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.
> 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?
>
> 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@xxxxxxxxxxxxxxx
> Signed-off-by: Calvin Owens <calvin@xxxxxxxxxx>
> ---
> Changes in v2:
> - Don't move pr_debug() from pps_device_destruct() to pps_unregister_cdev()
> - Actually add stable@xxxxxxxxxxxxxxx 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 :(
Please move the structure to be embedded in and then it should be
simpler.
thanks,
greg k-h
