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Tue, 06 Jan 2026 02:26:50 -0800 (PST) From: Qing Wang To: oleg@redhat.com Cc: Liam.Howlett@oracle.com, akpm@linux-foundation.org, brauner@kernel.org, bsegall@google.com, david@kernel.org, dietmar.eggemann@arm.com, jack@suse.cz, joel.granados@kernel.org, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, lorenzo.stoakes@oracle.com, mingo@redhat.com, mjguzik@gmail.com, peterz@infradead.org, rostedt@goodmis.org, rppt@kernel.org, syzbot+e0378d4f4fe57aa2bdd0@syzkaller.appspotmail.com, vbabka@suse.cz, vincent.guittot@linaro.org, wangqing7171@gmail.com Subject: Re: [PATCH] fork/pid: Fix use-after-free in __task_pid_nr_ns Date: Tue, 6 Jan 2026 18:26:42 +0800 Message-Id: <20260106102642.2765480-1-wangqing7171@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Tue, 06 Jan 2026 at 17:04, Oleg Nesterov wrote: > At first glance this is racy. Can't task->signal be freed right after > the check? > > And... Can't we make another fix? If copy_process() fails and does > free_signal_struct(), the child has not been added to rcu protected > lists and init_task_pid(child) was not called yet. > > So perhaps something like the patch below can work? > > Oleg. > --- > > --- x/kernel/events/core.c > +++ x/kernel/events/core.c > @@ -1422,16 +1422,17 @@ unclone_ctx(struct perf_event_context *c > static u32 perf_event_pid_type(struct perf_event *event, struct task_struct *p, > enum pid_type type) > { > - u32 nr; > + u32 nr = 0; > /* > * only top level events have the pid namespace they were created in > */ > if (event->parent) > event = event->parent; > > - nr = __task_pid_nr_ns(p, type, event->ns); > + if (pid_alive(p)) > + nr = __task_pid_nr_ns(p, type, event->ns); > /* avoid -1 if it is idle thread or runs in another ns */ > - if (!nr && !pid_alive(p)) > + if (!nr) > nr = -1; > return nr; > } Sorry, please ignore my previous reply. I've reconsidered your code, and using pid_alive() to check the validity of tsk->signal is actually correct. The pid is assigned after copy_signal(), so if a task has a PID, its tsk->signal memory is guaranteed to be valid.