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

[Munehisa Kamata]

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. 

> >>>  # 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:
>