From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 544D6C43387 for ; Tue, 15 Jan 2019 21:58:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F354420866 for ; Tue, 15 Jan 2019 21:58:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="M74wSh0k" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387401AbfAOV6v (ORCPT ); Tue, 15 Jan 2019 16:58:51 -0500 Received: from mail-yb1-f193.google.com ([209.85.219.193]:33198 "EHLO mail-yb1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1733253AbfAOV6v (ORCPT ); Tue, 15 Jan 2019 16:58:51 -0500 Received: by mail-yb1-f193.google.com with SMTP id o73so1713361ybc.0 for ; Tue, 15 Jan 2019 13:58:49 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc:content-transfer-encoding; bh=x6hciQX0pfhOjeSy7fJ9TxOfwAFNAo2LKbBl+80ZTpI=; b=M74wSh0kC2dZZ5sxN+gcbredyAJaFRI7sT1NxOlgLjPVBFtFxNB2cpQHvsjqeEoUB1 4T/cb2ykKb0NXWs259Ww25/1zAkk/rcpbGv/XnclSx/FKS6/boQqNCTi4NJ7F7sZm1Pd 3QpDb1ltJ9Cfl2g0+uCrCWQGEyWuvZ2PkwQ24= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc:content-transfer-encoding; bh=x6hciQX0pfhOjeSy7fJ9TxOfwAFNAo2LKbBl+80ZTpI=; b=VzCJ8LtziLgOCFwTn4uYJCuKV1BTvxFNumVd1niC4pmumH95IeiDVDUm+qtfD7d2UC EMiYAQPaOf9IYVlVSsu2oY8s/oCTsE7mlqar+o+8OkN4NsYGaiAuIVd6vzAS87TNnU/A wJEqBx/pjaZqPejo6CKs9ErL3y18YXEfQUiNOfHUwwhXGO6I+ji0qo75y+lciCUyzZRq B+MMRc4aF/Oe0k0Xsd924jFW+2wGQhnu0+Sksbv55WDZ/IkBg27GUQpbuVF7NFuFMQTj l0VcuQTAn7MYk3NzP80oDFAV7HImCN5U9yr2ZANnUhu3BIUNJlubYA62yLPfa74voPCE DeJA== X-Gm-Message-State: AJcUukdrh+7Nc4uig7arL8rslBfnM7F4FFdUbF4Pn02mY3K/92B8D+xS QcsG/AxfvQ4qe9ZucQAMMPgfS6Wl4pvHqF26za4jPA== X-Google-Smtp-Source: ALg8bN63UIh6DyUM0RmCdjgEPM8OyE5NRoq1Dv7xBizjWowGRuQYyyXgDbZLzXlCFTg7nW3j8BwRzxUVcLm/cgqGTsE= X-Received: by 2002:a25:20c6:: with SMTP id g189mr4941611ybg.303.1547589528937; Tue, 15 Jan 2019 13:58:48 -0800 (PST) MIME-Version: 1.0 References: <20190115180436.102826-1-mortonm@chromium.org> In-Reply-To: From: Micah Morton Date: Tue, 15 Jan 2019 13:58:37 -0800 Message-ID: Subject: Re: [PATCH v3 2/2] LSM: add SafeSetID module that gates setid calls To: Kees Cook Cc: James Morris , "Serge E. Hallyn" , Casey Schaufler , Stephen Smalley , linux-security-module Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On Tue, Jan 15, 2019 at 11:44 AM Kees Cook wrote: > > On Tue, Jan 15, 2019 at 10:04 AM wrote: > > > > From: Micah Morton > > > > SafeSetID gates the setid family of syscalls to restrict UID/GID > > transitions from a given UID/GID to only those approved by a > > system-wide whitelist. These restrictions also prohibit the given > > UIDs/GIDs from obtaining auxiliary privileges associated with > > CAP_SET{U/G}ID, such as allowing a user to set up user namespace UID > > mappings. For now, only gating the set*uid family of syscalls is > > supported, with support for set*gid coming in a future patch set. > > > > Signed-off-by: Micah Morton > > --- > > Changes since the last patch set: Pulled out the "no-op" changes that > > mark setid call sites in kernel/sys.c into a separate patch, and made > > other small mods proposed by Kees Cook. NOTE: this patch is still using > > do_exit(SIGKILL) to kill the process in check_uid_transition in lsm.c. > > This may need to change, pending further discussion. > > Documentation/admin-guide/LSM/SafeSetID.rst | 107 ++++++++ > > Documentation/admin-guide/LSM/index.rst | 1 + > > security/Kconfig | 1 + > > security/Makefile | 2 + > > security/safesetid/Kconfig | 12 + > > security/safesetid/Makefile | 7 + > > security/safesetid/lsm.c | 266 ++++++++++++++++++++ > > security/safesetid/lsm.h | 30 +++ > > security/safesetid/securityfs.c | 185 ++++++++++++++ > > 9 files changed, 611 insertions(+) > > create mode 100644 Documentation/admin-guide/LSM/SafeSetID.rst > > create mode 100644 security/safesetid/Kconfig > > create mode 100644 security/safesetid/Makefile > > create mode 100644 security/safesetid/lsm.c > > create mode 100644 security/safesetid/lsm.h > > create mode 100644 security/safesetid/securityfs.c > > > > diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentatio= n/admin-guide/LSM/SafeSetID.rst > > new file mode 100644 > > index 000000000000..ffb64be67f7a > > --- /dev/null > > +++ b/Documentation/admin-guide/LSM/SafeSetID.rst > > @@ -0,0 +1,107 @@ > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D > > +SafeSetID > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D > > +SafeSetID is an LSM module that gates the setid family of syscalls to = restrict > > +UID/GID transitions from a given UID/GID to only those approved by a > > +system-wide whitelist. These restrictions also prohibit the given UIDs= /GIDs > > +from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, su= ch as > > +allowing a user to set up user namespace UID mappings. > > + > > + > > +Background > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > +In absence of file capabilities, processes spawned on a Linux system t= hat need > > +to switch to a different user must be spawned with CAP_SETUID privileg= es. > > +CAP_SETUID is granted to programs running as root or those running as = a non-root > > +user that have been explicitly given the CAP_SETUID runtime capability= . It is > > +often preferable to use Linux runtime capabilities rather than file > > +capabilities, since using file capabilities to run a program with elev= ated > > +privileges opens up possible security holes since any user with access= to the > > +file can exec() that program to gain the elevated privileges. > > + > > +While it is possible to implement a tree of processes by giving full > > +CAP_SET{U/G}ID capabilities, this is often at odds with the goals of r= unning a > > +tree of processes under non-root user(s) in the first place. Specifica= lly, > > +since CAP_SETUID allows changing to any user on the system, including = the root > > +user, it is an overpowered capability for what is needed in this scena= rio, > > +especially since programs often only call setuid() to drop privileges = to a > > +lesser-privileged user -- not elevate privileges. Unfortunately, there= is no > > +generally feasible way in Linux to restrict the potential UIDs that a = user can > > +switch to through setuid() beyond allowing a switch to any user on the= system. > > +This SafeSetID LSM seeks to provide a solution for restricting setid > > +capabilities in such a way. > > + > > +The main use case for this LSM is to allow a non-root program to trans= ition to > > +other untrusted uids without full blown CAP_SETUID capabilities. The n= on-root > > +program would still need CAP_SETUID to do any kind of transition, but = the > > +additional restrictions imposed by this LSM would mean it is a "safer"= version > > +of CAP_SETUID since the non-root program cannot take advantage of CAP_= SETUID to > > +do any unapproved actions (e.g. setuid to uid 0 or create/enter new us= er > > +namespace). The higher level goal is to allow for uid-based sandboxing= of system > > +services without having to give out CAP_SETUID all over the place just= so that > > +non-root programs can drop to even-lesser-privileged uids. This is esp= ecially > > +relevant when one non-root daemon on the system should be allowed to s= pawn other > > +processes as different uids, but its undesirable to give the daemon a > > +basically-root-equivalent CAP_SETUID. > > + > > + > > +Other Approaches Considered > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D > > + > > +Solve this problem in userspace > > +------------------------------- > > +For candidate applications that would like to have restricted setid ca= pabilities > > +as implemented in this LSM, an alternative option would be to simply t= ake away > > +setid capabilities from the application completely and refactor the pr= ocess > > +spawning semantics in the application (e.g. by using a privileged