Re: [PATCH] dvb: usb: fix use after free in dvb_usb_device_exit

[Ben Hutchings <ben@decadent.org.uk>]

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On Tue, 2019-04-30 at 15:07 +0200, Oliver Neukum wrote:
> dvb_usb_device_exit() frees and uses the device name in that order
> Fix by storing the name in a buffer before freeing it
> 
> v2: fixed style issues
> v3: strscpy used and variable names changed
> v4: really use strscpy everywhere
> 
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> Reported-by: syzbot+26ec41e9f788b3eba396@syzkaller.appspotmail.com

This doesn't fix that bug (and I don't think it fixes a bug at all). 
The name string is static and doesn't get freed until the module it's
in is freed.

Look again at the stack traces in
<https://syzkaller.appspot.com/bug?extid=26ec41e9f788b3eba396>:

> Allocated by task 21:
[...]
>  kmemdup+0x23/0x50 mm/util.c:118
>
 kmemdup include/linux/string.h:428 [inline]
>  dw2102_probe+0x62c/0xc50
drivers/media/usb/dvb-usb/dw2102.c:2375
[...]
> Freed by task 21:
[...]
>
 kfree+0xce/0x290 mm/slub.c:3958
>  dw2102_probe+0x876/0xc50
drivers/media/usb/dvb-usb/dw2102.c:2409

So, d->desc was freed during probe, and is a dangling pointer before
dvb_usb_device_exit() runs at all.

The bug seems to have been introduced by:

commit 299c7007e93645067e1d2743f4e50156de78c4ff
Author: Anton Vasilyev <vasilyev@ispras.ru>
Date:   Mon Jul 23 13:04:54 2018 -0400

    media: dw2102: Fix memleak on sequence of probes

Ben.

> ---
>  drivers/media/usb/dvb-usb/dvb-usb-init.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/usb/dvb-usb/dvb-usb-init.c b/drivers/media/usb/dvb-usb/dvb-usb-init.c
> index 99951e02a880..dd063a736df5 100644
> --- a/drivers/media/usb/dvb-usb/dvb-usb-init.c
> +++ b/drivers/media/usb/dvb-usb/dvb-usb-init.c
> @@ -287,12 +287,15 @@ EXPORT_SYMBOL(dvb_usb_device_init);
>  void dvb_usb_device_exit(struct usb_interface *intf)
>  {
>  	struct dvb_usb_device *d = usb_get_intfdata(intf);
> -	const char *name = "generic DVB-USB module";
> +	const char *default_name = "generic DVB-USB module";
> +	char name[40];
>  
>  	usb_set_intfdata(intf, NULL);
>  	if (d != NULL && d->desc != NULL) {
> -		name = d->desc->name;
> +		strscpy(name, d->desc->name, sizeof(name));
>  		dvb_usb_exit(d);
> +	} else {
> +		strscpy(name, default_name, sizeof(name));
>  	}
>  	info("%s successfully deinitialized and disconnected.", name);
>  
-- 
Ben Hutchings
Experience is what causes a person to make new mistakes
instead of old ones.


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