Re: [PATCH] soundwire: fix error handling

[Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>]

On 8/31/20 12:47 PM, Nick Desaulniers wrote:
> On Sat, Aug 29, 2020 at 8:35 AM <trix@redhat.com> wrote:
>>
>> 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>
>> ---
>>   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;
> 
> This fix looks correct to me because L668 sets `bus->defer_msg.msg =
> wr_msg;`, but on error L719 frees `wr_msg`, so now
> `bus->defer_msg.msg` is a dangling pointer.
> 
>>          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);
>> +               }
> 
> I'd prefer a conditional check for each, but sdw_ml_sync_bank_switch()
> has this same pattern, so it looks like the lifetime of these two
> match.
> 
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Also looks good to me.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>