Re: [PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

[Eric W. Biederman]

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Thu, Dec 03, 2020 at 04:02:12PM -0800, Stephen Brennan wrote:
>> -void pid_update_inode(struct task_struct *task, struct inode *inode)
>> +static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
>> +			       unsigned int flags)
>
> I'm really nitpicking here, but this function only _updates_ the inode
> if flags says it should.  So I was thinking something like this
> (compile tested only).
>
> I'd really appreocate feedback from someone like Casey or Stephen on
> what they need for their security modules.

Just so we don't have security module questions confusing things
can we please make this a 2 patch series?  With the first
patch removing security_task_to_inode?

The justification for the removal is that all security_task_to_inode
appears to care about is the file type bits in inode->i_mode.  Something
that never changes.  Having this in a separate patch would make that
logical change easier to verify.

Eric

>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index b362523a9829..771f330bfce7 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1968,6 +1968,25 @@ void pid_update_inode(struct task_struct *task, struct inode *inode)
>  	security_task_to_inode(task, inode);
>  }
>  
> +/* See if we can avoid the above call.  Assumes RCU lock held */
> +static bool inode_needs_pid_update(struct task_struct *task,
> +		const struct inode *inode)
> +{
> +	kuid_t uid;
> +	kgid_t gid;
> +
> +	if (inode->i_mode & (S_ISUID | S_ISGID))
> +		return true;
> +	task_dump_owner(task, inode->i_mode, &uid, &gid);
> +	if (!uid_eq(uid, inode->i_uid) || !gid_eq(gid, inode->i_gid))
> +		return true;
> +	/*
> +	 * XXX: Do we need to call the security system here to see if
> +	 * there's a pending update?
> +	 */
> +	return false;
> +}
> +
>  /*
>   * Rewrite the inode's ownerships here because the owning task may have
>   * performed a setuid(), etc.
> @@ -1978,8 +1997,15 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
>  	struct inode *inode;
>  	struct task_struct *task;
>  
> -	if (flags & LOOKUP_RCU)
> +	if (flags & LOOKUP_RCU) {
> +		inode = d_inode_rcu(dentry);
> +		task = pid_task(proc_pid(inode), PIDTYPE_PID);
> +		if (!task)
> +			return 0;
> +		if (!inode_needs_pid_update(task, inode))
> +			return 1;
>  		return -ECHILD;
> +	}
>  
>  	inode = d_inode(dentry);
>  	task = get_proc_task(inode);