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

On Mon, Sep 1, 2025 at 4:06=E2=80=AFAM Jann Horn <[email protected]> wrote:
>
> On Thu, Aug 28, 2025 at 11:01=E2=80=AFPM Serge E. Hallyn <[email protected]=
m> wrote:
> > On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski wrote:
> > > On Wed, Aug 27, 2025 at 5:14=E2=80=AFPM Aleksa Sarai <[email protected]=
om> wrote:
> > > >
> > > > On 2025-08-26, Micka=C3=ABl Sala=C3=BCn <[email protected]> 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 st=
uffing
> > > > > > the VFS APIs full of special-purpose behavior to work around th=
e fact
> > > > > > that this is work that needs to be done in userspace. Change th=
e apps,
> > > > > > stop pushing more and more cruft into the VFS that has no busin=
ess
> > > > > > there.
> > > > >
> > > > > It would be interesting to know how to patch user space to get th=
e same
> > > > > guarantees...  Do you think I would propose a kernel patch otherw=
ise?
> > > >
> > > > You could mmap the script file with MAP_PRIVATE. This is the *actua=
l*
> > > > protection the kernel uses against overwriting binaries (yes, ETXTB=
SY 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 =3D tempfile.TemporaryFile()
> > >
> > > In [3]: f.write(b'initial contents')
> > > Out[3]: 16
> > >
> > > In [4]: f.flush()
> > >
> > > In [5]: map =3D mmap.mmap(f.fileno(), f.tell(), flags=3Dmmap.MAP_PRIV=
ATE,
> > > prot=3Dmmap.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.
>
> > P1:
> > f =3D open("/tmp/3", "w")
> > f.write('initial contents')
> > f.flush()
> >
> > P2:
> > import mmap
> > f =3D open("/tmp/3", "r")
> > map =3D mmap.mmap(f.fileno(), f.tell(), flags=3Dmmap.MAP_PRIVATE, prot=
=3Dmmap.PROT_READ)
> >
> > Back to P1:
> > f.seek(0)
> > f.write('changed')
> >
> > Back to P2:
> > map[:]
> >
> > Then P2 gives me:
> >
> > b'initial contents'
>
> Because when you executed `f.write('changed')`, Python internally
> buffered the write. "changed" is never actually written into the file
> in your example. If you add a `f.flush()` in P1 after this, running
> `map[:]` in P2 again will show you the new data.
>

These days, one can type in Python, ask an LLM to translate to C, and
get almost-correct output :)  Or one can use os.write(), which is
exactly what I should have done.

--Andy