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

--t3uldpk5vmc35wcr
Content-Type: text/plain; protected-headers=v1; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable
Subject: Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)
MIME-Version: 1.0

On 2025-08-27, Andy Lutomirski <[email protected]> wrote:
> On Wed, Aug 27, 2025 at 5:14=E2=80=AFPM Aleksa Sarai <[email protected]> =
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 stuffi=
ng
> > > > the VFS APIs full of special-purpose behavior to work around the fa=
ct
> > > > that this is work that needs to be done in userspace. Change the ap=
ps,
> > > > 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 sa=
me
> > > 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).
>=20
> 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.

Oh I guess you're right -- that's news to me. And from mmap(2):

> MAP_PRIVATE
> [...] It is unspecified whether changes made to the file after the
> mmap() call are visible in the mapped region.

But then what is the protection mechanism (in the absence of -ETXTBSY)
that stops you from overwriting the live text of a binary by just
writing to it?

I would need to go trawling through my old scripts to find the
reproducer that let you get around -ETXTBSY (I think it involved
executable memfds) but I distinctly remember that even if you overwrote
the binary you would not see the live process's mapped mm change value.
(Ditto for the few kernels when we removed -ETXTBSY.) I found this
surprising, but assumed that it was because of MAP_PRIVATE.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

--t3uldpk5vmc35wcr
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----

iJEEABYKADkWIQS2TklVsp+j1GPyqQYol/rSt+lEbwUCaK+oWhsUgAAAAAAEAA5t
YW51MiwyLjUrMS4xMSwyLDIACgkQKJf60rfpRG9YOwD+IgeIsIsIT209gc6p0UJ+
RpiujJ3PHk79u8piCbNs9GIA/2I0ge/RGvNochLLI17BW1sMdgZdclwN/rf+cUxC
cnwF
=FhJ0
-----END PGP SIGNATURE-----

--t3uldpk5vmc35wcr--