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

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Date:	Wed, 26 May 2010 14:40:06 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	David Howells <dhowells@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.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, Mike Frysinger wrote:
>
> On Tue, May 25, 2010 at 06:23, Oleg Nesterov wrote:
> >        - 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?
>
> i dont believe there are any code paths in UP systems where this would
> be a practical problem because sram_list is only updated by syscalls
> from userspace.

Yes sure, UP && !PREEMPT is safe.

> we probably should add proper locking to this
> structure though.

Agreed. I'll try to make the trivial patch tomorrow. I think we
can just use mm->mmap_sem, is_user_addr_valid() needs this lock
for find_vma() anyway.

Oleg.

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