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

[Casey Schaufler]

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.

>
> 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?

>  # 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;
> +		put_pid(pid);
> +		pid_update_inode(current, &ei->vfs_inode);
> +		rcu_read_unlock();
> +	}
> +
>  out_free:
>  	kfree(page);
>  out: