linux-kernel - Re: [PATCHv2] ptrace: fix PTRACE_LISTEN race corrupting task->state

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Date:   Wed, 5 Apr 2017 14:36:32 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     bsegall@...gle.com, linux-kernel@...r.kernel.org,
        Roland McGrath <roland@...k.frob.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCHv2] ptrace: fix PTRACE_LISTEN race corrupting task->state

On 04/04, Andrew Morton wrote:
>
> On Tue, 04 Apr 2017 14:47:34 -0700 bsegall@...gle.com wrote:
>
> > In PT_SEIZED + LISTEN mode STOP/CONT signals cause a wakeup against
> > __TASK_TRACED. If this races with the ptrace_unfreeze_traced at the end
> > of a PTRACE_LISTEN, this can wake the task /after/ the check against
> > __TASK_TRACED, but before the reset of state to TASK_TRACED. This causes
> > it to instead clobber TASK_WAKING, allowing a subsequent wakeup against
> > TRACED while the task is still on the rq wake_list, corrupting it.
>
> The changelog doesn't convey the urgency of the fix.  To understand
> this we'll need to know the user-visible impact of the bug and the
> likelihood of someone hitting it.

The kernel can crash or this can lead to other hard-to-debug problems.
In short, "task->state = TASK_TRACED" in ptrace_unfreeze_traced() assumes
that nobody else can wake it up, but PTRACE_LISTEN breaks the contract.
Obviusly it is veru wrong to manipulate task->state if this task is already
running, or WAKING, or it sleeps again.

> Also your suggestion regarding which kernel version(s) should be fixed
> (and the reasoning) is always valuable.

This fixes 9899d11f "ptrace: ensure arch_ptrace/ptrace_request can never
race with SIGKILL"

Oleg.