Re: [PATCH] isolation: keep CAP_DAC_OVERRIDE initially

[Stefano Brivio <sbrivio@redhat.com>]

[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?

> 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