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

[Munehisa Kamata]

Hi Alexey,

On Fri, 2023-12-01 09:30:00 +0000, Alexey Dobriyan wrote:
>
> On Wed, Nov 29, 2023 at 05:11:22PM -0800, Andrew Morton wrote:
> > 
> > fyi...
> > 
> > (yuk!)
> > 
> > 
> > 
> > Begin forwarded message:
> > 
> > Date: Thu, 30 Nov 2023 00:37:04 +0000
> > From: Munehisa Kamata <kamatam at amazon.com>
> > To: <linux-fsdevel at vger.kernel.org>, <linux-security-module at vger.kernel.org>
> > Cc: <linux-kernel at vger.kernel.org>, <akpm at linux-foundation.org>, "Munehisa Kamata" <kamatam at amazon.com>
> > Subject: [PATCH] proc: Update inode upon changing task security attribute
> > 
> > 
> > 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.
> > 
> > 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
> >  # 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@sun.com/
> > [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 at amazon.com>
> > ---
> >  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;
> 
> Should this "break;"? Why is only the last inode in the list updated?
> Should it be the first? All of them?

If it picks up the first node, it may end up updating /proc/[pid]/task/[tid]
rather than /proc/[pid] (when pid == tid) and the task may be denied access
to its own /proc/[pid] afterward.

I think updating all of them won't hurt. But, as long as /proc/[pid] is
accessible, the rest of the inodes should be updated upon path lookup via
revalidation as usual.

When pid != tid, it only updates /proc/[pid]/task/[tid] and the thread may
lose an access to /proc/[pid], but I think it's okay as it's a matter of
security policy enforced by security modules. Casey, do you have any
comments here?  


Regards,
Munehisa

 
> > +		put_pid(pid);
> > +		pid_update_inode(current, &ei->vfs_inode);
> > +		rcu_read_unlock();
> > +	}
>