Re: KASAN: invalid-free in snd_card_new

[Takashi Iwai <tiwai@suse.de>]

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@suse.de>
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@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAB7eexL1zBnB636hwS27d-LdPYZ_R1-5fJS_h=ZbCWYU=UPWJg@mail.gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 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