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pps: Fix a use-after-free
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commit c79a39d upstream.

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.
    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 d953e0e ("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, using __register_chrdev() with pps_idr
becoming the source of truth for which minor corresponds to which
device.

But now that pps_idr defines userspace visibility instead of cdev_add(),
we need to be sure the pps->dev 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 pps->dev.

    pps_core: source serial1 got cdev (251:1)
    <...>
    pps pps1: removed
    pps_core: unregistering pps1
    pps_core: deallocating pps1

Fixes: d953e0e ("pps: Fix a use-after free bug when unregistering a source.")
Cc: [email protected]
Signed-off-by: Calvin Owens <[email protected]>
Reviewed-by: Michal Schmidt <[email protected]>
Link: https://lore.kernel.org/r/a17975fd5ae99385791929e563f72564edbcf28f.1731383727.git.calvin@wbinvd.org
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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@jcalvinowens @gregkh
jcalvinowens authored and gregkh committed Feb 8, 2025
1 parent d8e63dd commit 85241f7
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Showing 9 changed files with 87 additions and 83 deletions.
4 changes: 2 additions & 2 deletions drivers/pps/clients/pps-gpio.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -214,8 +214,8 @@ static int pps_gpio_probe(struct platform_device *pdev)
return -EINVAL;
}

dev_info(data->pps->dev, "Registered IRQ %d as PPS source\n",
data->irq);
dev_dbg(&data->pps->dev, "Registered IRQ %d as PPS source\n",
data->irq);

return 0;
}
Expand Down
4 changes: 2 additions & 2 deletions drivers/pps/clients/pps-ktimer.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -56,7 +56,7 @@ static struct pps_source_info pps_ktimer_info = {

static void __exit pps_ktimer_exit(void)
{
dev_info(pps->dev, "ktimer PPS source unregistered\n");
dev_dbg(&pps->dev, "ktimer PPS source unregistered\n");

del_timer_sync(&ktimer);
pps_unregister_source(pps);
Expand All @@ -74,7 +74,7 @@ static int __init pps_ktimer_init(void)
timer_setup(&ktimer, pps_ktimer_event, 0);
mod_timer(&ktimer, jiffies + HZ);

dev_info(pps->dev, "ktimer PPS source registered\n");
dev_dbg(&pps->dev, "ktimer PPS source registered\n");

return 0;
}
Expand Down
6 changes: 3 additions & 3 deletions drivers/pps/clients/pps-ldisc.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -32,7 +32,7 @@ static void pps_tty_dcd_change(struct tty_struct *tty, bool active)
pps_event(pps, &ts, active ? PPS_CAPTUREASSERT :
PPS_CAPTURECLEAR, NULL);

dev_dbg(pps->dev, "PPS %s at %lu\n",
dev_dbg(&pps->dev, "PPS %s at %lu\n",
active ? "assert" : "clear", jiffies);
}

Expand Down Expand Up @@ -69,7 +69,7 @@ static int pps_tty_open(struct tty_struct *tty)
goto err_unregister;
}

dev_info(pps->dev, "source \"%s\" added\n", info.path);
dev_dbg(&pps->dev, "source \"%s\" added\n", info.path);

return 0;

Expand All @@ -89,7 +89,7 @@ static void pps_tty_close(struct tty_struct *tty)
if (WARN_ON(!pps))
return;

dev_info(pps->dev, "removed\n");
dev_info(&pps->dev, "removed\n");
pps_unregister_source(pps);
}

Expand Down
4 changes: 2 additions & 2 deletions drivers/pps/clients/pps_parport.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -81,7 +81,7 @@ static void parport_irq(void *handle)
/* check the signal (no signal means the pulse is lost this time) */
if (!signal_is_set(port)) {
local_irq_restore(flags);
dev_err(dev->pps->dev, "lost the signal\n");
dev_err(&dev->pps->dev, "lost the signal\n");
goto out_assert;
}

