linux-kernel - Re: [PATCH -mm 1/1] ptrace: PTRACE_GETFDPIC: fix the unsafe usage of child->mm

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Date:	Tue, 25 May 2010 12:23:45 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Frysinger <vapier@...too.org>, linux-sh@...r.kernel.org,
	Paul Mundt <lethal@...ux-sh.org>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH -mm 1/1] ptrace: PTRACE_GETFDPIC: fix the unsafe usage
	of child->mm

On 05/25, David Howells wrote:
>
> Roland McGrath <roland@...hat.com> wrote:
>
> > The mm pointer is only used by these uncommon ptrace operations
>
> Like PEEKTEXT and POKETEXT?

They use access_process_vm().

According to grep, mm is only use to read a couple of members.

Perhaps can even add the simple helper

	struct mm_xxx {
		unsigned long start_code, end_code, start_data, end_data;
		... some more ...
	};

	int get_mm_xxx(struct task_struct *tracee, struct mm_xxx *mm_xxx)
	{
		struct mm_struct *mm = get_task_mm(tracee);
		...
	}

Except:

	- arch/um/kernel/ptrace.c PTRACE_SWITCH_MM does something
	  really strange

	- arch/blackfin/kernel/ptrace.c:is_user_addr_valid()
	  needs mmap_sem around find_vma()

	  The lockless access to mm->context.sram_list doesn't look
	  safe to me.

	  If we add get_task_mm() - this protects us against
	  destroy_context() only. What is the tracee's sub-thread
	  does sys_sram_alloc() or sys_sram_free() in parallel?

Oleg.

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