[v1,2/2] Show /proc/self/net only for CAP_NET_ADMIN
diff mbox series

Message ID 20200727141411.203770-3-gladkov.alexey@gmail.com
State New
Headers show
Series
  • proc: Relax check of mount visibility
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Commit Message

Alexey Gladkov July 27, 2020, 2:14 p.m. UTC
Show /proc/self/net only for CAP_NET_ADMIN if procfs is mounted with
subset=pid option in user namespace. This is done to avoid possible
information leakage.

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
 fs/proc/proc_net.c | 6 ++++++
 1 file changed, 6 insertions(+)

Comments

Eric W. Biederman July 27, 2020, 4:29 p.m. UTC | #1
Alexey Gladkov <gladkov.alexey@gmail.com> writes:

> Show /proc/self/net only for CAP_NET_ADMIN if procfs is mounted with
> subset=pid option in user namespace. This is done to avoid possible
> information leakage.
>
> Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> ---
>  fs/proc/proc_net.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
> index dba63b2429f0..11fa2c4b3529 100644
> --- a/fs/proc/proc_net.c
> +++ b/fs/proc/proc_net.c
> @@ -275,6 +275,12 @@ static struct net *get_proc_task_net(struct inode *dir)
>  	struct task_struct *task;
>  	struct nsproxy *ns;
>  	struct net *net = NULL;
> +	struct proc_fs_info *fs_info = proc_sb_info(dir->i_sb);
> +
> +	if ((fs_info->pidonly == PROC_PIDONLY_ON) &&
> +	    (current_user_ns() != &init_user_ns) &&
> +	    !capable(CAP_NET_ADMIN))
> +		return net;
>
>  	rcu_read_lock();
>  	task = pid_task(proc_pid(dir), PIDTYPE_PID);

Hmm.

I see 3 options going forward.

1) We just make PROC_PIDONLY_ON mean the net directory does not exist.
   No permission checks just always fail.

2) Move the permission checks into opendir/readdir and whichever
   is the appropriate method there and always allow the dentries
   to be cached.

3) Simply cache the mounters credentials and make access to the
   net directories contingent of the permisions of the mounter of
   proc.  Something like the code below.

static struct net *get_proc_task_net(struct inode *dir)
{
	struct task_struct *task;
	struct nsproxy *ns;
	struct net *net = NULL;

	rcu_read_lock();
	task = pid_task(proc_pid(dir), PIDTYPE_PID);
	if (task != NULL) {
		task_lock(task);
		ns = task->nsproxy;
		if (ns != NULL)
			net = get_net(ns->net_ns);
		task_unlock(task);
	}
	rcu_read_unlock();
	if ((fs_info->pidonly == PROC_PIDONLY_ON) &&
            !security_capable(fs_info->mounter_cred,
			      net->user_ns, CAP_SYS_ADMIN,
			      CAP_OPT_NONE)) {
		put_net(net);
		net = NULL;
	}
	return net;
}

Eric
Alexey Gladkov July 28, 2020, 1:54 p.m. UTC | #2
On Mon, Jul 27, 2020 at 11:29:36AM -0500, Eric W. Biederman wrote:
> Alexey Gladkov <gladkov.alexey@gmail.com> writes:
> 
> > Show /proc/self/net only for CAP_NET_ADMIN if procfs is mounted with
> > subset=pid option in user namespace. This is done to avoid possible
> > information leakage.
> >
> > Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> > ---
> >  fs/proc/proc_net.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
> > index dba63b2429f0..11fa2c4b3529 100644
> > --- a/fs/proc/proc_net.c
> > +++ b/fs/proc/proc_net.c
> > @@ -275,6 +275,12 @@ static struct net *get_proc_task_net(struct inode *dir)
> >  	struct task_struct *task;
> >  	struct nsproxy *ns;
> >  	struct net *net = NULL;
> > +	struct proc_fs_info *fs_info = proc_sb_info(dir->i_sb);
> > +
> > +	if ((fs_info->pidonly == PROC_PIDONLY_ON) &&
> > +	    (current_user_ns() != &init_user_ns) &&
> > +	    !capable(CAP_NET_ADMIN))
> > +		return net;
> >
> >  	rcu_read_lock();
> >  	task = pid_task(proc_pid(dir), PIDTYPE_PID);
> 
> Hmm.
> 
> I see 3 options going forward.
> 
> 1) We just make PROC_PIDONLY_ON mean the net directory does not exist.
>    No permission checks just always fail.

I think it's wrong. Now if someone mounts a fully visible procfs then they
can see this directory. Hiding this directory completely will change the
current behavior.

> 2) Move the permission checks into opendir/readdir and whichever
>    is the appropriate method there and always allow the dentries
>    to be cached.

At first I did so, but then I transferred this check to get_proc_task_net
because if this function does not return anything, then 'net' directory
will exist but will simply be empty.

This allowed us to get rid of unnecessary wrappers for opendir/lookup.

> 3) Simply cache the mounters credentials and make access to the
>    net directories contingent of the permisions of the mounter of
>    proc.  Something like the code below.

Interesting idea. I like that :)

> static struct net *get_proc_task_net(struct inode *dir)
> {
> 	struct task_struct *task;
> 	struct nsproxy *ns;
> 	struct net *net = NULL;
> 
> 	rcu_read_lock();
> 	task = pid_task(proc_pid(dir), PIDTYPE_PID);
> 	if (task != NULL) {
> 		task_lock(task);
> 		ns = task->nsproxy;
> 		if (ns != NULL)
> 			net = get_net(ns->net_ns);
> 		task_unlock(task);
> 	}
> 	rcu_read_unlock();
> 	if ((fs_info->pidonly == PROC_PIDONLY_ON) &&

Is this check necessary? I mean, isn't it worth extending this check to
other cases?

>             !security_capable(fs_info->mounter_cred,
> 			      net->user_ns, CAP_SYS_ADMIN,
> 			      CAP_OPT_NONE)) {
> 		put_net(net);
> 		net = NULL;
> 	}
> 	return net;
> }
> 
> Eric
>

Patch
diff mbox series

diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
index dba63b2429f0..11fa2c4b3529 100644
--- a/fs/proc/proc_net.c
+++ b/fs/proc/proc_net.c
@@ -275,6 +275,12 @@  static struct net *get_proc_task_net(struct inode *dir)
 	struct task_struct *task;
 	struct nsproxy *ns;
 	struct net *net = NULL;
+	struct proc_fs_info *fs_info = proc_sb_info(dir->i_sb);
+
+	if ((fs_info->pidonly == PROC_PIDONLY_ON) &&
+	    (current_user_ns() != &init_user_ns) &&
+	    !capable(CAP_NET_ADMIN))
+		return net;
 
 	rcu_read_lock();
 	task = pid_task(proc_pid(dir), PIDTYPE_PID);