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

[Oliver Neukum <oneukum@suse.com>]

Am Dienstag, den 20.08.2019, 19:55 +0100 schrieb Ben Hutchings:
> 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.

I see.

> 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.

In that case KASAN would have reported a double free in testing, which
it did not.

> 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

AFAICT this patch only does anything if probe() succeeds, which it does
not. Something is strange.

	Regards
		Oliver