help= er program > > +to do process spawning and UID/GID transitions). Unfortunately, there = are a > > +number of semantics around process spawning that would be affected by = this, such > > +as fork() calls where the program doesn=E2=80=99t immediately call exe= c() after the > > +fork(), parent processes specifying custom environment variables or co= mmand line > > +args for spawned child processes, or inheritance of file handles acros= s a > > +fork()/exec(). Because of this, as solution that uses a privileged hel= per in > > +userspace would likely be less appealing to incorporate into existing = projects > > +that rely on certain process-spawning semantics in Linux. > > + > > +Use user namespaces > > +------------------- > > +Another possible approach would be to run a given process tree in its = own user > > +namespace and give programs in the tree setid capabilities. In this wa= y, > > +programs in the tree could change to any desired UID/GID in the contex= t of their > > +own user namespace, and only approved UIDs/GIDs could be mapped back t= o the > > +initial system user namespace, affectively preventing privilege escala= tion. > > +Unfortunately, it is not generally feasible to use user namespaces in = isolation, > > +without pairing them with other namespace types, which is not always a= n option. > > +Linux checks for capabilities based off of the user namespace that =E2= =80=9Cowns=E2=80=9D some > > +entity. For example, Linux has the notion that network namespaces are = owned by > > +the user namespace in which they were created. A consequence of this i= s that > > +capability checks for access to a given network namespace are done by = checking > > +whether a task has the given capability in the context of the user nam= espace > > +that owns the network namespace -- not necessarily the user namespace = under > > +which the given task runs. Therefore spawning a process in a new user = namespace > > +effectively prevents it from accessing the network namespace owned by = the > > +initial namespace. This is a deal-breaker for any application that exp= ects to > > +retain the CAP_NET_ADMIN capability for the purpose of adjusting netwo= rk > > +configurations. Using user namespaces in isolation causes problems reg= arding > > +other system interactions, including use of pid namespaces and device = creation. > > + > > +Use an existing LSM > > +------------------- > > +None of the other in-tree LSMs have the capability to gate setid trans= itions, or > > +even employ the security_task_fix_setuid hook at all. SELinux says of = that hook: > > +"Since setuid only affects the current process, and since the SELinux = controls > > +are not based on the Linux identity attributes, SELinux does not need = to control > > +this operation." > > + > > + > > +Directions for use > > +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > > +This LSM hooks the setid syscalls to make sure transitions are allowed= if an > > +applicable restriction policy is in place. Policies are configured thr= ough > > +securityfs by writing to the safesetid/add_whitelist_policy and > > +safesetid/flush_whitelist_policies files at the location where securit= yfs is > > +mounted. The format for adding a policy is ':', using litera= l > > +numbers, such as '123:456'. To flush the policies, any write to the fi= le is > > +sufficient. Again, configuring a policy for a UID will prevent that UI= D from > > +obtaining auxiliary setid privileges, such as allowing a user to set u= p user > > +namespace UID mappings. > > diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/ad= min-guide/LSM/index.rst > > index 9842e21afd4a..a6ba95fbaa9f 100644 > > --- a/Documentation/admin-guide/LSM/index.rst > > +++ b/Documentation/admin-guide/LSM/index.rst > > @@ -46,3 +46,4 @@ subdirectories. > > Smack > > tomoyo > > Yama > > + SafeSetID > > diff --git a/security/Kconfig