linux-kernel - Re: [PATCH] pid: Add a judgment for ns null in pid_nr

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Message-ID: <20250804-gepfercht-delfin-0172b1ee9556@brauner>
Date: Mon, 4 Aug 2025 14:14:02 +0200
From: Christian Brauner <brauner@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: 高翔 <gaoxiang17@...omi.com>, 
	Al Viro <viro@...iv.linux.org.uk>, Xiang Gao <gxxa03070307@...il.com>, 
	"mjguzik@...il.com" <mjguzik@...il.com>, "Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>, 
	"joel.granados@...nel.org" <joel.granados@...nel.org>, "lorenzo.stoakes@...cle.com" <lorenzo.stoakes@...cle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] pid: Add a judgment for ns null in pid_nr_ns

On Mon, Aug 04, 2025 at 01:49:01PM +0200, Oleg Nesterov wrote:
> On 08/04, 高翔 wrote:
> >
> >     struct task_struct *tsk = current;
> >
> >     struct task_struct *parent;
> >
> >     ...
> >
> >     info->pid = task_pid_vnr(tsk);
> >     rcu_read_lock();
> >     parent = rcu_dereference(tsk->real_parent);
> >     get_task_struct(parent);
> >     rcu_read_unlock();
> >     info->ppid = task_tgid_vnr(parent);
> >     strncpy(info->ptask_name, parent->comm, TASK_COMM_LEN);
> >     put_task_struct(parent);
> 
> So I guess the kernel crashes when you try to obtain another process's pid, not
> the current process's pid. This is was I suspected.
> 
> This code is buggy. tsk->real_parent points to nowhere if tsk = current was reaped.
> rcu_read_lock() alone can't help. Even get_task_struct(parent) is not safe. And it
> is not needed.
> 
> You need something like
> 
> 	info->pid = info->ppid = 0;
> 
> 	rcu_read_lock();
> 	if (pid_alive(tsk)) {
> 		info->pid = task_pid_vnr(tsk);
> 		info->ppid = task_tgid_vnr(tsk->real_parent);
> 	}
> 	rcu_read_unlock();

I distinctly remember having seen a similar patch about 5 years ago that
did the exact same fix for some out-of-tree abuse:
20201201024811.GA72235@...172-31-62-0.us-west-2.compute.internal

Where in the kernel is that code supposed to live? Is this again an
out-of-tree module?

  __task_pid_nr_ns+0x74/0xd0
  get_common_info+0x9c/0x1c0 [io_xxx 39b55c95a0fe9416f7d7be396be0fd1d6f590f17]
  io_monitor_global_log+0x1a0/0x294 [io_xxx 39b55c95a0fe9416f7d7be396be0fd1d6f590f17]
  cb_android_vh_ufs_compl_command+0x304/0x578 [io_xxx 39b55c95a0fe9416f7d7be396be0fd1d6f590f17]
  __traceiter_android_vh_ufs_compl_command+0x54/0x78

all looks to me like some out-of-tree stuff...

So no, not taking that.

The only thing I can see as a sensible addition is:

diff --git a/kernel/pid.c b/kernel/pid.c
index c45a28c16cd2..f27cbc208c5e 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -491,6 +491,9 @@ pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
        struct upid *upid;
        pid_t nr = 0;

+       if (unlikely(WARN_ON_ONCE(!ns)))
+               return 0;
+
        if (pid && ns->level <= pid->level) {
                upid = &pid->numbers[ns->level];
                if (upid->ns == ns)

So that we yell at abusers of pid_nr_ns(). But even that...