linux-kernel - Re: [PATCH] copy_process(): Move fd_install() out of sighand->siglock critical section

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Date:   Wed, 9 Feb 2022 11:25:20 -0500
From:   Waiman Long <longman@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Christian Brauner <brauner@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>,
        Alexey Gladkov <legion@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] copy_process(): Move fd_install() out of sighand->siglock
 critical section

On 2/8/22 16:59, Eric W. Biederman wrote:
> Al Viro <viro@...iv.linux.org.uk> writes:
>
>> On Tue, Feb 08, 2022 at 01:51:35PM -0500, Waiman Long wrote:
>>> On 2/8/22 13:16, Al Viro wrote:
>>>> On Tue, Feb 08, 2022 at 11:39:12AM -0500, Waiman Long wrote:
>>>>
>>>>> One way to solve this problem is to move the fd_install() call out of
>>>>> the sighand->siglock critical section.
>>>>>
>>>>> Before commit 6fd2fe494b17 ("copy_process(): don't use ksys_close()
>>>>> on cleanups"), the pidfd installation was done without holding both
>>>>> the task_list lock and the sighand->siglock. Obviously, holding these
>>>>> two locks are not really needed to protect the fd_install() call.
>>>>> So move the fd_install() call down to after the releases of both locks.
>>>> 	Umm... That assumes we can delay it that far.  IOW, that nothing
>>>> relies upon having pidfd observable in /proc/*/fd as soon as the child
>>>> becomes visible there in the first place.
>>>>
>>>> 	What warranties are expected from CLONE_PIDFD wrt observation of
>>>> child's descriptor table?
>>>>
>>> I think the fd_install() call can be moved after the release of
>>> sighand->siglock but before the release the tasklist_lock. Will that be good
>>> enough?
>> Looks like it should, but I'd rather hear from the CLONE_PIDFD authors first...
>> Christian, could you comment on that?
> The tasklist_lock and the siglock provide no protection against
> being looked up in proc.
>
> The proc filesystem looks up process information with things only
> protected by the rcu_read_lock().  Which means that the process
> will be visible through proc after "attach_pid(p, PIDTYPE_PID".
>
> The fd is being installed in the fdtable of the parent process,
> and the siglock and tasklist_lock are held to protect the child.
>
>
> Further fd_install is exposing the fd to userspace where it can be used
> by the process_madvise and the process_mrelease system calls, from
> anything that shares the fdtable of the parent thread.  Which means it
> needs to be guaranteed that kernel_clone will call wake_up_process
> before it is safe to call fd_install.
>
>
> So it appears to me that moving fd_install earlier fundamentally unsafe,
> and the locks are meaningless from an fd_install perspective.
>
> Which means it should be perfectly fine to move the fd_install outside
> of the tasklist_lock and the outside siglock.
>
>
> I don't see how we could support the fd appearing in the fdtable sooner
> which seems to make the question moot as to weather userspace in some
> odd corner case expects the fd to appear in the fdtable sooner.
>
> So I say move fd_install down with proc_fork_connector and friends.

Right. Keeping fd_install() inside of tasklist_lock may also be 
problematic as a read lock can be taken at interrupt context which may 
cause similar lockdep splat. So I am keep this patch as is.

Cheers,
Longman