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(a)redhat.com>
Reviewed-by: David Gibson <david(a)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);
}
/**
--
2.39.2