linux-kernel - Re: [PATCH] fork: avoid inappropriate uprobe access to invalid mm

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Message-ID: <20241210172315.GC31266@redhat.com>
Date: Tue, 10 Dec 2024 18:23:19 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	linux-mm@...ck.org, Peng Zhang <zhangpeng.00@...edance.com>
Subject: Re: [PATCH] fork: avoid inappropriate uprobe access to invalid mm

On 12/10, Oleg Nesterov wrote:
>
> I must have missed something, but...
>
> On 12/10, Lorenzo Stoakes wrote:
> >
> > @@ -1746,9 +1741,11 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
> >  	if (!mm_init(mm, tsk, mm->user_ns))
> >  		goto fail_nomem;
> >
> > +	uprobe_start_dup_mmap();
> >  	err = dup_mmap(mm, oldmm);
> >  	if (err)
> > -		goto free_pt;
> > +		goto free_pt_end_uprobe;
> > +	uprobe_end_dup_mmap();
> >
> >  	mm->hiwater_rss = get_mm_rss(mm);
> >  	mm->hiwater_vm = mm->total_vm;
> > @@ -1758,6 +1755,8 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
> >
> >  	return mm;
> >
> > +free_pt_end_uprobe:
> > +	uprobe_end_dup_mmap();
>
> if dup_mmap() fails and "mm" is incomplete, then with this version dup_mmap_sem
> is dropped before __mmput/exit_mmap/etc. How can this help?

Easy to fix, but what ensures that another mmget(mm) is not possible until
dup_mm() calls mmput() and drops dup_mmap_sem? Sorry, my understanding of mm/
is very limited...

Oleg.