Re: [PATCH v10 4/9] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option

[Kees Cook <keescook@chromium.org>]

On Fri, Mar 27, 2020 at 06:23:26PM +0100, Alexey Gladkov wrote:
> If "hidepid=4" mount option is set then do not instantiate pids that
> we can not ptrace. "hidepid=4" means that procfs should only contain
> pids that the caller can ptrace.
> 
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Djalal Harouni <tixxdz@gmail.com>
> Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
> Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
> ---
>  fs/proc/base.c          | 15 +++++++++++++++
>  fs/proc/root.c          | 13 ++++++++++---
>  include/linux/proc_fs.h |  1 +
>  3 files changed, 26 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 43a28907baf9..1ebe9eba48ea 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -701,6 +701,14 @@ static bool has_pid_permissions(struct proc_fs_info *fs_info,
>  				 struct task_struct *task,
>  				 int hide_pid_min)
>  {
> +	/*
> +	 * If 'hidpid' mount option is set force a ptrace check,
> +	 * we indicate that we are using a filesystem syscall
> +	 * by passing PTRACE_MODE_READ_FSCREDS
> +	 */
> +	if (fs_info->hide_pid == HIDEPID_NOT_PTRACEABLE)
> +		return ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
> +
>  	if (fs_info->hide_pid < hide_pid_min)
>  		return true;
>  	if (in_group_p(fs_info->pid_gid))
> @@ -3319,7 +3327,14 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
>  	if (!task)
>  		goto out;
>  
> +	/* Limit procfs to only ptraceable tasks */
> +	if (fs_info->hide_pid == HIDEPID_NOT_PTRACEABLE) {
> +		if (!has_pid_permissions(fs_info, task, HIDEPID_NO_ACCESS))
> +			goto out_put_task;
> +	}
> +
>  	result = proc_pid_instantiate(dentry, task, NULL);
> +out_put_task:
>  	put_task_struct(task);
>  out:
>  	return result;
> diff --git a/fs/proc/root.c b/fs/proc/root.c
> index 616e8976185c..62eae22403d2 100644
> --- a/fs/proc/root.c
> +++ b/fs/proc/root.c
> @@ -47,6 +47,14 @@ static const struct fs_parameter_spec proc_fs_parameters[] = {
>  	{}
>  };
>  
> +static inline int valid_hidepid(unsigned int value)
> +{
> +	return (value == HIDEPID_OFF ||
> +		value == HIDEPID_NO_ACCESS ||
> +		value == HIDEPID_INVISIBLE ||
> +		value == HIDEPID_NOT_PTRACEABLE);

This likely easier to do with a ...MAX value? i.e.

	return (value < HIDEPID_OFF || value >= HIDEPID_MAX);

> +}
> +
>  static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  {
>  	struct proc_fs_context *ctx = fc->fs_private;
> @@ -63,10 +71,9 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  		break;
>  
>  	case Opt_hidepid:
> +		if (!valid_hidepid(result.uint_32))
> +			return invalf(fc, "proc: unknown value of hidepid.\n");
>  		ctx->hidepid = result.uint_32;
> -		if (ctx->hidepid < HIDEPID_OFF ||
> -		    ctx->hidepid > HIDEPID_INVISIBLE)
> -			return invalfc(fc, "hidepid value must be between 0 and 2.\n");
>  		break;
>  
>  	default:
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index 7d852dbca253..21d19353fdc7 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -32,6 +32,7 @@ enum {
>  	HIDEPID_OFF	  = 0,
>  	HIDEPID_NO_ACCESS = 1,
>  	HIDEPID_INVISIBLE = 2,
> +	HIDEPID_NOT_PTRACEABLE = 4, /* Limit pids to only ptraceable pids */

This isn't a bit field -- shouldn't this be "3"?

	...
	HIDEPID_NOT_PTRACEABLE = 3,
	HIDEPID_MAX

etc?

>  };
>  
>  struct proc_fs_info {
> -- 
> 2.25.2
> 

-- 
Kees Cook