Re: [PATCH] pps: fix warning in pps_register_cdev when register device fail

[Calvin Owens <calvin@wbinvd.org>]

On Wednesday 08/27 at 08:32 -0700, Calvin Owens wrote:
> On Wednesday 08/27 at 14:50 +0800, Wang Liang wrote:
> > Similar to previous commit 2a934fdb01db ("media: v4l2-dev: fix error
> > handling in __video_register_device()"), the release hook should be set
> > before device_register(). Otherwise, when device_register() return error
> > and put_device() try to callback the release function, the below warning
> > may happen.
> > 
> >   ------------[ cut here ]------------
> >   WARNING: CPU: 1 PID: 4760 at drivers/base/core.c:2567 device_release+0x1bd/0x240 drivers/base/core.c:2567
> >   Modules linked in:
> >   CPU: 1 UID: 0 PID: 4760 Comm: syz.4.914 Not tainted 6.17.0-rc3+ #1 NONE
> >   RIP: 0010:device_release+0x1bd/0x240 drivers/base/core.c:2567
> >   Call Trace:
> >    <TASK>
> >    kobject_cleanup+0x136/0x410 lib/kobject.c:689
> >    kobject_release lib/kobject.c:720 [inline]
> >    kref_put include/linux/kref.h:65 [inline]
> >    kobject_put+0xe9/0x130 lib/kobject.c:737
> >    put_device+0x24/0x30 drivers/base/core.c:3797
> >    pps_register_cdev+0x2da/0x370 drivers/pps/pps.c:402
> >    pps_register_source+0x2f6/0x480 drivers/pps/kapi.c:108
> >    pps_tty_open+0x190/0x310 drivers/pps/clients/pps-ldisc.c:57
> >    tty_ldisc_open+0xa7/0x120 drivers/tty/tty_ldisc.c:432
> >    tty_set_ldisc+0x333/0x780 drivers/tty/tty_ldisc.c:563
> >    tiocsetd drivers/tty/tty_io.c:2429 [inline]
> >    tty_ioctl+0x5d1/0x1700 drivers/tty/tty_io.c:2728
> >    vfs_ioctl fs/ioctl.c:51 [inline]
> >    __do_sys_ioctl fs/ioctl.c:598 [inline]
> >    __se_sys_ioctl fs/ioctl.c:584 [inline]
> >    __x64_sys_ioctl+0x194/0x210 fs/ioctl.c:584
> >    do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >    do_syscall_64+0x5f/0x2a0 arch/x86/entry/syscall_64.c:94
> >    entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >    </TASK>
> > 
> > Before commit c79a39dc8d06 ("pps: Fix a use-after-free"),
> > pps_register_cdev() call device_create() to create pps->dev, which will
> > init dev->release to device_create_release(). Now the comment is outdated,
> > just remove it.
> 
> Hi Wang,
> 
> I'm curious why pps_register_cdev() is failing, is there possibly a
> second issue to investigate there? Or was it fault injection?
> 
> Otherwise, makes perfect sense to me. I'm new to this code, so grain of
> salt, but since I exposed it:
> 
> Reviewed-by: Calvin Owens <calvin@wbinvd.org>

I apologize Wang, I missed something when I looked at this yesterday:

I think your patch introduces a double-free, because the pps object is
already freed by pps_register_source() when pps_register_cdev() returns
an error. Because put_device() was a nop with the missing release_fn, it
wasn't actually doing anything. That's very confusing, and my fault.

Your patch is clearly an improvement. But I think you additionally need
to remove the 'kfree_pps' error logic at the bottom of
pps_register_source() to avoid a double free in the failure case:

diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
index 92d1b62ea239..4d0ba35590a3 100644
--- a/drivers/pps/kapi.c
+++ b/drivers/pps/kapi.c
@@ -116,9 +116,6 @@ struct pps_device *pps_register_source(struct pps_source_info *info,
 
 	return pps;
 
-kfree_pps:
-	kfree(pps);
-
 pps_register_source_exit:
 	pr_err("%s: unable to register source\n", info->name);
 

The only reason I put that kfree() there is because I noticed the memory
was being leaked in the failure case. Doh. Your patch makes this right.

> Thanks,
> Calvin
> 
> > Fixes: c79a39dc8d06 ("pps: Fix a use-after-free")
> > Signed-off-by: Wang Liang <wangliang74@huawei.com>
> > ---
> >  drivers/pps/pps.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> > index 9463232af8d2..0d2d57250575 100644
> > --- a/drivers/pps/pps.c
> > +++ b/drivers/pps/pps.c
> > @@ -383,13 +383,11 @@ int pps_register_cdev(struct pps_device *pps)
> >  	pps->dev.devt = MKDEV(pps_major, pps->id);
> >  	dev_set_drvdata(&pps->dev, pps);
> >  	dev_set_name(&pps->dev, "pps%d", pps->id);
> > +	pps->dev.release = pps_device_destruct;
> >  	err = device_register(&pps->dev);
> >  	if (err)
> >  		goto free_idr;
> >  
> > -	/* Override the release function with our own */
> > -	pps->dev.release = pps_device_destruct;
> > -
> >  	pr_debug("source %s got cdev (%d:%d)\n", pps->info.name, pps_major,
> >  		 pps->id);
> >  
> > -- 
> > 2.33.0
> >