Linux-Security-Module Archive on lore.kernel.org
 help / color / Atom feed
From: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
To: Christian Brauner <christian.brauner@ubuntu.com>,
	Adrian Reber <areber@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	"Casey Schaufler" <casey@schaufler-ca.com>,
	"Pavel Emelyanov" <ovzxemul@gmail.com>,
	"Oleg Nesterov" <oleg@redhat.com>,
	"Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Andrei Vagin" <avagin@gmail.com>,
	"Michał Cłapiński" <mclapinski@google.com>,
	"Kamil Yurtsever" <kyurtsever@google.com>,
	"Dirk Petersen" <dipeit@gmail.com>,
	"Christine Flood" <chf@redhat.com>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Radostin Stoyanov" <rstoyanov1@gmail.com>,
	"Cyrill Gorcunov" <gorcunov@openvz.org>,
	"Serge Hallyn" <serge@hallyn.com>,
	"Stephen Smalley" <stephen.smalley.work@gmail.com>,
	"Sargun Dhillon" <sargun@sargun.me>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"linux-security-module@vger.kernel.org"
	<linux-security-module@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"selinux@vger.kernel.org" <selinux@vger.kernel.org>,
	"Eric Paris" <eparis@parisplace.org>,
	"Jann Horn" <jannh@google.com>
Subject: RE: [PATCH] capabilities: Introduce CAP_RESTORE
Date: Wed, 27 May 2020 18:05:55 +0000
Message-ID: <d5ecde0c94014a4fad090e44377e9852@EXMBDFT11.ad.twosigma.com> (raw)
In-Reply-To: <20200527152955.jbbipgb6icb4nwgv@wittgenstein>

> > Also in this thread Kamil mentioned that they also need calling prctl 
> > with PR_SET_MM during restore in their production setup.
>
> We're using that as well but it really feels like this:
>
>	prctl_map = (struct prctl_mm_map){
>	    .start_code = start_code,
>	    .end_code = end_code,
>	    .start_stack = start_stack,
>	    .start_data = start_data,
>	    .end_data = end_data,
>	    .start_brk = start_brk,
>	    .brk = brk_val,
>	    .arg_start = arg_start,
>	    .arg_end = arg_end,
>	    .env_start = env_start,
>	    .env_end = env_end,
>	    .auxv = NULL,
>	    .auxv_size = 0,
>	    .exe_fd = -1,
>	};
>
> should belong under ns_capable(CAP_SYS_ADMIN). Why is that necessary to relax?

When the prctl(PR_SET_MM_MAP...), the only privileged operation is to change the symlink of /proc/self/exe via set_mm_exe_file().
See https://github.com/torvalds/linux/blob/444fc5cde64330661bf59944c43844e7d4c2ccd8/kernel/sys.c#L2001-L2004
It needs CAP_SYS_ADMIN of the current namespace.

I would argue that setting the current process exe file check should just be reduced to a "can you ptrace a children" check.
Here's why: any process can masquerade into another executable with ptrace.
One can fork a child, ptrace it, have the child execve("target_exe"), then replace its memory content with an arbitrary program.
With CRIU's libcompel parasite mechanism (https://criu.org/Compel) this is fairly easy to implement.
In fact, we could modify CRIU to do just that (but with a fair amount of efforts due to the way CRIU is written),
and not rely on being able to SET_MM_EXE_FILE via prctl(). In turn, that would give an easy way to masquerade any process
into another one, provided that one can ptrace a child.

When not using PR_SET_MM_MAP, but using SET_MM_EXE_FILE, the CAP_RESOURCES at the root namespace level is required:
https://github.com/torvalds/linux/blob/444fc5cde64330661bf59944c43844e7d4c2ccd8/kernel/sys.c#L2109
This seems inconsistent. Also for some reason changing auxv is not privileged if using prctl via the MM_MAP mechanism, but is privileged otherwise.

  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-22  5:53 Adrian Reber
2020-05-22  7:53 ` Christian Brauner
2020-05-22 18:02   ` Andrei Vagin
2020-05-22 13:41 ` Christian Brauner
2020-05-22 16:40 ` Casey Schaufler
2020-05-23  4:27   ` Andrei Vagin
2020-05-25  2:01     ` Casey Schaufler
2020-05-25  8:05   ` Adrian Reber
2020-05-25 18:55     ` Casey Schaufler
2020-05-27 13:48       ` Adrian Reber
2020-05-27 15:57         ` Casey Schaufler
2020-05-27 16:37           ` Nicolas Viennot
2020-05-27 16:46             ` Casey Schaufler
2020-05-26 13:59     ` Eric W. Biederman
     [not found]       ` <CALKUemw0UZ67yaDwAomHh0n8QZfjd52QvgEXTJ4R3JSrQjZX9g@mail.gmail.com>
2020-05-26 19:19         ` Casey Schaufler
2020-05-26 19:51         ` Jann Horn
2020-05-27 14:14       ` Adrian Reber
2020-05-27 15:29         ` Christian Brauner
2020-05-27 18:05           ` Nicolas Viennot [this message]
2020-05-28  9:48             ` Christian Brauner
2020-06-08  2:09               ` Andrei Vagin
2020-05-25 21:53 ` Jann Horn
2020-05-26  9:09   ` Radostin Stoyanov
2020-06-12  0:17 ` Matt Helsley
2020-06-12 14:39   ` Christian Brauner

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=d5ecde0c94014a4fad090e44377e9852@EXMBDFT11.ad.twosigma.com \
    --to=nicolas.viennot@twosigma.com \
    --cc=0x7f454c46@gmail.com \
    --cc=areber@redhat.com \
    --cc=arnd@arndb.de \
    --cc=avagin@gmail.com \
    --cc=casey@schaufler-ca.com \
    --cc=chf@redhat.com \
    --cc=christian.brauner@ubuntu.com \
    --cc=dipeit@gmail.com \
    --cc=ebiederm@xmission.com \
    --cc=eparis@parisplace.org \
    --cc=gorcunov@openvz.org \
    --cc=jannh@google.com \
    --cc=kyurtsever@google.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-security-module@vger.kernel.org \
    --cc=mclapinski@google.com \
    --cc=oleg@redhat.com \
    --cc=ovzxemul@gmail.com \
    --cc=rppt@linux.ibm.com \
    --cc=rstoyanov1@gmail.com \
    --cc=sargun@sargun.me \
    --cc=selinux@vger.kernel.org \
    --cc=serge@hallyn.com \
    --cc=stephen.smalley.work@gmail.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Security-Module Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-security-module/0 linux-security-module/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-security-module linux-security-module/ https://lore.kernel.org/linux-security-module \
		linux-security-module@vger.kernel.org
	public-inbox-index linux-security-module

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-security-module


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git