Re: [PATCH] proc: Update inode upon changing task security attribute

[Casey Schaufler]

On 11/30/2023 12:35 PM, Munehisa Kamata wrote:
> On Thu, 2023-11-30 18:00:13 +0000, Casey Schaufler wrote:
>> On 11/29/2023 7:07 PM, Munehisa Kamata wrote:
>>> Hi Casey,
>>>
>>> On Wed, 2023-11-29 18:28:55 -0800, Casey Schaufler wrote:
>>>> On 11/29/2023 4:37 PM, Munehisa Kamata wrote:
>>>>> I'm not clear whether VFS is a better (or worse) place[1] to fix the
>>>>> problem described below and would like to hear opinion.
>>>> Please To: or at least Cc: me on all Smack related issues.
>>> Will do that next.
>>>
>>>>> If the /proc/[pid] directory is bind-mounted on a system with Smack
>>>>> enabled, and if the task updates its current security attribute, the task
>>>>> may lose access to files in its own /proc/[pid] through the mountpoint.
>>>>>
>>>>>  $ sudo capsh --drop=cap_mac_override --
>>>>>  # mkdir -p dir
>>>>>  # mount --bind /proc/$$ dir
>>>>>  # echo AAA > /proc/$$/task/current		# assuming built-in echo
>>>> I don't see "current" in /proc/$$/task. Did you mean /proc/$$/attr?
>>> Ahh, yes, I meant /proc/$$/attr/current. Sorry about that...
>>>
>>>>>  # cat /proc/$$/task/current			# revalidate
>>>>>  AAA
>>>>>  # echo BBB > dir/attr/current
>>>>>  # cat dir/attr/current
>>>>>  cat: dir/attr/current: Permission denied
>>>>>  # ls dir/
>>>>>  ls: cannot access dir/: Permission denied
>> I don't see this behavior. What kernel version are you using?
>> I have a 6.5 kernel.
> I verified the behavior with 6.7-rc3. 
>
> Here is more "raw" log from my machine:
>
>  [ec2-user@ip-10-0-32-198 ~]$ uname -r
>  6.7.0-rc3-proc-fix+
>  [ec2-user@ip-10-0-32-198 ~]$ sudo capsh --drop=cap_mac_override --
>  [root@ip-10-0-32-198 ec2-user]# mount --bind /proc/$$ dir
>  [root@ip-10-0-32-198 ec2-user]# echo AAA > /proc/$$/attr/current
>  [root@ip-10-0-32-198 ec2-user]# cat /proc/$$/attr/current; echo
>  AAA
>  [root@ip-10-0-32-198 ec2-user]# echo BBB > dir/attr/current
>  [root@ip-10-0-32-198 ec2-user]# cat dir/attr/current
>  cat: dir/attr/current: Permission denied
>
> If something frequently scans /proc, such as ps, top or whatever, on your
> machine, the inode may get updated quickly (i.e. revalidated during path
> lookup) and then you may only have a short window to observe the behavior. 

I was able to reproduce the issue with a 6.5 kernel. The window seems
to be really short.

Would it be completely unreasonable for your sandboxing application to
call syncfs(2) after writing to current?

>
>>>>>  # cat /proc/$$/attr/current			# revalidate
>>>>>  BBB
>>>>>  # cat dir/attr/current
>>>>>  BBB
>>>>>  # echo CCC > /proc/$$/attr/current
>>>>>  # cat dir/attr/current
>>>>>  cat: dir/attr/current: Permission denied
>>>>>
>>>>> This happens because path lookup doesn't revalidate the dentry of the
>>>>> /proc/[pid] when traversing the filesystem boundary, so the inode security
>>>>> blob of the /proc/[pid] doesn't get updated with the new task security
>>>>> attribute. Then, this may lead security modules to deny an access to the
>>>>> directory. Looking at the code[2] and the /proc/pid/attr/current entry in
>>>>> proc man page, seems like the same could happen with SELinux. Though, I
>>>>> didn't find relevant reports.
>>>>>
>>>>> The steps above are quite artificial. I actually encountered such an
>>>>> unexpected denial of access with an in-house application sandbox
>>>>> framework; each app has its own dedicated filesystem tree where the
>>>>> process's /proc/[pid] is bind-mounted to and the app enters into via
>>>>> chroot.
>>>>>
>>>>> With this patch, writing to /proc/[pid]/attr/current (and its per-security
>>>>> module variant) updates the inode security blob of /proc/[pid] or
>>>>> /proc/[pid]/task/[tid] (when pid != tid) with the new attribute.
>>>>>
>>>>> [1] https://lkml.kernel.org/linux-fsdevel/4A2D15AF.8090000@xxxxxxx/
>>>>> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/security/selinux/hooks.c#n4220
>>>>>
>>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>>> Signed-off-by: Munehisa Kamata <kamatam@xxxxxxxxxx>
>>>>> ---
>>>>>  fs/proc/base.c | 23 ++++++++++++++++++++---
>>>>>  1 file changed, 20 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>>>> index dd31e3b6bf77..bdb7bea53475 100644
>>>>> --- a/fs/proc/base.c
>>>>> +++ b/fs/proc/base.c
>>>>> @@ -2741,6 +2741,7 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>>>>>  {
>>>>>  	struct inode * inode = file_inode(file);
>>>>>  	struct task_struct *task;
>>>>> +	const char *name = file->f_path.dentry->d_name.name;
>>>>>  	void *page;
>>>>>  	int rv;
>>>>>  
>>>>> @@ -2784,10 +2785,26 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
>>>>>  	if (rv < 0)
>>>>>  		goto out_free;
>>>>>  
>>>>> -	rv = security_setprocattr(PROC_I(inode)->op.lsm,
>>>>> -				  file->f_path.dentry->d_name.name, page,
>>>>> -				  count);
>>>>> +	rv = security_setprocattr(PROC_I(inode)->op.lsm, name, page, count);
>>>>>  	mutex_unlock(&current->signal->cred_guard_mutex);
>>>>> +
>>>>> +	/*
>>>>> +	 *  Update the inode security blob in advance if the task's security
>>>>> +	 *  attribute was updated
>>>>> +	 */
>>>>> +	if (rv > 0 && !strcmp(name, "current")) {
>>>>> +		struct pid *pid;
>>>>> +		struct proc_inode *cur, *ei;
>>>>> +
>>>>> +		rcu_read_lock();
>>>>> +		pid = get_task_pid(current, PIDTYPE_PID);
>>>>> +		hlist_for_each_entry(cur, &pid->inodes, sibling_inodes)
>>>>> +			ei = cur;
>>>>> +		put_pid(pid);
>>>>> +		pid_update_inode(current, &ei->vfs_inode);
>>>>> +		rcu_read_unlock();
>>>>> +	}
>>>>> +
>>>>>  out_free:
>>>>>  	kfree(page);
>>>>>  out: