[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CALmYWFsAZjU5sMcXTT23Mtw2Y30ewc94FAjKsnuSv1Ex=7fgLQ@mail.gmail.com>
Date: Fri, 19 Jul 2024 08:27:18 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>,
Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>,
Paul Moore <paul@...l-moore.com>, "Theodore Ts'o" <tytso@....edu>, Alejandro Colomar <alx@...nel.org>,
Aleksa Sarai <cyphar@...har.com>, Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
Casey Schaufler <casey@...aufler-ca.com>, Christian Heimes <christian@...hon.org>,
Dmitry Vyukov <dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>,
Eric Chiang <ericchiang@...gle.com>, Fan Wu <wufan@...ux.microsoft.com>,
Florian Weimer <fweimer@...hat.com>, Geert Uytterhoeven <geert@...ux-m68k.org>,
James Morris <jamorris@...ux.microsoft.com>, Jan Kara <jack@...e.cz>,
Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
Jordan R Abrahams <ajordanr@...gle.com>, Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
Luca Boccassi <bluca@...ian.org>, Luis Chamberlain <mcgrof@...nel.org>,
"Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>, Matt Bobrowski <mattbobrowski@...gle.com>,
Matthew Garrett <mjg59@...f.ucam.org>, Matthew Wilcox <willy@...radead.org>,
Miklos Szeredi <mszeredi@...hat.com>, Mimi Zohar <zohar@...ux.ibm.com>,
Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>,
Shuah Khan <shuah@...nel.org>, Stephen Rothwell <sfr@...b.auug.org.au>,
Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>,
Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
Vincent Strubel <vincent.strubel@....gouv.fr>, Xiaoming Ni <nixiaoming@...wei.com>,
Yin Fengwei <fengwei.yin@...el.com>, kernel-hardening@...ts.openwall.com,
linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org, Elliott Hughes <enh@...gle.com>
Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)
On Fri, Jul 19, 2024 at 8:04 AM Mickaël Salaün <mic@...ikod.net> wrote:
>
> On Fri, Jul 19, 2024 at 07:16:55AM -0700, Jeff Xu wrote:
> > On Fri, Jul 19, 2024 at 1:45 AM Mickaël Salaün <mic@...ikod.net> wrote:
> > >
> > > On Thu, Jul 18, 2024 at 06:29:54PM -0700, Jeff Xu wrote:
> > > > On Thu, Jul 18, 2024 at 5:24 AM Mickaël Salaün <mic@...ikod.net> wrote:
> > > > >
> > > > > On Wed, Jul 17, 2024 at 07:08:17PM -0700, Jeff Xu wrote:
> > > > > > On Wed, Jul 17, 2024 at 3:01 AM Mickaël Salaün <mic@...ikod.net> wrote:
> > > > > > >
> > > > > > > On Tue, Jul 16, 2024 at 11:33:55PM -0700, Jeff Xu wrote:
> > > > > > > > On Thu, Jul 4, 2024 at 12:02 PM Mickaël Salaün <mic@...ikod.net> wrote:
> > > > > > > > >
> > > > > > > > > Add a new AT_CHECK flag to execveat(2) to check if a file would be
> > > > > > > > > allowed for execution. The main use case is for script interpreters and
> > > > > > > > > dynamic linkers to check execution permission according to the kernel's
> > > > > > > > > security policy. Another use case is to add context to access logs e.g.,
> > > > > > > > > which script (instead of interpreter) accessed a file. As any
> > > > > > > > > executable code, scripts could also use this check [1].
> > > > > > > > >
> > > > > > > > > This is different than faccessat(2) which only checks file access
> > > > > > > > > rights, but not the full context e.g. mount point's noexec, stack limit,
> > > > > > > > > and all potential LSM extra checks (e.g. argv, envp, credentials).
> > > > > > > > > Since the use of AT_CHECK follows the exact kernel semantic as for a
> > > > > > > > > real execution, user space gets the same error codes.
> > > > > > > > >
> > > > > > > > So we concluded that execveat(AT_CHECK) will be used to check the
> > > > > > > > exec, shared object, script and config file (such as seccomp config),
> > >
> > > > > > > > I think binfmt_elf.c in the kernel needs to check the ld.so to make
> > > > > > > > sure it passes AT_CHECK, before loading it into memory.
> > > > > > >
> > > > > > > All ELF dependencies are opened and checked with open_exec(), which
> > > > > > > perform the main executability checks (with the __FMODE_EXEC flag).
> > > > > > > Did I miss something?
> > > > > > >
> > > > > > I mean the ld-linux-x86-64.so.2 which is loaded by binfmt in the kernel.
> > > > > > The app can choose its own dynamic linker path during build, (maybe
> > > > > > even statically link one ?) This is another reason that relying on a
> > > > > > userspace only is not enough.
> > > > >
> > > > > The kernel calls open_exec() on all dependencies, including
> > > > > ld-linux-x86-64.so.2, so these files are checked for executability too.
> > > > >
> > > > This might not be entirely true. iiuc, kernel calls open_exec for
> > > > open_exec for interpreter, but not all its dependency (e.g. libc.so.6)
> > >
> > > Correct, the dynamic linker is in charge of that, which is why it must
> > > be enlighten with execveat+AT_CHECK and securebits checks.
> > >
> > > > load_elf_binary() {
> > > > interpreter = open_exec(elf_interpreter);
> > > > }
> > > >
> > > > libc.so.6 is opened and mapped by dynamic linker.
> > > > so the call sequence is:
> > > > execve(a.out)
> > > > - open exec(a.out)
> > > > - security_bprm_creds(a.out)
> > > > - open the exec(ld.so)
> > > > - call open_exec() for interruptor (ld.so)
> > > > - call execveat(AT_CHECK, ld.so) <-- do we want ld.so going through
> > > > the same check and code path as libc.so below ?
> > >
> > > open_exec() checks are enough. LSMs can use this information (open +
> > > __FMODE_EXEC) if needed. execveat+AT_CHECK is only a user space
> > > request.
> > >
> > Then the ld.so doesn't go through the same security_bprm_creds() check
> > as other .so.
>
> Indeed, but...
>
My point is: we will want all the .so going through the same code
path, so security_ functions are called consistently across all the
objects, And in the future, if we want to develop additional LSM
functionality based on AT_CHECK, it will be applied to all objects.
Another thing to consider is: we are asking userspace to make
additional syscall before loading the file into memory/get executed,
there is a possibility for future expansion of the mechanism, without
asking user space to add another syscall again.
I m still not convinced yet that execveat(AT_CHECK) fits more than
faccessat(AT_CHECK)
> >
> > As my previous email, the ChromeOS LSM restricts executable mfd
> > through security_bprm_creds(), the end result is that ld.so can still
> > be executable memfd, but not other .so.
>
> The chromeOS LSM can check that with the security_file_open() hook and
> the __FMODE_EXEC flag, see Landlock's implementation. I think this
> should be the only hook implementation that chromeOS LSM needs to add.
>
> >
> > One way to address this is to refactor the necessary code from
> > execveat() code patch, and make it available to call from both kernel
> > and execveat() code paths., but if we do that, we might as well use
> > faccessat2(AT_CHECK)
>
> That's why I think it makes sense to rely on the existing __FMODE_EXEC
> information.
>
> >
> >
> > > > - transfer the control to ld.so)
> > > > - ld.so open (libc.so)
> > > > - ld.so call execveat(AT_CHECK,libc.so) <-- proposed by this patch,
> > > > require dynamic linker change.
> > > > - ld.so mmap(libc.so,rx)
> > >
> > > Explaining these steps is useful. I'll include that in the next patch
> > > series.
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
