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From:   Alexey Gladkov <gladkov.alexey@gmail.com>
To:     LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
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Cc:     Akinobu Mita <akinobu.mita@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Alexey Gladkov <gladkov.alexey@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Daniel Micay <danielmicay@gmail.com>,
        Djalal Harouni <tixxdz@gmail.com>,
        "Dmitry V . Levin" <ldv@altlinux.org>,
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        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Jeff Layton <jlayton@poochiereds.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Solar Designer <solar@openwall.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: [PATCH v6 08/10] proc: instantiate only pids that we can ptrace on 'hidepid=3' mount option
Date:   Wed, 25 Dec 2019 13:51:49 +0100
Message-Id: <20191225125151.1950142-9-gladkov.alexey@gmail.com>
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If "hidepid=3" mount option is set then do not instantiate pids that
we can not ptrace. "hidepid=3" 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>
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
---
 fs/proc/base.c          | 15 +++++++++++++++
 fs/proc/root.c          |  4 ++--
 include/linux/proc_fs.h |  1 +
 3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index f4f1bcb28603..b55d205a7f6e 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)))
@@ -3274,7 +3282,14 @@ 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 3bb8df360cf7..3c7b29140293 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -70,8 +70,8 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_hidepid:
 		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");
+		    ctx->hidepid > HIDEPID_NOT_PTRACABLE)
+			return invalf(fc, "proc: hidepid value must be between 0 and 3.\n");
 		break;
 
 	default:
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index e349fcafd729..83c87ea65505 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 = 3, /* Limit pids to only ptracable pids */
 };
 
 struct proc_fs_info {
-- 
2.24.1