Re: [PATCH] isolation: keep CAP_DAC_OVERRIDE initially

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Oct 07, 2025 at 06:02:32PM +0200, Stefano Brivio wrote:
> [Cc: Yumei as this is somewhat related to
>  https://archives.passt.top/passt-dev/20250926011714.5978-1-yuhuang@redhat.com/,
>  and David as he wrote most of this part]
> 
> On Tue,  7 Oct 2025 08:16:39 -0400
> Cole Robinson <crobinso@redhat.com> wrote:
> 
> > Reproducer that I'd expect to work
> > 
> >   $ cd $HOME
> >   $ sudo passt --runas $UID --socket foo.sock
> >   Failed to bind UNIX domain socket: Permission denied
> > 
> > A more practical example is for libguestfs apps when run as user=root.
> > 
> > + libguestfs connects to libvirt qemu:///system
> > + libvirt qemu:///system defaults to user=qemu.
> >   + chowns passt runtime dir to user=qemu
> > + libguestfs instead requests the VM run as user=root
> >   + patches in progress but we are blocked by this issue
> > + passt is launched as root, but can't open socket in passt dir.
> > 
> > Obviously libvirt needs improvements too.
> > But it seems like this is a defect as well.
> 
> Thanks for the patch! I think it's absolutely unproblematic to keep
> CAP_DAC_OVERRIDE for a moment at the beginning. Did you figure out
> exactly why it's needed by the way?

It's because the socket directory is chmod to qemu (by libvirt).
Without CAP_DAC_OVERRIDE, root can't open/write to a non-root
file/directory.

Cole explains a bit more at the end of this comment:
https://github.com/libguestfs/libguestfs/pull/218#issuecomment-3376943380

Rich.

> > Signed-off-by: Cole Robinson <crobinso@redhat.com>
> 
> Should we add:
> 
> Link: https://github.com/libguestfs/libguestfs/pull/218
> 
> ? Or it's misleading, or you omitted it for any other reason?
> 
> > ---
> >  isolation.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/isolation.c b/isolation.c
> > index bbcd23b..b25f349 100644
> > --- a/isolation.c
> > +++ b/isolation.c
> > @@ -188,6 +188,9 @@ void isolate_initial(int argc, char **argv)
> >  	 * We have to keep CAP_SETUID and CAP_SETGID at this stage, so
> >  	 * that we can switch user away from root.
> >  	 *
> > +	 * CAP_DAC_OVERRIDE may be required for socket setup when combined
> > +	 * with --runas.
> > +	 *
> >  	 * We have to keep some capabilities for the --netns-only case:
> >  	 *  - CAP_SYS_ADMIN, so that we can setns() to the netns.
> >  	 *  - Keep CAP_NET_ADMIN, so that we can configure interfaces
> > @@ -198,7 +201,7 @@ void isolate_initial(int argc, char **argv)
> >  	 * isolate_prefork().
> >  	 */
> >  	keep = BIT(CAP_NET_BIND_SERVICE) | BIT(CAP_SETUID) | BIT(CAP_SETGID) |
> > -	       BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN);
> > +	       BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN) | BIT(CAP_DAC_OVERRIDE);
> >  
> >  	/* Since Linux 5.12, if we want to update /proc/self/uid_map to create
> >  	 * a mapping from UID 0, which only happens with pasta spawning a child
> 
> -- 
> Stefano

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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