linux-kernel - Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task breakpoints

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Date:	Mon, 11 Apr 2011 11:47:57 +0100
From:	Will Deacon <will.deacon@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"v2.6.33.." <stable@...nel.org>
Subject: Re: [PATCH 1/5] ptrace: Prepare to fix racy accesses on task
 breakpoints

Hi Frederic,

On Fri, 2011-04-08 at 18:34 +0100, Frederic Weisbecker wrote:
> When a task is traced and is in a stopped state, the tracer
> may execute a ptrace request to examine the tracee state and
> get its task struct. Right after, the tracee can be killed
> and thus its breakpoints released.
> This can happen concurrently when the tracer is in the middle
> of reading or modifying these breakpoints, leading to dereferencing
> a freed pointer.

Oo, that's nasty. Would an alternative solution be to free the
breakpoints only when the task_struct usage count is zero?

> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 0fc1eed..dc7ab65 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -22,6 +22,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/uaccess.h>
>  #include <linux/regset.h>
> +#include <linux/hw_breakpoint.h>
> 
> 
>  /*
> @@ -879,3 +880,19 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
>         return ret;
>  }
>  #endif /* CONFIG_COMPAT */
> +
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +int ptrace_get_breakpoints(struct task_struct *tsk)
> +{
> +       if (atomic_inc_not_zero(&tsk->ptrace_bp_refcnt))
> +               return 0;
> +
> +       return -1;
> +}


Would it be better to return -ESRCH here instead?

Will

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