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Message-ID: <ZxUzORuz4r4HATOe@mozart.vkv.me>
Date: Sun, 20 Oct 2024 09:43:37 -0700
From: Calvin Owens <calvin@...nvd.org>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-kernel@...r.kernel.org, Rodolfo Giometti <giometti@...eenne.com>,
George Spelvin <linux@...izon.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] pps: Remove embedded cdev to fix a use-after-free
Hi Greg,
On Sunday 10/13 at 17:04 +0200, Greg Kroah-Hartman wrote:
> On Fri, Sep 13, 2024 at 05:24:29PM -0700, Calvin Owens wrote:
> > On a board running ntpd and gpsd, I'm seeing a consistent use-after-free
> > in sys_exit() from gpsd when rebooting:
> >
> > pps pps1: removed
> > ------------[ cut here ]------------
> > kobject: '(null)' (00000000db4bec24): is not initialized, yet kobject_put() is being called.
>
> Something is wrong with the reference counting here...
>
> > WARNING: CPU: 2 PID: 440 at lib/kobject.c:734 kobject_put+0x120/0x150
> > CPU: 2 UID: 299 PID: 440 Comm: gpsd Not tainted 6.11.0-rc6-00308-gb31c44928842 #1
> > Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
> > pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > pc : kobject_put+0x120/0x150
> > lr : kobject_put+0x120/0x150
> > sp : ffffffc0803d3ae0
> > x29: ffffffc0803d3ae0 x28: ffffff8042dc9738 x27: 0000000000000001
> > x26: 0000000000000000 x25: ffffff8042dc9040 x24: ffffff8042dc9440
> > x23: ffffff80402a4620 x22: ffffff8042ef4bd0 x21: ffffff80405cb600
> > x20: 000000000008001b x19: ffffff8040b3b6e0 x18: 0000000000000000
> > x17: 0000000000000000 x16: 0000000000000000 x15: 696e6920746f6e20
> > x14: 7369203a29343263 x13: 205d303434542020 x12: 0000000000000000
> > x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
> > x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
> > x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
> > x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000000000
> > Call trace:
> > kobject_put+0x120/0x150
> > cdev_put+0x20/0x3c
> > __fput+0x2c4/0x2d8
> > ____fput+0x1c/0x38
> > task_work_run+0x70/0xfc
> > do_exit+0x2a0/0x924
> > do_group_exit+0x34/0x90
> > get_signal+0x7fc/0x8c0
> > do_signal+0x128/0x13b4
> > do_notify_resume+0xdc/0x160
> > el0_svc+0xd4/0xf8
> > el0t_64_sync_handler+0x140/0x14c
> > el0t_64_sync+0x190/0x194
> > ---[ end trace 0000000000000000 ]---
> >
> > ...followed by more symptoms of corruption, with similar stacks:
> >
> > refcount_t: underflow; use-after-free.
> > kernel BUG at lib/list_debug.c:62!
> > Kernel panic - not syncing: Oops - BUG: Fatal exception
> >
> > This happens because pps_device_destruct() frees the pps_device with the
> > embedded cdev immediately after calling cdev_del(), but, as the comment
> > above cdev_del() notes, fops for previously opened cdevs are still
> > callable even after cdev_del() returns. I think this bug has always
> > been there: I can't explain why it suddenly started happening every time
> > I reboot this particular board.
> >
> > In commit d953e0e837e6 ("pps: Fix a use-after free bug when
> > unregistering a source."), George Spelvin suggested removing the
> > embedded cdev. That seems like the simplest way to fix this, so I've
> > implemented his suggestion, with pps_idr becoming the source of truth
> > for which minor corresponds to which device.
>
> You remove it, but now the structure has no reference counting at all,
> so you should make it a real "struct device" not just containing a
> pointer to one.
It still uses the device refcount. I didn't change that, see the
kobject_get() (which is confusing and should be get_device() as you note
below) in pps_cdev_open() before my patch.
> > But now that pps_idr defines userspace visibility instead of cdev_add(),
> > we need to be sure the pps->dev kobject refcount can't reach zero while
> > userspace can still find it again. So, the idr_remove() call moves to
> > pps_unregister_cdev(), and pps_idr now holds a reference to the pps->dev
> > kobject.
>
> An idr shouldn't be doing the reference counting here, the struct device
> should be doing it, right?
Right, I was trying to explain that it is now necessary to call
get_device() to acquire a reference while the device minor is indexed by
the idr.
Before, the minor was deindexed in the idr in ->release(). But now,
since it is visible the moment it is indexed by the idr, deindexing it
in ->release() means this could happen during removal:
CPU1 CPU2
--- ---
pps_unregister_cdev()
<...>
sys_open()
<...>
idr_lookup()
idr_remove()
put_device() <--- ref goes to zero
get_device() <--- BOOM
pps_device_destruct()
pps_idr_remove()
By calling get_device() in pps_idr_get(), and by moving the idr
deindexing from ->release() to pps_unregister_cdev(), we ensure that
can't happen.
> >
> > pps_core: source serial1 got cdev (251:1)
> > <...>
> > pps pps1: removed
> > pps_core: unregistering pps1
> > pps_core: deallocating pps1
> >
> > Fixes: d953e0e837e6 ("pps: Fix a use-after free bug when unregistering a source.")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Calvin Owens <calvin@...nvd.org>
> > ---
> > Changes in v2:
> > - Don't move pr_debug() from pps_device_destruct() to pps_unregister_cdev()
> > - Actually add stable@...r.kernel.org to CC
> > ---
> > drivers/pps/pps.c | 83 ++++++++++++++++++++------------------
> > include/linux/pps_kernel.h | 1 -
> > 2 files changed, 44 insertions(+), 40 deletions(-)
> >
> > diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> > index 5d19baae6a38..6980ab17f314 100644
> > --- a/drivers/pps/pps.c
> > +++ b/drivers/pps/pps.c
> > @@ -25,7 +25,7 @@
> > * Local variables
> > */
> >
> > -static dev_t pps_devt;
> > +static int pps_major;
> > static struct class *pps_class;
> >
> > static DEFINE_MUTEX(pps_idr_lock);
> > @@ -296,19 +296,35 @@ static long pps_cdev_compat_ioctl(struct file *file,
> > #define pps_cdev_compat_ioctl NULL
> > #endif
> >
> > +static struct pps_device *pps_idr_get(unsigned long id)
> > +{
> > + struct pps_device *pps;
> > +
> > + mutex_lock(&pps_idr_lock);
> > + pps = idr_find(&pps_idr, id);
> > + if (pps)
> > + kobject_get(&pps->dev->kobj);
>
> A driver should never call "raw" kobject calls, this alone makes this
> not ok :(
Oh this is silly, sorry: it's simply open coding put_device() and
get_device(), I'll fix that.
> Please move the structure to be embedded in and then it should be
> simpler.
I actually tried to do that, but got hung up on the API: what's the
right way to do what device_create() does for an embedded device struct?
Today, the pps core does:
pps = kzalloc(sizeof(*pps));
...then later in pps_register_cdev():
idr_alloc(&newmin, ...);
pps->dev = device_create(..., MKDEV(maj, newmin), ...);
To embed the device struct, I guess this would become something like:
pps = kzalloc(sizeof(*pps));
device_initialize(&pps->dev);
idr_alloc(&newmin, ...);
/* ??? */
device_register(&pps->dev);
...but the question marks are what I'm not seeing: what's the proper way
to accomplish what device_create() does, but in an embedded dev struct?
Obviously I could just copy device_create(), but I'm sure that's not
what you want.
Thanks,
Calvin
> thanks,
>
> greg k-h
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