linux-kernel - Re: perf: use-after-free in perf

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Date:   Tue, 14 Mar 2017 14:24:27 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: perf: use-after-free in perf_release

On 03/14, Peter Zijlstra wrote:
>
> On Tue, Mar 07, 2017 at 05:51:32PM +0100, Oleg Nesterov wrote:
>
> > inherit_event() returns NULL under is_orphaned_event() check, not ERR_PTR().
> > Is it correct?
>
> Yes. This is all a tad tricky, but it seems to be correct.
>
> By returning NULL, not an error, we affect the silent discard of
> orphaned events. This is correct, because otherwise
> perf_event_release_kernel() would have come by and explicitly discarded
> those events for us anyway.

Thanks... I'll try to understand this later.

> @@ -10608,7 +10627,6 @@ inherit_task_group(struct perf_event *event, struct task_struct *parent,
>  		 * First allocate and initialize a context for the
>  		 * child.
>  		 */
> -
>  		child_ctx = alloc_perf_context(parent_ctx->pmu, child);
>  		if (!child_ctx)
>  			return -ENOMEM;
> @@ -10670,7 +10688,7 @@ static int perf_event_init_context(struct task_struct *child, int ctxn)
>  		ret = inherit_task_group(event, parent, parent_ctx,
>  					 child, ctxn, &inherited_all);
>  		if (ret)
> -			break;
> +			goto out_unlock;
>  	}
>  
>  	/*
> @@ -10686,7 +10704,7 @@ static int perf_event_init_context(struct task_struct *child, int ctxn)
>  		ret = inherit_task_group(event, parent, parent_ctx,
>  					 child, ctxn, &inherited_all);
>  		if (ret)
> -			break;
> +			goto out_unlock;

With this change you can also simplify inherit_task_group() a little bit,
it no longer needs to nullify *inherited_all if inherit_group() fails.

Oleg.