Re: [PATCH] misc: fastrpc: Fix double free of 'buf' in error path

[Dan Carpenter <dan.carpenter@linaro.org>]

On Wed, May 31, 2023 at 09:45:38PM -0700, Sukrut Bellary wrote:
> On Fri, May 19, 2023 at 03:57:59PM -0700, Sukrut Bellary wrote:
> > On Fri, May 19, 2023 at 11:39:59AM +0100, Srinivas Kandagatla wrote:
> > > 
> > > 
> > > On 19/05/2023 11:22, Dan Carpenter wrote:
> > > > > ----------------------->cut<---------------------------
> > > > > diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> > > > > index f60bbf99485c..3fdd326e1ae8 100644
> > > > > --- a/drivers/misc/fastrpc.c
> > > > > +++ b/drivers/misc/fastrpc.c
> > > > > @@ -1891,7 +1891,8 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl,
> > > > > char __user *argp)
> > > > >                                        &args[0]);
> > > > >          if (err) {
> > > > >                  dev_err(dev, "mmap error (len 0x%08llx)\n", buf->size);
> > > > > -               goto err_invoke;
> > > > > +               fastrpc_buf_free(buf);
> > > > > +               return err;
> > > > >          }
> > > > > 
> > > > >          /* update the buffer to be able to deallocate the memory on the DSP
> > > > > */
> > > > > @@ -1930,11 +1931,7 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl,
> > > > > char __user *argp)
> > > > >          return 0;
> > > > > 
> > > > >   err_assign:
> > > > > -       fastrpc_req_munmap_impl(fl, buf);
> > > > > -err_invoke:
> > > > > -       fastrpc_buf_free(buf);
> > > > > -
> > > > > -       return err;
> > > > > +       return fastrpc_req_munmap_impl(fl, buf);
> > > > 
> > > > This will return success if copy_to_user() fails.
> > > > 
> > > that is true, using return value of fastrpc_req_munmap_impl does not really
> > > make sense here we should just return err in either case to the user.
> > >
> >
> 
> I have one follow-up question before I send the v2 patch.
> With the following approach, I do see one issue.
> 
> ----------------------->cut<---------------------------
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index f60bbf99485c..3fdd326e1ae8 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -1891,7 +1891,8 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl,
> char __user *argp)
>                                       &args[0]);
>         if (err) {
>                 dev_err(dev, "mmap error (len 0x%08llx)\n", buf->size);
> -               goto err_invoke;
> +               fastrpc_buf_free(buf);
> +               return err;
>         }
> 
>         /* update the buffer to be able to deallocate the memory on the DSP */
> @@ -1930,11 +1931,7 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl,
> char __user *argp)
>         return 0;
> 
>  err_assign:
> -       fastrpc_req_munmap_impl(fl, buf);
> -err_invoke:
> -       fastrpc_buf_free(buf);
> -
> -       return err;
> +       fastrpc_req_munmap_impl(fl, buf);
> +       return err;
>  }
> ----------------------->cut<---------------------------
> 
> In this, if qcom_scm_assign_mem() fails, the buf is not added to the list.
> But the call to fastrpc_req_munmap_impl() tries to delete the buf
> from the list.
> 
> To avoid this, we can use the following approach.

The list is initialized in __fastrpc_buf_alloc().

	INIT_LIST_HEAD(&buf->node);

So calling list_del(&buf->node); is fine.  It's not necessary but it's
not harmful.

regards,
dan carpenter