
> >   
> > -	if (unlikely(&folio_memcg(folio)->css != wb->memcg_css))
> > +	memcg = folio_memcg(folio);
> > +	if (unlikely(memcg && &memcg->css != wb->memcg_css))
> >   		mem_cgroup_track_foreign_dirty_slowpath(folio, wb);
> >   }
>
> Has this null deref actually been observed, or is this from code
> inspection?  (This is why it's nice to include the Link: after a
> Reported-by!)

It does occurs in our internal test and report by wupeng(based v5.10),

BUG: KASAN: user-memory-access in 
mem_cgroup_track_foreign_dirty_slowpath+0xc0/0x480 mm/memcontrol.c:4708
Read of size 8 at addr 0000000000001000 by task syz-executor.2/28325

CPU: 2 PID: 28325 Comm: syz-executor.2 Tainted: G        W 
5.10.0-03333-g48e46a146cbc-dirty #1
Hardware name: linux,dummy-virt (DT)
Call trace:
 ...
 mem_cgroup_track_foreign_dirty_slowpath+0xc0/0x480 mm/memcontrol.c:4708
 mem_cgroup_track_foreign_dirty include/linux/memcontrol.h:1880 [inline]
 account_page_dirtied+0x9a0/0xa90 mm/page-writeback.c:2436
 __set_page_dirty+0x1f8/0x4b0 fs/buffer.c:608
 __set_page_dirty_buffers+0x3d0/0x550 fs/buffer.c:668
 set_page_dirty+0x158/0x500 mm/page-writeback.c:2575
 filemap_page_mkwrite+0x3dc/0x490 mm/filemap.c:3224
 do_page_mkwrite+0xc4/0x3d0 mm/memory.c:2786
 wp_page_shared+0x14c/0x980 mm/memory.c:3118
 do_wp_page+0x930/0xbc0 mm/memory.c:3219
 handle_pte_fault+0x5e0/0x630 mm/memory.c:4570
 __handle_mm_fault+0x41c/0x910 mm/memory.c:4690
 handle_mm_fault+0x25c/0x484 mm/memory.c:4788
 __do_page_fault arch/arm64/mm/fault.c:440 [inline]
 do_page_fault+0x3ac/0x9d4 arch/arm64/mm/fault.c:539

> Do we have any theories why this took so many years to surface?

After google, I found a similar issue[1], maybe 
hwpoison/mem_cgroup_track_foreign_dirty concurrency is uncommon.

[1] https://syzkaller.appspot.com/bug?extid=0c84bf23aed8ee0d8399

> I'm confused about the mention of 18365225f044, but the Fixes: target
> is a different commit.  Please explain this?

18365225f044 said that it will uncharge it manually from its memcg in 
hwpison handler, but when the new feature "writeback, memcg: Implement 
foreign dirty flushing" is introduced, we don't consider this, when
folio's memcg is cleared by hwpison handler, the issue occurs.

> Do you think the fix should be backported into earlier -stable kernels?

it's better to send stable kernel.

> If so, it will need some rework due to the subsequent folio
> conversion.

When the patch is merged, I could refresh and send to stable maillist.

>