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 [thread overview]
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.
next prev parent reply other threads:[~2020-05-27 18:06 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-22 5:53 [PATCH] capabilities: Introduce CAP_RESTORE 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
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