Re: KASAN: invalid-free in snd_card_new

[Rondreis]

Hello,

I tested this patch with the reproducer and the crash did not trigger again.

On Mon, Sep 19, 2022 at 5:24 PM Takashi Iwai <tiwai@xxxxxxx> wrote:
>
> On Mon, 19 Sep 2022 10:30:54 +0200,
> Takashi Iwai wrote:
> >
> > On Sun, 18 Sep 2022 15:01:11 +0200,
> > Rondreis wrote:
> > >
> > > Hello,
> > >
> > > When fuzzing the Linux kernel driver v6.0-rc4, the following crash was
> > > triggered.
> > >
> > > HEAD commit: 7e18e42e4b280c85b76967a9106a13ca61c16179
> > > git tree: upstream
> > >
> > > kernel config: https://pastebin.com/raw/xtrgsXP3
> > > C reproducer: https://pastebin.com/raw/w2sdQWYj
> > > console output: https://pastebin.com/raw/Yyf7zw2d
> > >
> > > Basically, in the c reproducer, we use the gadget module to emulate
> > > attaching a USB device(vendor id: 0x1bc7, product id: 0x1206, with the
> > > midi function) and executing some simple sequence of system calls.
> > > To reproduce this crash, we utilize a third-party library to emulate
> > > the attaching process: https://github.com/linux-usb-gadgets/libusbgx.
> > > Just clone this repository, install it, and compile the c
> > > reproducer with ``` gcc crash.c -lusbgx -lconfig -o crash ``` will do
> > > the trick.
> > >
> > > I would appreciate it if you have any idea how to solve this bug.
> >
> > Could you try the patch below?  It looks like a simple double-free in
> > the code.
>
> A more proper patch is below.  Please give it a try.
>
>
> thanks,
>
> Takashi
>
> -- 8< --
> From: Takashi Iwai <tiwai@xxxxxxx>
> Subject: [PATCH] ALSA: core: Fix double-free at snd_card_new()
>
> During the code change to add the support for devres-managed card
> instance, we put an explicit kfree(card) call at the error path in
> snd_card_new().  This is needed for the early error path before the
> card is initialized with the device, but is rather superfluous and
> causes a double-free at the error path after the card instance is
> initialized, as the destructor of the card object already contains a
> kfree() call.
>
> This patch fixes the double-free situation by removing the superfluous
> kfree().  Meanwhile we need to call kfree() explicitly for the early
> error path, so it's added there instead.
>
> Fixes: e8ad415b7a55 ("ALSA: core: Add managed card creation")
> Reported-by: Rondreis <linhaoguo86@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/CAB7eexL1zBnB636hwS27d-LdPYZ_R1-5fJS_h=ZbCWYU=UPWJg@xxxxxxxxxxxxxx
> Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> ---
>  sound/core/init.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/sound/core/init.c b/sound/core/init.c
> index 193dae361fac..5377f94eb211 100644
> --- a/sound/core/init.c
> +++ b/sound/core/init.c
> @@ -178,10 +178,8 @@ int snd_card_new(struct device *parent, int idx, const char *xid,
>                 return -ENOMEM;
>
>         err = snd_card_init(card, parent, idx, xid, module, extra_size);
> -       if (err < 0) {
> -               kfree(card);
> -               return err;
> -       }
> +       if (err < 0)
> +               return err; /* card is freed by error handler */
>
>         *card_ret = card;
>         return 0;
> @@ -233,7 +231,7 @@ int snd_devm_card_new(struct device *parent, int idx, const char *xid,
>         card->managed = true;
>         err = snd_card_init(card, parent, idx, xid, module, extra_size);
>         if (err < 0) {
> -               devres_free(card);
> +               devres_free(card); /* in managed mode, we need to free manually */
>                 return err;
>         }
>
> @@ -297,6 +295,8 @@ static int snd_card_init(struct snd_card *card, struct device *parent,
>                 mutex_unlock(&snd_card_mutex);
>                 dev_err(parent, "cannot find the slot for index %d (range 0-%i), error: %d\n",
>                          idx, snd_ecards_limit - 1, err);
> +               if (!card->managed)
> +                       kfree(card); /* manually free here, as no destructor called */
>                 return err;
>         }
>         set_bit(idx, snd_cards_lock);           /* lock it */
> --
> 2.35.3
>