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From: "Serge E. Hallyn" <serge@hallyn.com>
To: Jann Horn <jannh@google.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Mickaël Salaün" <mic@digikod.net>,
	"Christian Brauner" <brauner@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Kees Cook" <keescook@chromium.org>,
	"Paul Moore" <paul@paul-moore.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Christian Heimes" <christian@python.org>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Elliott Hughes" <enh@google.com>,
	"Fan Wu" <wufan@linux.microsoft.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Jeff Xu" <jeffxu@google.com>, "Jonathan Corbet" <corbet@lwn.net>,
	"Jordan R Abrahams" <ajordanr@google.com>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"Luca Boccassi" <bluca@debian.org>,
	"Matt Bobrowski" <mattbobrowski@google.com>,
	"Miklos Szeredi" <mszeredi@redhat.com>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Nicolas Bouchinet" <nicolas.bouchinet@oss.cyber.gouv.fr>,
	"Robert Waite" <rowait@microsoft.com>,
	"Roberto Sassu" <roberto.sassu@huawei.com>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org
Subject: Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)
Date: Mon, 1 Sep 2025 08:18:04 -0500	[thread overview]
Message-ID: <aLWdDMtMR9i1nMRy@mail.hallyn.com> (raw)
In-Reply-To: <CAG48ez0p1B9nmG3ZyNRywaSYTtEULSpbxueia912nVpg2Q7WYA@mail.gmail.com>

On Mon, Sep 01, 2025 at 01:05:16PM +0200, Jann Horn wrote:
> On Thu, Aug 28, 2025 at 11:01 PM Serge E. Hallyn <serge@hallyn.com> wrote:
> > On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski wrote:
> > > On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > >
> > > > On 2025-08-26, Mickaël Salaün <mic@digikod.net> wrote:
> > > > > On Tue, Aug 26, 2025 at 11:07:03AM +0200, Christian Brauner wrote:
> > > > > > Nothing has changed in that regard and I'm not interested in stuffing
> > > > > > the VFS APIs full of special-purpose behavior to work around the fact
> > > > > > that this is work that needs to be done in userspace. Change the apps,
> > > > > > stop pushing more and more cruft into the VFS that has no business
> > > > > > there.
> > > > >
> > > > > It would be interesting to know how to patch user space to get the same
> > > > > guarantees...  Do you think I would propose a kernel patch otherwise?
> > > >
> > > > You could mmap the script file with MAP_PRIVATE. This is the *actual*
> > > > protection the kernel uses against overwriting binaries (yes, ETXTBSY is
> > > > nice but IIRC there are ways to get around it anyway).
> > >
> > > Wait, really?  MAP_PRIVATE prevents writes to the mapping from
> > > affecting the file, but I don't think that writes to the file will
> > > break the MAP_PRIVATE CoW if it's not already broken.
> > >
> > > IPython says:
> > >
> > > In [1]: import mmap, tempfile
> > >
> > > In [2]: f = tempfile.TemporaryFile()
> > >
> > > In [3]: f.write(b'initial contents')
> > > Out[3]: 16
> > >
> > > In [4]: f.flush()
> > >
> > > In [5]: map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE,
> > > prot=mmap.PROT_READ)
> > >
> > > In [6]: map[:]
> > > Out[6]: b'initial contents'
> > >
> > > In [7]: f.seek(0)
> > > Out[7]: 0
> > >
> > > In [8]: f.write(b'changed')
> > > Out[8]: 7
> > >
> > > In [9]: f.flush()
> > >
> > > In [10]: map[:]
> > > Out[10]: b'changed contents'
> >
> > That was surprising to me, however, if I split the reader
> > and writer into different processes, so
> 
> Testing this in python is a terrible idea because it obfuscates the
> actual syscalls from you.

Hah, I was just trying to fit in :), but of course you're right.
Redoing it in straight c, I'm getting the updates.

