[RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)

[Andy Lutomirski]

On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai <cyphar at cyphar.com> wrote:
>
> On 2025-08-26, Mickaël Salaün <mic at 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'