linux-kernel - Re: [PATCH 1/2] ftrace: Fix NULL pointer dereference in t_probe

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Message-ID: <20190830164706.453a119e@gandalf.local.home>
Date:   Fri, 30 Aug 2019 16:47:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     <linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] ftrace: Fix NULL pointer dereference in
 t_probe_next()

On Thu,  4 Jul 2019 20:04:41 +0530
"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:


>  kernel/trace/ftrace.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 7b037295a1f1..0791eafb693d 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3093,6 +3093,10 @@ t_probe_next(struct seq_file *m, loff_t *pos)
>  		hnd = &iter->probe_entry->hlist;
>  
>  	hash = iter->probe->ops.func_hash->filter_hash;
> +
> +	if (!hash)
> +		return NULL;
> +
>  	size = 1 << hash->size_bits;
>  
>   retry:

OK, I added this, but I'm also adding this on top:

-- Steve

>From 372e0d01da71c84dcecf7028598a33813b0d5256 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Date: Fri, 30 Aug 2019 16:30:01 -0400
Subject: [PATCH] ftrace: Check for empty hash and comment the race with
 registering probes

The race between adding a function probe and reading the probes that exist
is very subtle. It needs a comment. Also, the issue can also happen if the
probe has has the EMPTY_HASH as its func_hash.

Cc: stable@...r.kernel.org
Fixes: 7b60f3d876156 ("ftrace: Dynamically create the probe ftrace_ops for the trace_array")
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 kernel/trace/ftrace.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 80beed2cf0da..6200a6fe10e3 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3096,7 +3096,11 @@ t_probe_next(struct seq_file *m, loff_t *pos)
 
 	hash = iter->probe->ops.func_hash->filter_hash;
 
-	if (!hash)
+	/*
+	 * A probe being registered may temporarily have an empty hash
+	 * and it's at the end of the func_probes list.
+	 */
+	if (!hash || hash == EMPTY_HASH)
 		return NULL;
 
 	size = 1 << hash->size_bits;
@@ -4324,6 +4328,10 @@ register_ftrace_function_probe(char *glob, struct trace_array *tr,
 
 	mutex_unlock(&ftrace_lock);
 
+	/*
+	 * Note, there's a small window here that the func_hash->filter_hash
+	 * may be NULL or empty. Need to be carefule when reading the loop.
+	 */
 	mutex_lock(&probe->ops.func_hash->regex_lock);
 
 	orig_hash = &probe->ops.func_hash->filter_hash;
-- 
2.20.1