Expand All @@ -98,7 +98,7 @@ static void parport_irq(void *handle)
/* timeout */
dev->cw_err++;
if (dev->cw_err >= CLEAR_WAIT_MAX_ERRORS) {
dev_err(dev->pps->dev, "disabled clear edge capture after %d"
dev_err(&dev->pps->dev, "disabled clear edge capture after %d"
" timeouts\n", dev->cw_err);
dev->cw = 0;
dev->cw_err = 0;
Expand Down
10 changes: 5 additions & 5 deletions drivers/pps/kapi.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,7 @@ static void pps_add_offset(struct pps_ktime *ts, struct pps_ktime *offset)
static void pps_echo_client_default(struct pps_device *pps, int event,
void *data)
{
dev_info(pps->dev, "echo %s %s\n",
dev_info(&pps->dev, "echo %s %s\n",
event & PPS_CAPTUREASSERT ? "assert" : "",
event & PPS_CAPTURECLEAR ? "clear" : "");
}
Expand Down Expand Up @@ -112,7 +112,7 @@ struct pps_device *pps_register_source(struct pps_source_info *info,
goto kfree_pps;
}

dev_info(pps->dev, "new PPS source %s\n", info->name);
dev_dbg(&pps->dev, "new PPS source %s\n", info->name);

return pps;

Expand Down Expand Up @@ -166,7 +166,7 @@ void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
/* check event type */
BUG_ON((event & (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR)) == 0);

dev_dbg(pps->dev, "PPS event at %lld.%09ld\n",
dev_dbg(&pps->dev, "PPS event at %lld.%09ld\n",
(s64)ts->ts_real.tv_sec, ts->ts_real.tv_nsec);

timespec_to_pps_ktime(&ts_real, ts->ts_real);
Expand All @@ -188,7 +188,7 @@ void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
/* Save the time stamp */
pps->assert_tu = ts_real;
pps->assert_sequence++;
dev_dbg(pps->dev, "capture assert seq #%u\n",
dev_dbg(&pps->dev, "capture assert seq #%u\n",
pps->assert_sequence);

captured = ~0;
Expand All @@ -202,7 +202,7 @@ void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
/* Save the time stamp */
pps->clear_tu = ts_real;
pps->clear_sequence++;
dev_dbg(pps->dev, "capture clear seq #%u\n",
dev_dbg(&pps->dev, "capture clear seq #%u\n",
pps->clear_sequence);

captured = ~0;
Expand Down
10 changes: 5 additions & 5 deletions drivers/pps/kc.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -43,11 +43,11 @@ int pps_kc_bind(struct pps_device *pps, struct pps_bind_args *bind_args)
pps_kc_hardpps_mode = 0;
pps_kc_hardpps_dev = NULL;
spin_unlock_irq(&pps_kc_hardpps_lock);
dev_info(pps->dev, "unbound kernel"
dev_info(&pps->dev, "unbound kernel"
" consumer\n");
} else {
spin_unlock_irq(&pps_kc_hardpps_lock);
dev_err(pps->dev, "selected kernel consumer"
dev_err(&pps->dev, "selected kernel consumer"
" is not bound\n");
return -EINVAL;
}
Expand All @@ -57,11 +57,11 @@ int pps_kc_bind(struct pps_device *pps, struct pps_bind_args *bind_args)
pps_kc_hardpps_mode = bind_args->edge;
pps_kc_hardpps_dev = pps;
spin_unlock_irq(&pps_kc_hardpps_lock);
dev_info(pps->dev, "bound kernel consumer: "
dev_info(&pps->dev, "bound kernel consumer: "
"edge=0x%x\n", bind_args->edge);
} else {
spin_unlock_irq(&pps_kc_hardpps_lock);
dev_err(pps->dev, "another kernel consumer"
dev_err(&pps->dev, "another kernel consumer"
" is already bound\n");
return -EINVAL;
}
Expand All @@ -83,7 +83,7 @@ void pps_kc_remove(struct pps_device *pps)
pps_kc_hardpps_mode = 0;
pps_kc_hardpps_dev = NULL;
spin_unlock_irq(&pps_kc_hardpps_lock);
dev_info(pps->dev, "unbound kernel consumer"
dev_info(&pps->dev, "unbound kernel consumer"
" on device removal\n");
} else
spin_unlock_irq(&pps_kc_hardpps_lock);
Expand Down
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