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bh=RdvAMuUQxe0Qi+V1vQERcuk1lbOQES8EHOAEiADwjx4=; b=m5Q9r71oR3IXcx5IJ3/AiFO8JPXJjXS7KoyAGdGRARXfodwgrkNaesBy57EbrKdvP+ qI4gy3A/rFkORc/q2+760jcYquLKcrDlc2sLQE0gPiXVIJOKgSmP9A42t8H84sgwNLDe 27Va7rYL0Nee24g+kLIutSN7kZHtjPAnTRUkJmaDua+v1njO/Rh4qRj2pz4j+6MYKtAf oc7M9v9K5xLar0RH7FWiGK5sDE0Q73qjeV82eNryDmGUVIzr0OfSqq05UALoIquT5Ebb 0jGx5UT0lBYzqeoSVfFdkBFmo/I+1oNR5ANVU+EAJsTZ+aRbzp9hiCjKxJwnfEP2M7Se mSeg== X-Gm-Message-State: APjAAAVHKb2WMs8jNBpxRwX6f2fHPh6bFhsYScy3Zx5wELyP6sb724ec 45BYLY78v4tF//UH//HFQBHIWT8tm6rvVIVD2Ybxjg== X-Google-Smtp-Source: APXvYqyXTwYgrj0c7UYR5UOFwxokPf4vTsxbTx3fy2BDq5QidLoMSOhiLMdFoee85BdD5EQHBPP1Vxuuj+PwahJ4k7s= X-Received: by 2002:a9d:724e:: with SMTP id a14mr2516551otk.23.1570621719468; Wed, 09 Oct 2019 04:48:39 -0700 (PDT) MIME-Version: 1.0 References: <20191008154418.GA16972@andrea> <20191009113134.5171-1-christian.brauner@ubuntu.com> In-Reply-To: <20191009113134.5171-1-christian.brauner@ubuntu.com> From: Marco Elver Date: Wed, 9 Oct 2019 13:48:27 +0200 Message-ID: Subject: Re: [PATCH] taskstats: fix data-race To: Christian Brauner Cc: Andrea Parri , bsingharora@gmail.com, Dmitry Vyukov , LKML , stable@vger.kernel.org, syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 9 Oct 2019 at 13:31, Christian Brauner wrote: > > When assiging and testing taskstats in taskstats_exit() there's a race > when writing and reading sig->stats when a thread-group with more than > one thread exits: > > cpu0: > thread catches fatal signal and whole thread-group gets taken down > do_exit() > do_group_exit() > taskstats_exit() > taskstats_tgid_alloc() > The tasks reads sig->stats without holding sighand lock seeing garbage. > > cpu1: > task calls exit_group() > do_exit() > do_group_exit() > taskstats_exit() > taskstats_tgid_alloc() > The task takes sighand lock and assigns new stats to sig->stats. > > Fix this by using smp_load_acquire() and smp_store_release(). > > Reported-by: syzbot+c5d03165a1bd1dead0c1@syzkaller.appspotmail.com > Fixes: 34ec12349c8a ("taskstats: cleanup ->signal->stats allocation") > Cc: stable@vger.kernel.org > Signed-off-by: Christian Brauner > Reviewed-by: Dmitry Vyukov > --- > /* v1 */ > Link: https://lore.kernel.org/r/20191005112806.13960-1-christian.brauner@ubuntu.com > > /* v2 */ > Link: https://lore.kernel.org/r/20191006235216.7483-1-christian.brauner@ubuntu.com > - Dmitry Vyukov , Marco Elver : > - fix the original double-checked locking using memory barriers > > /* v3 */ > Link: https://lore.kernel.org/r/20191007110117.1096-1-christian.brauner@ubuntu.com/ > - Andrea Parri : > - document memory barriers to make checkpatch happy > > /* v4 */ > - Andrea Parri : > - use smp_load_acquire(), not READ_ONCE() > - update commit message Acked-by: Marco Elver Note that this now looks almost like what I suggested, except the return at the end of the function is accessing sig->stats again. In this case, it seems it's fine assuming sig->stats cannot be written elsewhere. Just wanted to point it out to make sure it's considered. Thanks! > --- > kernel/taskstats.c | 24 +++++++++++++++--------- > 1 file changed, 15 insertions(+), 9 deletions(-) > > diff --git a/kernel/taskstats.c b/kernel/taskstats.c > index 13a0f2e6ebc2..e6b45315607a 100644 > --- a/kernel/taskstats.c > +++ b/kernel/taskstats.c > @@ -554,24 +554,30 @@ static int taskstats_user_cmd(struct sk_buff *skb, struct genl_info *info) > static struct taskstats *taskstats_tgid_alloc(struct task_struct *tsk) > { > struct signal_struct *sig = tsk->signal; > - struct taskstats *stats; > + struct taskstats *stats_new, *stats; > > - if (sig->stats || thread_group_empty(tsk)) > - goto ret; > + /* Pairs with smp_store_release() below. */ > + stats = smp_load_acquire(sig->stats); > + if (stats || thread_group_empty(tsk)) > + return stats; > > /* No problem if kmem_cache_zalloc() fails */ > - stats = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > + stats_new = kmem_cache_zalloc(taskstats_cache, GFP_KERNEL); > > spin_lock_irq(&tsk->sighand->siglock); > if (!sig->stats) { > - sig->stats = stats; > - stats = NULL; > + /* > + * Pairs with smp_store_release() above and order the > + * kmem_cache_zalloc(). > + */ > + smp_store_release(&sig->stats, stats_new); > + stats_new = NULL; > } > spin_unlock_irq(&tsk->sighand->siglock); > > - if (stats) > - kmem_cache_free(taskstats_cache, stats); > -ret: > + if (stats_new) > + kmem_cache_free(taskstats_cache, stats_new); > + > return sig->stats; > } > > -- > 2.23.0 >