-serge

// mmap-w.c (creates an overwrites)
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define FIRST "Initial contents"
#define SECOND "updated contents"

int main() {
	int fd, rc;
	char c;

	fd = open("/tmp/m", O_CREAT | O_RDWR, 0644);
	if (fd < 0) {
		printf("failed to open /tmp/m: %m\n");
		_exit(1);
	}
	rc = write(fd, FIRST, sizeof(FIRST));
	if (rc < 0) {
		printf("write failed: %m\n");
		_exit(1);
	}
	rc = fsync(fd);
	if (rc < 0) {
		printf("flush failed: %m\n");
		_exit(1);
	}

	read(STDIN_FILENO, &c, 1);

	printf("updating the contents\n");

	rc = lseek(fd, 0, SEEK_SET);
	if (rc < 0) {
		printf("seek failed; %m\n");
		_exit(1);
	}

	rc = write(fd, SECOND, sizeof(SECOND));
	if (fd < 0) {
		printf("write failed: %m\n");
		_exit(1);
	}
	rc = close(fd);
	if (rc < 0) {
		printf("close failed: %m\n");
		_exit(1);
	}
	printf("done\n");
}

// mmap-r.c (checks and re-checks contents)
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>

#define FIRST "Initial contents"
#define SECOND "Updated contents"

int main() {
	int fd, rc;
	char *m;
	char c;

	fd = open("/tmp/m", O_RDONLY);
	if (fd < 0) {
		printf("failed to open /tmp/m: %m\n");
		_exit(1);
	}

	m = mmap(NULL, 40, PROT_READ, MAP_PRIVATE, fd, 0);
	if (m == MAP_FAILED) {
		printf("mmap failed: %m\n");
		_exit(1);
	}

	if (strncmp(m, FIRST, 7) != 0) {
		printf("m is %c%c%c%c%c%c%c\n",
			m[0], m[1], m[2], m[3], m[4], m[5], m[6]);
		_exit(1);
	}

	read(STDIN_FILENO, &c, 1);

	if (strncmp(m, SECOND, 7) != 0) {
		printf("m is %c%c%c%c%c%c%c\n",
			m[0], m[1], m[2], m[3], m[4], m[5], m[6]);
		_exit(1);
	}

	printf("done\n");
}

  reply	other threads:[~2025-09-01 13:18 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-08-22 17:07 [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK) Mickaël Salaün
2025-08-22 17:07 ` [RFC PATCH v1 1/2] fs: Add O_DENY_WRITE Mickaël Salaün
2025-08-22 19:45   ` Jann Horn
2025-08-24 11:03     ` Mickaël Salaün
2025-08-24 18:04       ` Andy Lutomirski
2025-08-25  9:31         ` Mickaël Salaün
2025-08-25  9:39           ` Florian Weimer
2025-08-26 12:35             ` Mickaël Salaün
2025-08-25 16:43           ` Andy Lutomirski
2025-08-25 18:10             ` Jeff Xu
2025-08-25 17:57           ` Jeff Xu
2025-08-26 12:39             ` Mickaël Salaün
2025-08-26 20:29               ` Jeff Xu
2025-08-27  8:19                 ` Mickaël Salaün
2025-08-28 20:17                   ` Jeff Xu
2025-08-27 10:18     ` Aleksa Sarai
2025-08-27 10:29   ` Aleksa Sarai
2025-08-22 17:08 ` [RFC PATCH v1 2/2] selftests/exec: Add O_DENY_WRITE tests Mickaël Salaün
2025-08-26  9:07 ` [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK) Christian Brauner
2025-08-26 11:23   ` Mickaël Salaün
2025-08-26 12:30     ` Theodore Ts'o
2025-08-26 17:47       ` Mickaël Salaün
2025-08-26 20:50         ` Theodore Ts'o
2025-08-27  8:19           ` Mickaël Salaün
2025-08-27 17:35         ` Andy Lutomirski
2025-08-27 19:07           ` Mickaël Salaün
2025-08-27 20:35             ` Andy Lutomirski
2025-08-28  0:14     ` Aleksa Sarai
2025-08-28  0:32       ` Andy Lutomirski
2025-08-28  0:52         ` Aleksa Sarai
2025-08-28 21:01         ` Serge E. Hallyn
2025-09-01 11:05           ` Jann Horn
2025-09-01 13:18             ` Serge E. Hallyn [this message]
2025-09-01 16:01             ` Andy Lutomirski
2025-09-01  9:24       ` Roberto Sassu
2025-09-01 16:25         ` Andy Lutomirski
2025-09-01 17:01           ` Roberto Sassu
2025-09-02  8:57             ` Roberto Sassu

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