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

[Hillf Danton]

Wed,  2 Sep 2020 13:26:50 -0700
> From: Tom Rix <trix at 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 at redhat.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>
> Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart at 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

Looks like it needs also to release the current buf before putting the new
one into place.

--- a/drivers/soundwire/stream.c
+++ b/drivers/soundwire/stream.c
@@ -665,6 +665,10 @@ static int sdw_bank_switch(struct sdw_bu
 	if (!wr_msg)
 		return -ENOMEM;
 
+	if (bus->defer_msg.msg) {
+		kfree(bus->defer_msg.msg->buf);
+		kfree(bus->defer_msg.msg);
+	}
 	bus->defer_msg.msg = wr_msg;
 
 	wbuf = kzalloc(sizeof(*wbuf), GFP_KERNEL);