b/security/Kconfig > > index 78dc12b7eeb3..9efc7a5e3280 100644 > > --- a/security/Kconfig > > +++ b/security/Kconfig > > @@ -236,6 +236,7 @@ source "security/tomoyo/Kconfig" > > source "security/apparmor/Kconfig" > > source "security/loadpin/Kconfig" > > source "security/yama/Kconfig" > > +source "security/safesetid/Kconfig" > > > > source "security/integrity/Kconfig" > > > > diff --git a/security/Makefile b/security/Makefile > > index 4d2d3782ddef..c598b904938f 100644 > > --- a/security/Makefile > > +++ b/security/Makefile > > @@ -10,6 +10,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO) +=3D tomoyo > > subdir-$(CONFIG_SECURITY_APPARMOR) +=3D apparmor > > subdir-$(CONFIG_SECURITY_YAMA) +=3D yama > > subdir-$(CONFIG_SECURITY_LOADPIN) +=3D loadpin > > +subdir-$(CONFIG_SECURITY_SAFESETID) +=3D safesetid > > > > # always enable default capabilities > > obj-y +=3D commoncap.o > > @@ -25,6 +26,7 @@ obj-$(CONFIG_SECURITY_TOMOYO) +=3D tomoyo/ > > obj-$(CONFIG_SECURITY_APPARMOR) +=3D apparmor/ > > obj-$(CONFIG_SECURITY_YAMA) +=3D yama/ > > obj-$(CONFIG_SECURITY_LOADPIN) +=3D loadpin/ > > +obj-$(CONFIG_SECURITY_SAFESETID) +=3D safesetid/ > > obj-$(CONFIG_CGROUP_DEVICE) +=3D device_cgroup.o > > Given the refactoring of the LSM enabling logic, you'll need to do > some minor merging with the linux-next tree to get this to apply to > security-next. That would make James's life easier, I think, though > maybe James can speak to that, since I'm not sure how the trees are > split right now. These patches apply cleanly to security-next at the moment (unless I'm doing something weird -- the last commit I see in the git log is mine from last week: c1a85a00ea66cb6f0bd0f14e47c28c2b0999799f) > > > > > # Object integrity file lists > > diff --git a/security/safesetid/Kconfig b/security/safesetid/Kconfig > > new file mode 100644 > > index 000000000000..bf89a47ffcc8 > > --- /dev/null > > +++ b/security/safesetid/Kconfig > > @@ -0,0 +1,12 @@ > > +config SECURITY_SAFESETID > > + bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilit= ies" > > + default n > > + help > > + SafeSetID is an LSM module that gates the setid family of sy= scalls to > > + restrict UID/GID transitions from a given UID/GID to only th= ose > > + approved by a system-wide whitelist. These restrictions also= prohibit > > + the given UIDs/GIDs from obtaining auxiliary privileges asso= ciated > > + with CAP_SET{U/G}ID, such as allowing a user to set up user = namespace > > + UID mappings. > > + > > + If you are unsure how to answer this question, answer N. > > diff --git a/security/safesetid/Makefile b/security/safesetid/Makefile > > new file mode 100644 > > index 000000000000..6b0660321164 > > --- /dev/null > > +++ b/security/safesetid/Makefile > > @@ -0,0 +1,7 @@ > > +# SPDX-License-Identifier: GPL-2.0 > > +# > > +# Makefile for the safesetid LSM. > > +# > > + > > +obj-$(CONFIG_SECURITY_SAFESETID) :=3D safesetid.o > > +safesetid-y :=3D lsm.o securityfs.o > > diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c > > new file mode 100644 > > index 000000000000..aa7bd3323751 > > --- /dev/null > > +++ b/security/safesetid/lsm.c > > @@ -0,0 +1,266 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +/* > > + * SafeSetID Linux Security Module > > + * > > + * Author: Micah Morton > > + * > > + * Copyright (C) 2018 The Chromium OS Authors. > > + * > > + * This program is free software; you can redistribute it and/or modif= y > > + * it under the terms of the GNU General Public License version 2, as > > + * published by the Free Software Foundation. > > + * > > + */ > > + > > +#define pr_fmt(fmt) "SafeSetID: " fmt > > + > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > + > > +#define NUM_BITS 8 /* 128 buckets in hash table */ > > + > > +static DEFINE_HASHTABLE(safesetid_whitelist_hashtable, NUM_BITS); > > + > > +/* > > + * Hash table entry to store safesetid policy signifying that 'parent'= user > > + * can setid to 'child' user. > > + */ > > +struct entry { > > + struct hlist_node next; > > + struct hlist_node dlist; /* for deletion cleanup */ > > + uint64_t parent_kuid; > > + uint64_t child_kuid; > > +}; > > + > > +static DEFINE_SPINLOCK(safesetid_whitelist_hashtable_spinlock); > > + > > +static bool check_setuid_policy_hashtable_key(kuid_t parent) > > +{ > > + struct entry *entry; > > + > > + rcu_read_lock(); > > + hash_for_each_possible_rcu(safesetid_whitelist_hashtable, > > + entry, next, __kuid_val(parent)) { > > + if (entry->parent_kuid =3D=3D __kuid_val(parent)) { > > + rcu_read_unlock(); > > + return true; > > + } > > + } > > + rcu_read_unlock(); > > + > > + return false; > > +} > > + > > +static bool check_setuid_policy_hashtable_key_value(kuid_t parent, > > + kuid_t child) > > +{ > > + struct entry *entry; > > + > > + rcu_read_lock(); > > + hash_for_each_possible_rcu(safesetid_whitelist_hashtable, > > + entry, next, __kuid_val(parent)) { > > + if (entry->parent_kuid =3D=3D __kuid_val(parent) && > > + entry->child_kuid =3D=3D __kuid_val(child)) { > > + rcu_read_unlock(); > > + return true; > > + } > > + } > > + rcu_read_unlock(); > > + > > + return false; > > +} > > + > > +static int safesetid_security_capable(const struct cred *cred, > > + struct user_namespace *ns, > > + int cap, > > + unsigned int opts) > > +{ > > + if (cap =3D=3D CAP_SETUID && > > + check_setuid_policy_hashtable_key(cred->uid)) { > > + if (!(opts & CAP_OPT_INSETID)) { > > + /* > > + * Deny if we're not in a set*uid() syscall to = avoid > > + * giving powers gated by CAP_SETUID that are r= elated > > + * to functionality other than calling set*uid(= ) (e.g. > > + * allowing user to set up userns uid mappings)= . > > + */ > > + pr_warn("Operation requires CAP_SETUID, which i= s not available to UID %u for operations besides approved set*uid transitio= ns", > > + __kuid_val(cred->uid)); > > + return -1; > > + } > > + } > > + return 0; > > +} > > + > > +static int check_uid_transition(kuid_t parent, kuid_t child) > > +{ > > + if (check_setuid_policy_hashtable_key_value(parent, child)) > > + return 0; > > + pr_warn("UID transition (%d -> %d) blocked", > > + __kuid_val(parent), > > + __kuid_val(child)); > > + /* > > + * Kill this process to avoid potential security vulnerabiliti= es > > + * that could arise from a missing whitelist entry preventing = a > > + * privileged process from dropping to a lesser-privileged one= . > > + */ > > + do_exit(SIGKILL); > > +} > > This needs double-checking, but I think you want this, to avoid > missing various process clean-up steps (like performing a core dump if > desired, etc): > > force_sig(SIGKILL, current); > return -EACCES; > > But please double-check that a rejected setuid() syscall never > completes and the process does die with SIGKILL. Yep, this looks good. I changed those lines and see the following strace output from a process that isn't allowed to setuid to root per the whitelist policies: ... setgid(0) =3D 0 setuid(0) =3D ? +++ killed by SIGKILL +++ FWIW, I checked this with the following command on a ChromeOS device in dev mode: localhost ~ # strace -ff -o /tmp/strace /sbin/minijail0 -u shill -g shill -c 0xc0 -- /sbin/capsh --user=3Droot -- -c /usr/bin/whoami > > > + > > +/* > > + * Check whether there is either an exception for user under old cred = struct to > > + * set*uid to user under new cred struct, or the UID transition is all= owed (by > > + * Linux set*uid rules) even without CAP_SETUID. > > + */ > > +static int safesetid_task_fix_setuid(struct cred *new, > > + const struct cred *old, > > + int flags) > > +{ > > + > > + /* Do nothing if there are no setuid restrictions for this UID.= */ > > + if (!check_setuid_policy_hashtable_key(old->uid)) > > + return 0; > > + > > + switch (flags) { > > + case LSM_SETID_RE: > > + /* > > + * Users for which setuid restrictions exist can only s= et the > > + * real UID to the real UID or the effective UID, unles= s an > > + * explicit whitelist policy allows the transition. > > + */ > > + if (!uid_eq(old->uid, new->uid) && > > + !uid_eq(old->euid, new->uid)) { > > + return check_uid_transition(old->uid, new->uid)= ; > > + } > > + /* > > + * Users for which setuid restrictions exist can only s= et the > > + * effective UID to the real UID, the effective UID, or= the > > + * saved set-UID, unless an explicit whitelist policy a= llows > > + * the transition. > > + */ > > + if (!uid_eq(old->uid, new->euid) && > > + !uid_eq(old->euid, new->euid) && > > + !uid_eq(old->suid, new->euid)) { > > + return check_uid_transition(old->euid, new->eui= d); > > + } > > + break; > > + case LSM_SETID_ID: > > + /* > > + * Users for which setuid restrictions exist cannot cha= nge the > > + * real UID or saved set-UID unless an explicit whiteli= st > > + * policy allows the transition. > > + */ > > + if (!uid_eq(old->uid, new->uid)) > > + return check_uid_transition(old->uid, new->uid)= ; > > + if (!uid_eq(old->suid, new->suid)) > > + return check_uid_transition(old->suid, new->sui= d); > > + break; > > + case LSM_SETID_RES: > > + /* > > + * Users for which setuid restrictions exist cannot cha= nge the > > + * real UID, effective UID, or saved set-UID to anythin= g but > > + * one of: the current real UID, the current effective = UID or > > + * the current saved set-user-ID unless an explicit whi= telist > > + * policy allows the transition. > > + */ > > + if (!uid_eq(new->uid, old->uid) && > > + !uid_eq(new->uid, old->euid) && > > + !uid_eq(new->uid, old->suid)) { > > + return check_uid_transition(old->uid, new->uid)= ; > > + } > > + if (!uid_eq(new->euid, old->uid) && > > + !uid_eq(new->euid, old->euid) && > > + !uid_eq(new->euid, old->suid)) { > > + return check_uid_transition(old->euid, new->eui= d); > > + } > > + if (!uid_eq(new->suid, old->uid) && > > + !uid_eq(new->suid, old->euid) && > > + !uid_eq(new->suid, old->suid)) { > > + return check_uid_transition(old->suid, new->sui= d); > > + } > > + break; > > + case LSM_SETID_FS: > > + /* > > + * Users for which setuid restrictions exist cannot cha= nge the > > + * filesystem UID to anything but one of: the current r= eal UID, > > + * the current effective UID or the current saved set-U= ID > > + * unless an explicit whitelist policy allows the trans= ition. > > + */ > > + if (!uid_eq(new->fsuid, old->uid) && > > + !uid_eq(new->fsuid, old->euid) && > > + !uid_eq(new->fsuid, old->suid) && > > + !uid_eq(new->fsuid, old->fsuid)) { > > + return check_uid_transition(old->fsuid, new->fs= uid); > > + } > > + break; > > As a robustness measure can you add a default case here that will > "fail closed"? Something like: > > default: > WARN_ON_ONCE("Unknown setid state %d\n", flags); > force_sig(SIGKILL, current); > return -EINVAL; Done. > > > + } > > + return 0; > > +} > > + > > +int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child) > > +{ > > + struct entry *new; > > + > > + /* Return if entry already exists */ > > + if (check_setuid_policy_hashtable_key_value(parent, child)) > > + return 0; > > + > > + new =3D kzalloc(sizeof(struct entry), GFP_KERNEL); > > + if (!new) > > + return -ENOMEM; > > + new->parent_kuid =3D __kuid_val(parent); > > + new->child_kuid =3D __kuid_val(child); > > + spin_lock(&safesetid_whitelist_hashtable_spinlock); > > + hash_add_rcu(safesetid_whitelist_hashtable, > > + &new->next, > > + __kuid_val(parent)); > > + spin_unlock(&safesetid_whitelist_hashtable_spinlock); > > + return 0; > > +} > > + > > +void flush_safesetid_whitelist_entries(void) > > +{ > > + struct entry *entry; > > + struct hlist_node *hlist_node; > > + unsigned int bkt_loop_cursor; > > + HLIST_HEAD(free_list); > > + > > + /* > > + * Could probably use hash_for_each_rcu here instead, but this = should > > + * be fine as well. > > + */ > > + spin_lock(&safesetid_whitelist_hashtable_spinlock); > > + hash_for_each_safe(safesetid_whitelist_hashtable, bkt_loop_curs= or, > > + hlist_node, entry, next) { > > + hash_del_rcu(&entry->next); > > + hlist_add_head(&entry->dlist, &free_list); > > + } > > + spin_unlock(&safesetid_whitelist_hashtable_spinlock); > > + synchronize_rcu(); > > + hlist_for_each_entry_safe(entry, hlist_node, &free_list, dlist)= { > > + hlist_del(&entry->dlist); > > + kfree(entry); > > + } > > +} > > + > > +static struct security_hook_list safesetid_security_hooks[] =3D { > > + LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid), > > + LSM_HOOK_INIT(capable, safesetid_security_capable) > > +}; > > + > > +static int __init safesetid_security_init(void) > > +{ > > + security_add_hooks(safesetid_security_hooks, > > + ARRAY_SIZE(safesetid_security_hooks), "safes= etid"); > > + > > + return 0; > > +} > > + > > +DEFINE_LSM(safesetid_security_init) =3D { > > + .init =3D safesetid_security_init, > > +}; > > diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h > > new file mode 100644 > > index 000000000000..bf78af9bf314 > > --- /dev/null > > +++ b/security/safesetid/lsm.h > > @@ -0,0 +1,30 @@ > > +/* SPDX-License-Identifier: GPL-2.0 */ > > +/* > > + * SafeSetID Linux Security Module > > + * > > + * Author: Micah Morton > > + * > > + * Copyright (C) 2018 The Chromium OS Authors. > > + * > > + * This program is free software; you can redistribute it and/or modif= y > > + * it under the terms of the GNU General Public License version 2, as > > + * published by the Free Software Foundation. > > + * > > + */ > > +#ifndef _SAFESETID_H > > +#define _SAFESETID_H > > + > > +#include > > + > > +/* Function type. */ > > +enum safesetid_whitelist_file_write_type { > > + SAFESETID_WHITELIST_ADD, /* Add whitelist policy. */ > > + SAFESETID_WHITELIST_FLUSH, /* Flush whitelist policies. */ > > +}; > > + > > +/* Add entry to safesetid whitelist to allow 'parent' to setid to 'chi= ld'. */ > > +int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child); > > + > > +void flush_safesetid_whitelist_entries(void); > > + > > +#endif /* _SAFESETID_H */ > > diff --git a/security/safesetid/securityfs.c b/security/safesetid/secur= ityfs.c > > new file mode 100644 > > index 000000000000..c3ce7b63b4af > > --- /dev/null > > +++ b/security/safesetid/securityfs.c > > @@ -0,0 +1,185 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +/* > > + * SafeSetID Linux Security Module > > + * > > + * Author: Micah Morton > > + * > > + * Copyright (C) 2018 The Chromium OS Authors. > > + * > > + * This program is free software; you can redistribute it and/or modif= y > > + * it under the terms of the GNU General Public License version 2, as > > + * published by the Free Software Foundation. > > + * > > + */ > > +#include > > +#include > > + > > +#include "lsm.h" > > + > > +static struct dentry *safesetid_policy_dir; > > + > > +struct safesetid_file_entry { > > + const char *name; > > + enum safesetid_whitelist_file_write_type type; > > + struct dentry *dentry; > > +}; > > + > > +static struct safesetid_file_entry safesetid_files[] =3D { > > + {.name =3D "add_whitelist_policy", > > + .type =3D SAFESETID_WHITELIST_ADD}, > > + {.name =3D "flush_whitelist_policies", > > + .type =3D SAFESETID_WHITELIST_FLUSH}, > > +}; > > + > > +/* > > + * In the case the input buffer contains one or more invalid UIDs, the= kuid_t > > + * variables pointed to by 'parent' and 'child' will get updated but t= his > > + * function will return an error. > > + */ > > +static int parse_safesetid_whitelist_policy(const char __user *buf, > > + size_t len, > > + kuid_t *parent, > > + kuid_t *child) > > +{ > > + char *kern_buf; > > + char *parent_buf; > > + char *child_buf; > > + const char separator[] =3D ":"; > > + int ret; > > + size_t first_substring_length; > > + long parsed_parent; > > + long