[PATCH] soundwire: fix error handling

[Nick Desaulniers]

On Sat, Aug 29, 2020 at 8:35 AM <trix at redhat.com> wrote:
>
> 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>
> ---
>  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 at google.com>

Thanks for the patch!

>         }
>
>  msg_unlock:
> --
> 2.18.1
>


-- 
Thanks,
~Nick Desaulniers