Re: [RFC] ipack: ipoctal: fix use-after-free and null-ptr-deref on remove

[Shuangpeng]

Hi Pei,

Thanks for the updated patch!

I applied it and reran the same reproducer. The previous crash in
ipoctal_write_tty() is no longer triggered, but the same reproducer now
hits another Oops after removing the device while the tty fd is still open.

The new report looks like this:

BUG: unable to handle page fault for address: ffffc9000617b005
#PF: supervisor write access in kernel mode
Oops: Oops: 0002 [#1] SMP KASAN PTI
RIP: 0010:iowrite8 (lib/iomap.c:207)

Call Trace:
  ipoctal_set_termios (drivers/ipack/devices/ipoctal.c:66 drivers/ipack/devices/ipoctal.c:517)
  tty_set_termios (drivers/tty/tty_ioctl.c:341)
  set_termios (drivers/tty/tty_ioctl.c:516)
  tty_mode_ioctl (drivers/tty/tty_ioctl.c:?)
  tty_ioctl (drivers/tty/tty_io.c:2801)
  __se_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:597 fs/ioctl.c:583)
  do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

The faulting access is from ipoctal_reset_channel():

drivers/ipack/devices/ipoctal.c:66
iowrite8(CR_DISABLE_RX | CR_DISABLE_TX, &channel->regs->w.cr);

called by ipoctal_set_termios():

drivers/ipack/devices/ipoctal.c:517
ipoctal_reset_channel(channel);


Best,
Shuangpeng


> On Jun 24, 2026, at 21:33, 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 the UAF 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.
> 
> However, keeping the ipoctal struct alive via kref exposed a null
> pointer dereference: __ipoctal_remove() calls tty_port_free_xmit_buf()
> which frees and NULLs xmit_buf, but a userspace process may still hold
> the tty fd and call write(), leading to a crash in
> ipoctal_copy_write_buffer() when it dereferences xmit_buf.
> 
> Fix this by checking for a NULL xmit_buf in ipoctal_write_tty() and
> returning 0 if the buffer has been freed.
> 
> 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 | 22 ++++++++++++++++++++--
> 1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/ipack/devices/ipoctal.c b/drivers/ipack/devices/ipoctal.c
> index 1bbefc6de708..6bbbfb651dea 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;
> @@ -462,6 +467,9 @@ static ssize_t ipoctal_write_tty(struct tty_struct *tty, const u8 *buf,
> struct ipoctal_channel *channel = tty->driver_data;
> size_t char_copied;
> 
> + if (!channel->tty_port.xmit_buf)
> + return 0;
> +
> char_copied = ipoctal_copy_write_buffer(channel, buf, count);
> 
> /* As the IP-OCTAL 485 only supports half duplex, do it manually */
> @@ -666,6 +674,7 @@ 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 +692,13 @@ 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 +708,8 @@ 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 +719,7 @@ static int ipoctal_probe(struct ipack_device *dev)
> return 0;
> 
> out_uninst:
> - kfree(ipoctal);
> + kref_put(&ipoctal->kref, ipoctal_release);
> return res;
> }
> 
> @@ -725,7 +743,7 @@ 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
>