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

[Serge E. Hallyn]

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 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'

That was surprising to me, however, if I split the reader
and writer into different processes, so

P1:
f = open("/tmp/3", "w")
f.write('initial contents')
f.flush()

P2:
import mmap
f = open("/tmp/3", "r")
map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE, prot=mmap.PROT_READ)

Back to P1:
f.seek(0)
f.write('changed')

Back to P2:
map[:]

Then P2 gives me:

b'initial contents'

-serge