Re: [PATCH 3/3] isolation: Initially Keep CAP_SETFCAP if running as UID 0 in non-init

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, May 22, 2023 at 01:41:58AM +0200, Stefano Brivio wrote:
> If pasta spawns a child process while running as UID 0, which is only
> allowed from a non-init namespace, we need to keep CAP_SETFCAP before
> pasta_start_ns() is called: otherwise, starting from Linux 5.12, we
> won't be able to update /proc/self/uid_map with the intended mapping
> (from 0 to 0). See user_namespaces(7).
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  isolation.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/isolation.c b/isolation.c
> index 5f89047..19932bf 100644
> --- a/isolation.c
> +++ b/isolation.c
> @@ -177,6 +177,8 @@ static void clamp_caps(void)
>   */
>  void isolate_initial(void)
>  {
> +	uint64_t keep;
> +
>  	/* We want to keep CAP_NET_BIND_SERVICE in the initial
>  	 * namespace if we have it, so that we can forward low ports
>  	 * into the guest/namespace
> @@ -193,9 +195,18 @@ void isolate_initial(void)
>  	 * further capabilites in isolate_user() and
>  	 * isolate_prefork().
>  	 */
> -	drop_caps_ep_except(BIT(CAP_NET_BIND_SERVICE) |
> -			    BIT(CAP_SETUID) | BIT(CAP_SETGID) |
> -			    BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN));
> +	keep = BIT(CAP_NET_BIND_SERVICE) | BIT(CAP_SETUID) | BIT(CAP_SETGID) |
> +	       BIT(CAP_SYS_ADMIN) | BIT(CAP_NET_ADMIN);
> +
> +	/* 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
> +	 * from a non-init user namespace (pasta can't run as root), we need to
> +	 * retain CAP_SETFCAP too.
> +	 */
> +	if (!ns_is_init() && !geteuid())
> +		keep |= BIT(CAP_SETFCAP);
> +
> +	drop_caps_ep_except(keep);
>  }
>  
>  /**

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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