Re: [PATCH] soundwire: fix error handling

[Pierre-Louis Bossart]

On 8/31/20 12:47 PM, Nick Desaulniers wrote:
> On Sat, Aug 29, 2020 at 8:35 AM <trix@xxxxxxxxxx> wrote:
> > From: Tom Rix <trix@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxx>

Also looks good to me.

Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>