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

[Stephen Brennan]

ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:

> 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.
>

I'll gladly split that out in v3 so we can continue the discussion
there.

I'll also include some changes with Matthew's suggestion of
inode_needs_pid_update(). This in combination with your suggestion to do
fewer flag checks in pid_revalidate() should cleanup the code a fair bit.

Stephen

> 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);