linux-kernel - Re: [syzbot] general protection fault in list_lru

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Date:   Thu, 24 Mar 2022 17:18:28 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        syzbot <syzbot+f8c45ccc7d5d45fc5965@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] general protection fault in list_lru_add

On Thu, 24 Mar 2022 at 17:13, Muchun Song <songmuchun@...edance.com> wrote:
>
> On Thu, Mar 24, 2022 at 4:50 PM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >
> > On Thu, 24 Mar 2022 at 09:44, Muchun Song <songmuchun@...edance.com> wrote:
> > >
> > > On Thu, Mar 24, 2022 at 11:05 AM Linus Torvalds
> > > <torvalds@...ux-foundation.org> wrote:
> > > >
> > > > On Wed, Mar 23, 2022 at 7:19 PM Muchun Song <songmuchun@...edance.com> wrote:
> > > > >
> > > > > After this commit, the rules of dentry allocations changed.
> > > > > The dentry should be allocated by kmem_cache_alloc_lru()
> > > >
> > > > Yeah, I looked at that, but I can't find any way there could be other
> > > > allocations - not only are there strict rules how to initialize
> > > > everything, but the dentries are free'd using
> > > >
> > > >         kmem_cache_free(dentry_cache, dentry);
> > > >
> > > > and as a result if they were allocated any other way I would expect
> > > > things would go south very quickly.
> > > >
> > > > The only other thing I could come up with is some breakage in the
> > > > superblock lifetime so that &dentry->d_sb->s_dentry_lru would have
> > > > problems, but again, this is *such* core code and not some unusual
> > > > path, that I would be very very surprised if it wouldn't have
> > > > triggered other issues long long ago.
> > > >
> > > > That's why I'd be more inclined to worry about the list_lru code being
> > > > somehow broken.
> > > >
> > >
> > > I also have the same concern.  I have been trying for a few hours to
> > > reproduce this issue, but it didn't oops on my test machine.  And I'll
> > > continue reproducing this.
> >
> > syzbot triggered it 222 times in a day, so it's most likely real:
> > https://syzkaller.appspot.com/bug?extid=f8c45ccc7d5d45fc5965
> >
> > There are 2 reproducers, but they look completely different. May be a race.
> > You may also try to use syzbot's patch testing feature to get some
> > additional debug info.
>
> Do you know how to tell the syzbot to test the following patch?
> I found some infos from github, it says "#syz test:", is it like the following?
> Thanks.
>
> #syz test: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
> master

Yes, this is correct. You can now see the request listed here:
https://syzkaller.appspot.com/bug?extid=f8c45ccc7d5d45fc5965

but the patch was truncated (probably you email client messed
whitespaces). In such case it's more reliable to attach the patch as
text file.



> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index c669d87001a6..ddb2ee627d32 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -67,6 +67,7 @@ list_lru_from_kmem(struct list_lru *lru, int nid, void *ptr,
>         struct list_lru_node *nlru = &lru->node[nid];
>         struct list_lru_one *l = &nlru->lru;
>         struct mem_cgroup *memcg = NULL;
> +       int kmemcg_id;
>
>         if (!list_lru_memcg_aware(lru))
>                 goto out;
> @@ -75,7 +76,13 @@ list_lru_from_kmem(struct list_lru *lru, int nid, void *ptr,
>         if (!memcg)
>                 goto out;
>
> -       l = list_lru_from_memcg_idx(lru, nid, memcg_kmem_id(memcg));
> +       kmemcg_id = memcg_kmem_id(memcg);
> +       l = list_lru_from_memcg_idx(lru, nid, kmemcg_id);
> +       if (!l) {
> +               pr_info("BUG: the memcg(%px)->objcg(%px), kmemcg_id: %d\n",
> +                       memcg, memcg->objcg, kmemcg_id);
> +               BUG();
> +       }
>  out:
>         if (memcg_ptr)
>                 *memcg_ptr = memcg;