Re: [RFC] ipack: ipoctal: fix use-after-free on remove

[Pei Xiao]

在 2026/6/25 03:47, Shuangpeng 写道:
> Hi Pei,
>
> Thanks for working on this!
>
> I applied the patch to commit:
>
> 390d73adf896bf4883c7d3bcd13c1b53d64351e3 (Jun 19 2026)
>
> and reran the same reproducer. The original slab-use-after-free report did
> not show up in the same form, but the reproducer still triggers another
> KASAN r
> eport in ipoctal_write_tty():
Hi shuangpeng，

I really appreciate that you've spent so much time helping me test this
patch. I thought it would just go unnoticed.
/I've made another revision. Could you please test it again?/
Thanks！
> [   73.728267] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
> [   73.730941] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> [   73.731780] CPU: 0 UID: 0 PID: 8289 Comm: ipoctal_fd_afte Not tainted 7.1.0-10997-g390d73adf896-dirty #33 PREEMPT(full)
> [   73.734223] RIP: 0010:ipoctal_write_tty (drivers/ipack/devices/ipoctal.c:453 drivers/ipack/devices/ipoctal.c:470)
>
> Call Trace:
> [   73.746486]  n_tty_write (drivers/tty/n_tty.c:2388)
> [   73.752256]  file_tty_write (drivers/tty/tty_io.c:1006 drivers/tty/tty_io.c:1081)
> [   73.752746]  vfs_write (fs/read_write.c:595 fs/read_write.c:687)
> [   73.754695]  ksys_write (fs/read_write.c:739)
> [   73.756837]  do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
> [   73.757309]  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>
> So the same open-tty-fd-after-tpci200-unbind reproducer still crashes the
> kernel after this patch.
>
> Best,
> Shuangpeng
>
>
>
>> On Jun 23, 2026, at 03:50, Pei Xiao <xiaopei01@xxxxxxxxxx> wrote:
>>
>> A use-after-free occurs when the device is removed while a tty
>> session is still active.  The remove callback frees the ipoctal
>> structure via kfree() while ipoctal_write_tty() and other tty ops
>> may still access it.
>>
>> Fix this by introducing kref-based lifetime management for the
>> ipoctal structure. A kref is taken in ipoctal_install() when a tty is
>> initialized, and released in ipoctal_cleanup() when the tty is finally
>> destroyed. The remove callback replaces direct kfree() with kref_put(),
>> ensuring the memory is only freed after all tty references have been
>> released.
>>
>> Reported-by: Shuangpeng Bai <shuangpeng.kernel@xxxxxxxxx>
>> Closes: https://lore.kernel.org/lkml/178144969601.60470.1257088106279546587@xxxxxxxxx/
>> Fixes: 05e5027efc9c ("Staging: ipack: move out of staging")
>> Signed-off-by: Pei Xiao <xiaopei01@xxxxxxxxxx>
>> ---
>> drivers/ipack/devices/ipoctal.c | 19 +++++++++++++++++--
>> 1 file changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/ipack/devices/ipoctal.c b/drivers/ipack/devices/ipoctal.c
>> index 1bbefc6de708..079bda93ad5a 100644
>> --- a/drivers/ipack/devices/ipoctal.c
>> +++ b/drivers/ipack/devices/ipoctal.c
>> @@ -10,6 +10,7 @@
>> #include <linux/device.h>
>> #include <linux/module.h>
>> #include <linux/interrupt.h>
>> +#include <linux/kref.h>
>> #include <linux/sched.h>
>> #include <linux/tty.h>
>> #include <linux/serial.h>
>> @@ -25,6 +26,8 @@
>>
>> static const struct tty_operations ipoctal_fops;
>>
>> +static void ipoctal_release(struct kref *kref);
>> +
>> struct ipoctal_channel {
>> struct ipoctal_stats stats;
>> unsigned int nb_bytes;
>> @@ -49,6 +52,7 @@ struct ipoctal {
>> struct tty_driver *tty_drv;
>> u8 __iomem *mem8_space;
>> u8 __iomem *int_space;
>> + struct kref kref;
>> };
>>
>> static inline struct ipoctal *chan_to_ipoctal(struct ipoctal_channel *chan,
>> @@ -95,6 +99,7 @@ static int ipoctal_install(struct tty_driver *driver, struct tty_struct *tty)
>> if (res)
>> goto err_put_carrier;
>>
>> + kref_get(&ipoctal->kref);
>> tty->driver_data = channel;
>>
>> return 0;
>> @@ -666,6 +671,7 @@ static void ipoctal_cleanup(struct tty_struct *tty)
>>
>> /* release the carrier driver */
>> ipack_put_carrier(ipoctal->dev);
>> + kref_put(&ipoctal->kref, ipoctal_release);
>> }
>>
>> static const struct tty_operations ipoctal_fops = {
>> @@ -683,6 +689,13 @@ static const struct tty_operations ipoctal_fops = {
>> .cleanup =              ipoctal_cleanup,
>> };
>>
>> +static void ipoctal_release(struct kref *kref)
>> +{
>> + struct ipoctal *ipoctal = container_of(kref, struct ipoctal, kref);
>> +
>> + kfree(ipoctal);
>> +}
>> +
>> static int ipoctal_probe(struct ipack_device *dev)
>> {
>> int res;
>> @@ -692,6 +705,8 @@ static int ipoctal_probe(struct ipack_device *dev)
>> if (ipoctal == NULL)
>> return -ENOMEM;
>>
>> + kref_init(&ipoctal->kref);
>> +
>> ipoctal->dev = dev;
>> res = ipoctal_inst_slot(ipoctal, dev->bus->bus_nr, dev->slot);
>> if (res)
>> @@ -701,7 +716,7 @@ static int ipoctal_probe(struct ipack_device *dev)
>> return 0;
>>
>> out_uninst:
>> - kfree(ipoctal);
>> + kref_put(&ipoctal->kref, ipoctal_release);
>> return res;
>> }
>>
>> @@ -725,7 +740,7 @@ static void __ipoctal_remove(struct ipoctal *ipoctal)
>> tty_unregister_driver(ipoctal->tty_drv);
>> kfree(ipoctal->tty_drv->name);
>> tty_driver_kref_put(ipoctal->tty_drv);
>> - kfree(ipoctal);
>> + kref_put(&ipoctal->kref, ipoctal_release);
>> }
>>
>> static void ipoctal_remove(struct ipack_device *idev)
>> -- 
>> 2.25.1
>>