linux-kernel - Re: [PATCH 1/3] uprobes: Change write_opcode() to use FOLL

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Date:	Tue, 25 Sep 2012 14:19:31 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] uprobes: Change write_opcode() to use FOLL_FORCE

* Oleg Nesterov <oleg@...hat.com> [2012-09-16 19:52:42]:

> write_opcode()->get_user_pages() needs FOLL_FORCE to ensure we can
> read the page even if the probed task did mprotect(PROT_NONE) after
> uprobe_register(). Without FOLL_WRITE, FOLL_FORCE doesn't have any
> side effect but allows to read the !VM_READ memory.
> 
> Otherwiese the subsequent uprobe_unregister()->set_orig_insn() fails
> and we leak "int3". If that task does mprotect(PROT_READ | EXEC) and
> execute the probed insn later it will be killed.
> 
> Note: in fact this is also needed for _register, see the next patch.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>

> ---
>  kernel/events/uprobes.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 14c2e99..fa1579a 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -221,7 +221,7 @@ static int write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
> 
>  retry:
>  	/* Read the page with vaddr into memory */
> -	ret = get_user_pages(NULL, mm, vaddr, 1, 0, 0, &old_page, &vma);
> +	ret = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &old_page, &vma);
>  	if (ret <= 0)
>  		return ret;
> 
> -- 
> 1.5.5.1
> 

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