[PATCH] ipv4: Return EINVAL when ping_group_range sysctl doesn't map to user ns

[Tyler Hicks <tyhicks@canonical.com>]

The low and high values of the net.ipv4.ping_group_range sysctl were
being silently forced to the default disabled state when a write to the
sysctl contained GIDs that didn't map to the associated user namespace.
Confusingly, the sysctl's write operation would return success and then
a subsequent read of the sysctl would indicate that the low and high
values are the overflowgid.

This patch changes the behavior by clearly returning an error when the
sysctl write operation receives a GID range that doesn't map to the
associated user namespace. In such a situation, the previous value of
the sysctl is preserved and that range will be returned in a subsequent
read of the sysctl.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
---

This patch stems from triaging the following report from a LXD user:

  https://discuss.linuxcontainers.org/t/setting-net-ipv4-ping-group-range-inside-an-lxd-container/2162

 net/ipv4/sysctl_net_ipv4.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index af0a857d8352..5fa335fd3852 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -189,8 +189,9 @@ static int ipv4_ping_group_range(struct ctl_table *table, int write,
 	if (write && ret == 0) {
 		low = make_kgid(user_ns, urange[0]);
 		high = make_kgid(user_ns, urange[1]);
-		if (!gid_valid(low) || !gid_valid(high) ||
-		    (urange[1] < urange[0]) || gid_lt(high, low)) {
+		if (!gid_valid(low) || !gid_valid(high))
+			return -EINVAL;
+		if (urange[1] < urange[0] || gid_lt(high, low)) {
 			low = make_kgid(&init_user_ns, 1);
 			high = make_kgid(&init_user_ns, 0);
 		}
-- 
2.7.4