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Date:   Fri, 18 Oct 2019 17:12:16 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Prateek Sood <prsood@codeaurora.org>
Cc:     mingo@redhat.com, linux-kernel@vger.kernel.org,
        kaushalk@codeaurora.org, Peter Zijlstra <peterz@infradead.org>,
        Song Liu <songliubraving@fb.com>
Subject: Re: [PATCH] trace: fix race in perf_trace_buf initialization
Message-ID: <20191018171216.3e446f1e@gandalf.local.home>
In-Reply-To: <1571120245-4186-1-git-send-email-prsood@codeaurora.org>
References: <1571120245-4186-1-git-send-email-prsood@codeaurora.org>
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Peter told me it was fine, but I'm also adding Song who is the author
of the code.

Also needs the tags (which I'll add)

Cc: stable@vger.kernel.org
Fixes: e12f03d7031a9 ("perf/core: Implement the 'perf_kprobe' PMU")

-- Steve


On Tue, 15 Oct 2019 11:47:25 +0530
Prateek Sood <prsood@codeaurora.org> wrote:

> [  943.034988] Unable to handle kernel paging request at virtual address 0000003106f2003c
> [  943.043653] Mem abort info:
> [  943.046679]   ESR = 0x96000045
> [  943.050428]   Exception class = DABT (current EL), IL = 32 bits
> [  943.056643]   SET = 0, FnV = 0
> [  943.060168]   EA = 0, S1PTW = 0
> [  943.063449] Data abort info:
> [  943.066474]   ISV = 0, ISS = 0x00000045
> [  943.070856]   CM = 0, WnR = 1
> [  943.074016] user pgtable: 4k pages, 39-bit VAs, pgdp = ffffffc034b9b000
> [  943.081446] [0000003106f2003c] pgd=0000000000000000, pud=0000000000000000
> [  943.088862] Internal error: Oops: 96000045 [#1] PREEMPT SMP
> [  943.141700] Process syz-executor (pid: 18393, stack limit = 0xffffffc093190000)
> [  943.164146] pstate: 80400005 (Nzcv daif +PAN -UAO)
> [  943.169119] pc : __memset+0x20/0x1ac
> [  943.172831] lr : memset+0x3c/0x50
> [  943.176269] sp : ffffffc09319fc50
> 
> [  943.557593]  __memset+0x20/0x1ac
> [  943.560953]  perf_trace_buf_alloc+0x140/0x1a0
> [  943.565472]  perf_trace_sys_enter+0x158/0x310
> [  943.569985]  syscall_trace_enter+0x348/0x7c0
> [  943.574413]  el0_svc_common+0x11c/0x368
> [  943.578394]  el0_svc_handler+0x12c/0x198
> [  943.582459]  el0_svc+0x8/0xc
> 
> In Ramdumps:
> total_ref_count = 3
> perf_trace_buf = (
>     0x0 -> NULL,
>     0x0 -> NULL,
>     0x0 -> NULL,
>     0x0 -> NULL)
> 
> event_call in perf_trace_sys_enter()
> event_call = 0xFFFFFF900CB511D8 -> (
>     list = (next = 0xFFFFFF900CB4E2E0, prev = 0xFFFFFF900CB512B0),
>     class = 0xFFFFFF900CDC8308,
>     name = 0xFFFFFF900CDDA1D8,
>     tp = 0xFFFFFF900CDDA1D8,
>     event = (
>       node = (next = 0x0, pprev = 0xFFFFFF900CB80210),
>       list = (next = 0xFFFFFF900CB512E0, prev = 0xFFFFFF900CB4E310),
>       type = 21,
>       funcs = 0xFFFFFF900CB51130),
>     print_fmt = 0xFFFFFF900CB51150,
>     filter = 0x0,
>     mod = 0x0,
>     data = 0x0,
>     flags = 18,
>     perf_refcount = 1,
>     perf_events = 0xFFFFFF8DB8E54158,
>     prog_array = 0x0,
>     perf_perm = 0x0)
> 
> perf_events added on CPU0
> (struct hlist_head *)(0xFFFFFF8DB8E54158+__per_cpu_offset[0]) -> (
>     first = 0xFFFFFFC0980FD0E0 -> (
>       next = 0x0,
>       pprev = 0xFFFFFFBEBFD74158))
> 
> Could you please confirm:
> 1) the race mentioned below exists or not.
> 2) if following patch fixes it.
> 
> 
> >8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8>8  
> 
> A race condition exists while initialiazing perf_trace_buf from
> perf_trace_init() and perf_kprobe_init().
> 
>       CPU0                                        CPU1
> perf_trace_init()
>   mutex_lock(&event_mutex)
>     perf_trace_event_init()
>       perf_trace_event_reg()
>         total_ref_count == 0
> 	buf = alloc_percpu()
>         perf_trace_buf[i] = buf
>         tp_event->class->reg() //fails       perf_kprobe_init()
> 	goto fail                              perf_trace_event_init()
>                                                  perf_trace_event_reg()
>         fail:
> 	  total_ref_count == 0
> 
>                                                    total_ref_count == 0
>                                                    buf = alloc_percpu()
>                                                    perf_trace_buf[i] = buf
>                                                    tp_event->class->reg()
>                                                    total_ref_count++
> 
>           free_percpu(perf_trace_buf[i])
>           perf_trace_buf[i] = NULL
> 
> Any subsequent call to perf_trace_event_reg() will observe total_ref_count > 0,
> causing the perf_trace_buf to be NULL always. This can result in perf_trace_buf
> getting accessed from perf_trace_buf_alloc() without being initialized. Acquiring
> event_mutex in perf_kprobe_init() before calling perf_trace_event_init() should
> fix this race.
> 
> Signed-off-by: Prateek Sood <prsood@codeaurora.org>
> ---
>  kernel/trace/trace_event_perf.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 4629a61..48ee92c 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -272,9 +272,11 @@ int perf_kprobe_init(struct perf_event *p_event, bool is_retprobe)
>  		goto out;
>  	}
>  
> +	mutex_lock(&event_mutex);
>  	ret = perf_trace_event_init(tp_event, p_event);
>  	if (ret)
>  		destroy_local_trace_kprobe(tp_event);
> +	mutex_unlock(&event_mutex);
>  out:
>  	kfree(func);
>  	return ret;
> @@ -282,8 +284,10 @@ int perf_kprobe_init(struct perf_event *p_event, bool is_retprobe)
>  
>  void perf_kprobe_destroy(struct perf_event *p_event)
>  {
> +	mutex_lock(&event_mutex);
>  	perf_trace_event_close(p_event);
>  	perf_trace_event_unreg(p_event);
> +	mutex_unlock(&event_mutex);
>  
>  	destroy_local_trace_kprobe(p_event->tp_event);
>  }