[PATCH v2] soundwire: fix double free of dangling pointer

From: trix@redhat.com
To: vkoul@kernel.org, yung-chuan.liao@linux.intel.com,
	pierre-louis.bossart@linux.intel.com, sanyog.r.kale@intel.com,
	natechancellor@gmail.com, ndesaulniers@google.com,
	shreyas.nc@intel.com
Cc: alsa-devel@alsa-project.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com, Tom Rix <trix@redhat.com>
Subject: [PATCH v2] soundwire: fix double free of dangling pointer
Date: Wed,  2 Sep 2020 13:26:50 -0700	[thread overview]
Message-ID: <20200902202650.14189-1-trix@redhat.com> (raw)

From: Tom Rix <trix@redhat.com>

clang static analysis flags this problem

stream.c:844:9: warning: Use of memory after
  it is freed
        kfree(bus->defer_msg.msg->buf);
              ^~~~~~~~~~~~~~~~~~~~~~~

This happens in an error handler cleaning up memory
allocated for elements in a list.

	list_for_each_entry(m_rt, &stream->master_list, stream_node) {
		bus = m_rt->bus;

		kfree(bus->defer_msg.msg->buf);
		kfree(bus->defer_msg.msg);
	}

And is triggered when the call to sdw_bank_switch() fails.
There are a two problems.

First, when sdw_bank_switch() fails, though it frees memory it
does not clear bus's reference 'defer_msg.msg' to that memory.

The second problem is the freeing msg->buf. In some cases
msg will be NULL so this will dereference a null pointer.
Need to check before freeing.

Fixes: 99b8a5d608a6 ("soundwire: Add bank switch routine")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
---
v2 : change title, was 'soundwire: fix error handling'
---
drivers/soundwire/stream.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/soundwire/stream.c b/drivers/soundwire/stream.c
index 37290a799023..6e36deb505b1 100644
--- a/drivers/soundwire/stream.c
+++ b/drivers/soundwire/stream.c
@@ -717,6 +717,7 @@ static int sdw_bank_switch(struct sdw_bus *bus, int m_rt_count)
 	kfree(wbuf);
 error_1:
 	kfree(wr_msg);
+	bus->defer_msg.msg = NULL;
 	return ret;
 }
 
@@ -840,9 +841,10 @@ static int do_bank_switch(struct sdw_stream_runtime *stream)
 error:
 	list_for_each_entry(m_rt, &stream->master_list, stream_node) {
 		bus = m_rt->bus;
-
-		kfree(bus->defer_msg.msg->buf);
-		kfree(bus->defer_msg.msg);
+		if (bus->defer_msg.msg) {
+			kfree(bus->defer_msg.msg->buf);
+			kfree(bus->defer_msg.msg);
+		}
 	}
 
 msg_unlock:
-- 
2.18.1

