[PATCH v3 1/1] fs: Allow no_new_privs tasks to call chroot(2)

[Kees Cook]

On Thu, Mar 11, 2021 at 11:52:42AM +0100, Mickaël Salaün wrote:
> [...]
> This change may not impact systems relying on other permission models
> than POSIX capabilities (e.g. Tomoyo).  Being able to use chroot(2) on
> such systems may require to update their security policies.
> 
> Only the chroot system call is relaxed with this no_new_privs check; the
> init_chroot() helper doesn't require such change.
> 
> Allowing unprivileged users to use chroot(2) is one of the initial
> objectives of no_new_privs:
> https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html
> This patch is a follow-up of a previous one sent by Andy Lutomirski:
> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/

I liked it back when Andy first suggested it, and I still like it now.
:) I'm curious, do you have a specific user in mind for this feature?

> [...]
> @@ -546,8 +547,18 @@ SYSCALL_DEFINE1(chroot, const char __user *, filename)
>  	if (error)
>  		goto dput_and_out;
>  
> +	/*
> +	 * Changing the root directory for the calling task (and its future
> +	 * children) requires that this task has CAP_SYS_CHROOT in its
> +	 * namespace, or be running with no_new_privs and not sharing its
> +	 * fs_struct and not escaping its current root (cf. create_user_ns()).
> +	 * As for seccomp, checking no_new_privs avoids scenarios where
> +	 * unprivileged tasks can affect the behavior of privileged children.
> +	 */
>  	error = -EPERM;
> -	if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT))
> +	if (!ns_capable(current_user_ns(), CAP_SYS_CHROOT) &&
> +			!(task_no_new_privs(current) && current->fs->users == 1
> +				&& !current_chrooted()))
>  		goto dput_and_out;
>  	error = security_path_chroot(&path);
>  	if (error)

I think the logic here needs to be rearranged to avoid setting
PF_SUPERPRIV, and I find the many negations hard to read. Perhaps:

static inline int current_chroot_allowed(void)
{
	/* comment here */
	if (task_no_new_privs(current) && current->fs->users == 1 &&
	    !current_chrooted())
		return 0;

	if (ns_capable(current_user_ns(), CAP_SYS_CHROOT))
		return 0;

	return -EPERM;
}

...

	error = current_chroot_allowed();
	if (error)
		goto dput_and_out;


I can't think of a way to race current->fs->users ...

-- 
Kees Cook