parsed_child; > > + > > + /* Duplicate string from user memory and NULL-terminate */ > > + kern_buf =3D memdup_user_nul(buf, len); > > + if (IS_ERR(kern_buf)) > > + return PTR_ERR(kern_buf); > > + > > + /* > > + * Format of |buf| string should be :. > > + * Find location of ":" in kern_buf (copied from |buf|). > > + */ > > + first_substring_length =3D strcspn(kern_buf, separator); > > + if (first_substring_length =3D=3D 0 || first_substring_length = =3D=3D len) { > > + ret =3D -EINVAL; > > + goto free_kern; > > + } > > + > > + parent_buf =3D kmemdup_nul(kern_buf, first_substring_length, GF= P_KERNEL); > > + if (!parent_buf) { > > + ret =3D -ENOMEM; > > + goto free_kern; > > + } > > + > > + ret =3D kstrtol(parent_buf, 0, &parsed_parent); > > + if (ret) > > + goto free_both; > > + > > + child_buf =3D kern_buf + first_substring_length + 1; > > + ret =3D kstrtol(child_buf, 0, &parsed_child); > > + if (ret) > > + goto free_both; > > + > > + *parent =3D make_kuid(current_user_ns(), parsed_parent); > > + if (!uid_valid(*parent)) { > > + ret =3D -EINVAL; > > + goto free_both; > > + } > > + > > + *child =3D make_kuid(current_user_ns(), parsed_child); > > + if (!uid_valid(*child)) { > > + ret =3D -EINVAL; > > + goto free_both; > > + } > > + > > +free_both: > > + kfree(parent_buf); > > +free_kern: > > + kfree(kern_buf); > > + return ret; > > +} > > + > > +static ssize_t safesetid_file_write(struct file *file, > > + const char __user *buf, > > + size_t len, > > + loff_t *ppos) > > +{ > > + struct safesetid_file_entry *file_entry =3D > > + file->f_inode->i_private; > > + kuid_t parent; > > + kuid_t child; > > + int ret; > > + > > + if (!ns_capable(current_user_ns(), CAP_MAC_ADMIN)) > > + return -EPERM; > > + > > + if (*ppos !=3D 0) > > + return -EINVAL; > > + > > + switch (file_entry->type) { > > + case SAFESETID_WHITELIST_FLUSH: > > + flush_safesetid_whitelist_entries(); > > missing break? Thanks. > > > + case SAFESETID_WHITELIST_ADD: > > + ret =3D parse_safesetid_whitelist_policy(buf, len, &pa= rent, > > + &chil= d); > > + if (ret) > > + return ret; > > + > > + ret =3D add_safesetid_whitelist_entry(parent, child); > > + if (ret) > > + return ret; > > And add a default here too, something like: > > default: > WARN_ON_ONCE("Unknown securityfs file %d!?\n", file_entry->type); > break; > Done. > > + } > > + > > + /* Return len on success so caller won't keep trying to write = */ > > + return len; > > +} > > + > > +static const struct file_operations safesetid_file_fops =3D { > > + .write =3D safesetid_file_write, > > +}; > > + > > +static void safesetid_shutdown_securityfs(void) > > +{ > > + int i; > > + > > + for (i =3D 0; i < ARRAY_SIZE(safesetid_files); ++i) { > > + struct safesetid_file_entry *entry =3D > > + &safesetid_files[i]; > > + securityfs_remove(entry->dentry); > > + entry->dentry =3D NULL; > > + } > > + > > + securityfs_remove(safesetid_policy_dir); > > + safesetid_policy_dir =3D NULL; > > +} > > + > > +static int __init safesetid_init_securityfs(void) > > +{ > > + int i; > > + int ret; > > + > > + safesetid_policy_dir =3D securityfs_create_dir("safesetid", NUL= L); > > + if (!safesetid_policy_dir) { > > + ret =3D PTR_ERR(safesetid_policy_dir); > > + goto error; > > + } > > + > > + for (i =3D 0; i < ARRAY_SIZE(safesetid_files); ++i) { > > + struct safesetid_file_entry *entry =3D > > + &safesetid_files[i]; > > + entry->dentry =3D securityfs_create_file( > > + entry->name, 0200, safesetid_policy_dir, > > + entry, &safesetid_file_fops); > > + if (IS_ERR(entry->dentry)) { > > + ret =3D PTR_ERR(entry->dentry); > > + goto error; > > + } > > + } > > + > > + return 0; > > + > > +error: > > + safesetid_shutdown_securityfs(); > > + return ret; > > +} > > +fs_initcall(safesetid_init_securityfs); > > -- > > 2.20.1.97.g81188d93c3-goog > > > > And if I didn't say it before, thank you for the docs on this too! :) > > -- > Kees Cook