Re: [PATCH v3] Implement /proc/pid/kill

[Tycho Andersen <tycho@tycho.ws>]

On Wed, Oct 31, 2018 at 03:59:12PM +0000, Daniel Colascione wrote:
> Add a simple proc-based kill interface. To use /proc/pid/kill, just
> write the signal number in base-10 ASCII to the kill file of the
> process to be killed: for example, 'echo 9 > /proc/$$/kill'.
> 
> Semantically, /proc/pid/kill works like kill(2), except that the
> process ID comes from the proc filesystem context instead of from an
> explicit system call parameter. This way, it's possible to avoid races
> between inspecting some aspect of a process and that process's PID
> being reused for some other process.
> 
> Note that the write(2) to the kill file descriptor works only if it
> happens in the security context as the call to open(2), where
> "security context" is defined as the set of all ambient user IDs
> (effective uid, fs uid, real uid, and saved uid) as well as the
> presence of the CAP_KILL capability.  This check prevents confused
> deputy attacks via, e.g., supplying a /proc/$(pidof httpd)/kill file
> descriptor as the standard output of setuid program and convincing
> that program to write a "9".
> 
> With /proc/pid/kill, it's possible to write a proper race-free and
> safe pkill(1). An approximation follows. A real program might use
> openat(2), having opened a process's /proc/pid directory explicitly,
> with the directory file descriptor serving as a sort of "process
> handle".
> 
>     #!/bin/bash
>     set -euo pipefail
>     pat=$1
>     for proc_status in /proc/*/status; do (
>         cd $(dirname $proc_status)
>         readarray proc_argv -d'' < cmdline
>         if ((${#proc_argv[@]} > 0)) &&
>                [[ ${proc_argv[0]} = *$pat* ]];
>         then
>             echo 15 > kill
>         fi
>     ) || true; done
> 
> Signed-off-by: Daniel Colascione <dancol@google.com>
> ---
> 
> Turns out that checking struct user isn't sufficient, since signal.c's
> permissions check also cares about effective UIDs. Let's be
> extra-paranoid and bail if _anything_ relevant in struct cred
> has changed.
> 
> Also, as Joel suggested, switch from goto-return to direct return.
> 
>  fs/proc/base.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 67 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 7e9f07bf260d..b0e7ded96af9 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -205,6 +205,72 @@ static int proc_root_link(struct dentry *dentry, struct path *path)
>  	return result;
>  }
>  
> +static ssize_t proc_pid_kill_write(struct file *file,
> +				   const char __user *buf,
> +				   size_t count, loff_t *ppos)
> +{
> +	ssize_t res;
> +	int sig;
> +	char buffer[4];
> +	const struct cred *cur_cred;
> +	const struct cred *open_cred;
> +	bool security_changed;
> +
> +	/* This check prevents a confused deputy attack in which an
> +	 * unprivileged process opens /proc/victim/kill and convinces
> +	 * a privileged process to write to that kill FD, effectively
> +	 * performing a kill with the privileges of the unwitting
> +	 * privileged process.  Here, we just fail the kill operation
> +	 * if someone calls write(2) with a real user ID that differs
> +	 * from the one used to open the kill FD.
> +	 */
> +	cur_cred = current_cred();
> +	open_cred = file->f_cred;
> +	security_changed =
> +		cur_cred->user_ns != open_cred->user_ns ||
> +		!uid_eq(cur_cred->euid, open_cred->euid) ||
> +		!uid_eq(cur_cred->fsuid, open_cred->fsuid) ||
> +		!uid_eq(cur_cred->suid, open_cred->suid) ||
> +		!uid_eq(cur_cred->uid, open_cred->uid) ||
> +		/* No audit: if we actually use the capability, we'll
> +		 * audit during the actual kill. Here, we're just
> +		 * checking whether our kill-FD has escaped its
> +		 * original security context and bailing if it has.
> +		 */
> +		(security_capable_noaudit(cur_cred,
> +					  cur_cred->user_ns,
> +					  CAP_KILL)
> +		 != security_capable_noaudit(open_cred,
> +					     open_cred->user_ns,
> +					     CAP_KILL));
> +	if (security_changed)
> +		return -EPERM;

Why not just use an ioctl() like Jann suggested instead of this big
security check? Then we avoid the whole setuid writer thing entirely,
and we can pass the fd around if we want to.

Tycho