[PATCH v8 08/11] proc: instantiate only pids that we can ptrace on 'hidepid=4' mount option

[Jordan Glover]

On Monday, February 10, 2020 3:05 PM, Alexey Gladkov <gladkov.alexey at gmail.com> 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 at chromium.org
> Cc: Andy Lutomirski luto at kernel.org
> Signed-off-by: Djalal Harouni tixxdz at gmail.com
> Signed-off-by: Alexey Gladkov gladkov.alexey at gmail.com
>
> fs/proc/base.c | 15 +++++++++++++++
> fs/proc/root.c | 14 +++++++++++---
> include/linux/proc_fs.h | 1 +
> 3 files changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 24b7c620ded3..49937d54e745 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -699,6 +699,14 @@ 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 (proc_fs_hide_pid(fs_info) == HIDEPID_NOT_PTRACABLE)
> -         return ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
>
>
> -   if (proc_fs_hide_pid(fs_info) < hide_pid_min)
>     return true;
>     if (in_group_p(proc_fs_pid_gid(fs_info)))
>     @@ -3271,7 +3279,14 @@ struct dentry *proc_pid_lookup(struct dentry *dentry, unsigned int flags)
>     if (!task)
>     goto out;
>
> -   /* Limit procfs to only ptracable tasks */
> -   if (proc_fs_hide_pid(fs_info) == HIDEPID_NOT_PTRACABLE) {
> -         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 e2bb015da1a8..5e27bb31f125 100644
>     --- a/fs/proc/root.c
>     +++ b/fs/proc/root.c
>     @@ -52,6 +52,15 @@ static const struct fs_parameter_description proc_fs_parameters = {
>     .specs = proc_param_specs,
>     };
>
>     +static inline int
>     +valid_hidepid(unsigned int value)
>     +{
>
> -   return (value == HIDEPID_OFF ||
> -         value == HIDEPID_NO_ACCESS ||
>
>
> -         value == HIDEPID_INVISIBLE ||
>
>
> -         value == HIDEPID_NOT_PTRACABLE);
>
>
>
> +}
> +
> static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
> {
> struct proc_fs_context *ctx = fc->fs_private;
> @@ -68,10 +77,9 @@ 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 invalf(fc, "proc: 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 f307940f8311..6822548405a7 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -17,6 +17,7 @@ enum {
> HIDEPID_OFF = 0,
> HIDEPID_NO_ACCESS = 1,
> HIDEPID_INVISIBLE = 2,
>
> -   HIDEPID_NOT_PTRACABLE = 4, /* Limit pids to only ptracable pids */

Is there a reason new option is "4" instead of "3"? The order 1..2..4 may be
confusing for people.

Jordan