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

[Munehisa Kamata]

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/$$/task/current			# revalidate
> >  AAA
> >  # echo BBB > dir/attr/current
> >  # cat dir/attr/current
> >  cat: dir/attr/current: Permission denied
> >  # ls dir/
> >  ls: cannot access dir/: Permission denied
> >  # cat /proc/$$/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:
>