linux-kernel - Re: [PATCH] char: lp: Fix NULL pointer dereference of cad

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Message-ID: <5e81851f-a7f4-5606-9e0d-b823aa5210e5@huaweicloud.com>
Date: Tue, 30 Dec 2025 09:51:43 +0800
From: Li Nan <linan666@...weicloud.com>
To: arnd@...db.de, gregkh@...uxfoundation.org
Cc: linux-kernel@...r.kernel.org, "wanghai (M)" <wanghai38@...wei.com>
Subject: Re: [PATCH] char: lp: Fix NULL pointer dereference of cad

Friendly ping...

在 2025/12/18 22:20, linan666@...weicloud.com 写道:
> From: Li Nan <linan122@...wei.com>
> 
> NULL pointer dereference occurs when accessing 'port->physport->cad'
> as below:
> 
>    KASAN: null-ptr-deref in range [0x00000000000003f0-0x00000000000003f7]
>    RIP: 0010:parport_wait_peripheral+0x130/0x4b0
>    Call Trace:
>     parport_ieee1284_write_compat+0x306/0xb70
>     ? __pfx_parport_ieee1284_write_compat+0x10/0x10
>     parport_write+0x1d6/0x660
>     lp_write+0x43e/0xbc0
>     ? __pfx_lp_write+0x10/0x10
>     vfs_write+0x21c/0x960
>     ksys_write+0x12e/0x260
>     ? __pfx_ksys_write+0x10/0x10
>     ? __audit_syscall_entry+0x39e/0x510
>     do_syscall_64+0x59/0x110
>     entry_SYSCALL_64_after_hwframe+0x78/0xe2
> 
> The root cause is other processes may set 'port->cad' to NULL during
> lp_write() operations. Process flow:
> 
>    T1					T2
>    lp_write
>     lock port_mutex *
>     lp_claim_parport_or_block
>      parport_claim
>       port->cad = dev;
>     parport_write
>      parport_ieee1284_write_compat
> 					lp_do_ioctl
> 					 lp_reset
> 					  lp_release_parport
> 					   parport_release
> 					    port->cad = NULL;
>       parport_wait_peripheral
>        port->physport->cad->timeout
> 		       |
> 		      NULL
> 
> Fix this issue by adding 'port_mutex' protection. Like read/write and
> ioctl LPGETSTATUS, use this lock to protect port access and modification
> to prevent concurrency problems.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Li Nan <linan122@...wei.com>
> ---
>   drivers/char/lp.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
> 
> diff --git a/drivers/char/lp.c b/drivers/char/lp.c
> index 24417a00dfe9..7e842dfc7722 100644
> --- a/drivers/char/lp.c
> +++ b/drivers/char/lp.c
> @@ -520,9 +520,14 @@ static int lp_open(struct inode *inode, struct file *file)
>   	   should most likely only ever be used by the tunelp application. */
>   	if ((LP_F(minor) & LP_ABORTOPEN) && !(file->f_flags & O_NONBLOCK)) {
>   		int status;
> +		if (mutex_lock_interruptible(&lp_table[minor].port_mutex)) {
> +			ret = -EINTR;
> +			goto out;
> +		}
>   		lp_claim_parport_or_block(&lp_table[minor]);
>   		status = r_str(minor);
>   		lp_release_parport(&lp_table[minor]);
> +		mutex_unlock(&lp_table[minor].port_mutex);
>   		if (status & LP_POUTPA) {
>   			printk(KERN_INFO "lp%d out of paper\n", minor);
>   			LP_F(minor) &= ~LP_BUSY;
> @@ -547,6 +552,10 @@ static int lp_open(struct inode *inode, struct file *file)
>   		goto out;
>   	}
>   	/* Determine if the peripheral supports ECP mode */
> +	if (mutex_lock_interruptible(&lp_table[minor].port_mutex)) {
> +		ret = -EINTR;
> +		goto out;
> +	}
>   	lp_claim_parport_or_block(&lp_table[minor]);
>   	if ((lp_table[minor].dev->port->modes & PARPORT_MODE_ECP) &&
>   	     !parport_negotiate(lp_table[minor].dev->port,
> @@ -559,6 +568,7 @@ static int lp_open(struct inode *inode, struct file *file)
>   	/* Leave peripheral in compatibility mode */
>   	parport_negotiate(lp_table[minor].dev->port, IEEE1284_MODE_COMPAT);
>   	lp_release_parport(&lp_table[minor]);
> +	mutex_unlock(&lp_table[minor].port_mutex);
>   	lp_table[minor].current_mode = IEEE1284_MODE_COMPAT;
>   out:
>   	mutex_unlock(&lp_mutex);
> @@ -569,10 +579,13 @@ static int lp_release(struct inode *inode, struct file *file)
>   {
>   	unsigned int minor = iminor(inode);
>   
> +	if (mutex_lock_interruptible(&lp_table[minor].port_mutex))
> +		return -EINTR;
>   	lp_claim_parport_or_block(&lp_table[minor]);
>   	parport_negotiate(lp_table[minor].dev->port, IEEE1284_MODE_COMPAT);
>   	lp_table[minor].current_mode = IEEE1284_MODE_COMPAT;
>   	lp_release_parport(&lp_table[minor]);
> +	mutex_unlock(&lp_table[minor].port_mutex);
>   	kfree(lp_table[minor].lp_buffer);
>   	lp_table[minor].lp_buffer = NULL;
>   	LP_F(minor) &= ~LP_BUSY;
> @@ -641,7 +654,10 @@ static int lp_do_ioctl(unsigned int minor, unsigned int cmd,
>   				return -EFAULT;
>   			break;
>   		case LPRESET:
> +			if (mutex_lock_interruptible(&lp_table[minor].port_mutex))
> +				return -EINTR;
>   			lp_reset(minor);
> +			mutex_unlock(&lp_table[minor].port_mutex);
>   			break;
>   #ifdef LP_STATS
>   		case LPGETSTATS:

-- 
Thanks,
Nan