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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 5DC98C433DF for ; Wed, 1 Jul 2020 08:56:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 39A232074D for ; Wed, 1 Jul 2020 08:56:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728961AbgGAI4F (ORCPT ); Wed, 1 Jul 2020 04:56:05 -0400 Received: from youngberry.canonical.com ([91.189.89.112]:51835 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726009AbgGAI4F (ORCPT ); Wed, 1 Jul 2020 04:56:05 -0400 Received: from ip5f5af08c.dynamic.kabel-deutschland.de ([95.90.240.140] helo=wittgenstein) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1jqYWo-0003mV-AJ; Wed, 01 Jul 2020 08:55:38 +0000 Date: Wed, 1 Jul 2020 10:55:37 +0200 From: Christian Brauner To: Adrian Reber Cc: Eric Biederman , Pavel Emelyanov , Oleg Nesterov , Dmitry Safonov <0x7f454c46@gmail.com>, Andrei Vagin , Nicolas Viennot , =?utf-8?B?TWljaGHFgiBDxYJhcGnFhHNraQ==?= , Kamil Yurtsever , Dirk Petersen , Christine Flood , Casey Schaufler , Mike Rapoport , Radostin Stoyanov , Cyrill Gorcunov , Serge Hallyn , Stephen Smalley , Sargun Dhillon , Arnd Bergmann , linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, selinux@vger.kernel.org, Eric Paris , Jann Horn , linux-fsdevel@vger.kernel.org Subject: Re: [PATCH v4 3/3] prctl: Allow ptrace capable processes to change /proc/self/exe Message-ID: <20200701085537.k7whjbn3icu5rpsq@wittgenstein> References: <20200701064906.323185-1-areber@redhat.com> <20200701064906.323185-4-areber@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200701064906.323185-4-areber@redhat.com> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Jul 01, 2020 at 08:49:06AM +0200, Adrian Reber wrote: > From: Nicolas Viennot > > Previously, the current process could only change the /proc/self/exe > link with local CAP_SYS_ADMIN. > This commit relaxes this restriction by permitting such change with > CAP_CHECKPOINT_RESTORE, and the ability to use ptrace. > > With access to ptrace facilities, a process can do the following: fork a > child, execve() the target executable, and have the child use ptrace() > to replace the memory content of the current process. This technique > makes it possible to masquerade an arbitrary program as any executable, > even setuid ones. > > Signed-off-by: Nicolas Viennot > Signed-off-by: Adrian Reber > --- > include/linux/lsm_hook_defs.h | 1 + > include/linux/security.h | 6 ++++++ > kernel/sys.c | 12 ++++-------- > security/commoncap.c | 26 ++++++++++++++++++++++++++ > security/security.c | 5 +++++ > security/selinux/hooks.c | 14 ++++++++++++++ > 6 files changed, 56 insertions(+), 8 deletions(-) > > diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h > index 0098852bb56a..90e51d5e093b 100644 > --- a/include/linux/lsm_hook_defs.h > +++ b/include/linux/lsm_hook_defs.h > @@ -211,6 +211,7 @@ LSM_HOOK(int, 0, task_kill, struct task_struct *p, struct kernel_siginfo *info, > int sig, const struct cred *cred) > LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2, > unsigned long arg3, unsigned long arg4, unsigned long arg5) > +LSM_HOOK(int, 0, prctl_set_mm_exe_file, struct file *exe_file) > LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p, > struct inode *inode) > LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag) > diff --git a/include/linux/security.h b/include/linux/security.h > index 2797e7f6418e..0f594eb7e766 100644 > --- a/include/linux/security.h > +++ b/include/linux/security.h > @@ -412,6 +412,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info, > int sig, const struct cred *cred); > int security_task_prctl(int option, unsigned long arg2, unsigned long arg3, > unsigned long arg4, unsigned long arg5); > +int security_prctl_set_mm_exe_file(struct file *exe_file); > void security_task_to_inode(struct task_struct *p, struct inode *inode); > int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag); > void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid); > @@ -1124,6 +1125,11 @@ static inline int security_task_prctl(int option, unsigned long arg2, > return cap_task_prctl(option, arg2, arg3, arg4, arg5); > } > > +static inline int security_prctl_set_mm_exe_file(struct file *exe_file) > +{ > + return cap_prctl_set_mm_exe_file(exe_file); > +} > + > static inline void security_task_to_inode(struct task_struct *p, struct inode *inode) > { } > > diff --git a/kernel/sys.c b/kernel/sys.c > index 00a96746e28a..bb53e8408c63 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -1851,6 +1851,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd) > if (err) > goto exit; > > + err = security_prctl_set_mm_exe_file(exe.file); > + if (err) > + goto exit; > + > /* > * Forbid mm->exe_file change if old file still mapped. > */ > @@ -2006,14 +2010,6 @@ static int prctl_set_mm_map(int opt, const void __user *addr, unsigned long data > } > > if (prctl_map.exe_fd != (u32)-1) { > - /* > - * Make sure the caller has the rights to > - * change /proc/pid/exe link: only local sys admin should > - * be allowed to. > - */ > - if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN)) > - return -EINVAL; > - > error = prctl_set_mm_exe_file(mm, prctl_map.exe_fd); > if (error) > return error; > diff --git a/security/commoncap.c b/security/commoncap.c > index 59bf3c1674c8..663d00fe2ecc 100644 > --- a/security/commoncap.c > +++ b/security/commoncap.c > @@ -1291,6 +1291,31 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3, > } > } > > +/** > + * cap_prctl_set_mm_exe_file - Determine whether /proc/self/exe can be changed > + * by the current process. > + * @exe_file: The new exe file > + * Returns 0 if permission is granted, -ve if denied. > + * > + * The current process is permitted to change its /proc/self/exe link via two policies: > + * 1) The current user can do checkpoint/restore. At the time of this writing, > + * this means CAP_SYS_ADMIN or CAP_CHECKPOINT_RESTORE capable. > + * 2) The current user can use ptrace. > + * > + * With access to ptrace facilities, a process can do the following: > + * fork a child, execve() the target executable, and have the child use > + * ptrace() to replace the memory content of the current process. > + * This technique makes it possible to masquerade an arbitrary program as the > + * target executable, even if it is setuid. > + */ > +int cap_prctl_set_mm_exe_file(struct file *exe_file) > +{ > + if (checkpoint_restore_ns_capable(current_user_ns())) > + return 0; > + > + return security_ptrace_access_check(current, PTRACE_MODE_ATTACH_REALCREDS); > +} What is the reason for having this be a new security hook? Doesn't look like it needs to be unless I'm missing something. This just seems more complex than it needs to be. I might be wrong here but if you look at the callsites for security_ptrace_access_check() right now, you'll see that it's only called from kernel/ptrace.c in __ptrace_may_access() and that function checks right at the top: /* Don't let security modules deny introspection */ if (same_thread_group(task, current)) return 0; since you're passing in same_thread_group(current, current) you're passing this check and never even hitting security_ptrace_access_check(). So the contract seems to be (as is obvious from the comment) that a task can't be denied ptrace introspection. But if you're using security_ptrace_access_check(current) here and _if_ there would be any lsm that would deny ptrace introspection to current you'd suddenly introduce a callsite where ptrace introspection is denied. That seems wrong. So either you meant to do something else here or you really just want: checkpoint_restore_ns_capable(current_user_ns()) and none of the rest. But I might be missing the big picture in this patch. > + if (checkpoint_restore_ns_capable(current_user_ns())) > + return 0; > + > /** > * cap_vm_enough_memory - Determine whether a new virtual mapping is permitted > * @mm: The VM space in which the new mapping is to be made > @@ -1356,6 +1381,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = { > LSM_HOOK_INIT(mmap_file, cap_mmap_file), > LSM_HOOK_INIT(task_fix_setuid, cap_task_fix_setuid), > LSM_HOOK_INIT(task_prctl, cap_task_prctl), > + LSM_HOOK_INIT(prctl_set_mm_exe_file, cap_prctl_set_mm_exe_file), > LSM_HOOK_INIT(task_setscheduler, cap_task_setscheduler), > LSM_HOOK_INIT(task_setioprio, cap_task_setioprio), > LSM_HOOK_INIT(task_setnice, cap_task_setnice), > diff --git a/security/security.c b/security/security.c > index 2bb912496232..13a1ed32f9e3 100644 > --- a/security/security.c > +++ b/security/security.c > @@ -1790,6 +1790,11 @@ int security_task_prctl(int option, unsigned long arg2, unsigned long arg3, > return rc; > } > > +int security_prctl_set_mm_exe_file(struct file *exe_file) > +{ > + return call_int_hook(prctl_set_mm_exe_file, 0, exe_file); > +} > + > void security_task_to_inode(struct task_struct *p, struct inode *inode) > { > call_void_hook(task_to_inode, p, inode); > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index ca901025802a..fca5581392b8 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -4156,6 +4156,19 @@ static int selinux_task_kill(struct task_struct *p, struct kernel_siginfo *info, > secid, task_sid(p), SECCLASS_PROCESS, perm, NULL); > } > > +static int selinux_prctl_set_mm_exe_file(struct file *exe_file) > +{ > + u32 sid = current_sid(); > + > + struct common_audit_data ad = { > + .type = LSM_AUDIT_DATA_FILE, > + .u.file = exe_file, > + }; > + > + return avc_has_perm(&selinux_state, sid, sid, > + SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad); > +} > + > static void selinux_task_to_inode(struct task_struct *p, > struct inode *inode) > { > @@ -7057,6 +7070,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = { > LSM_HOOK_INIT(task_getscheduler, selinux_task_getscheduler), > LSM_HOOK_INIT(task_movememory, selinux_task_movememory), > LSM_HOOK_INIT(task_kill, selinux_task_kill), > + LSM_HOOK_INIT(prctl_set_mm_exe_file, selinux_prctl_set_mm_exe_file), > LSM_HOOK_INIT(task_to_inode, selinux_task_to_inode), > > LSM_HOOK_INIT(ipc_permission, selinux_ipc_permission), > -- > 2.26.2 >