When handling proxy_ndp, if rtnl_net_trylock() fails, the operation is
retried but the position pointer was already advanced meaning that the
restarted sysctl will read from an incorrect offset.

Fix this by restoring the original position pointer before restarting
the syscall.

In addition, remove the redundant position pointer restoration at the
end of the function.

Fixes: f24987ef6959 ("ipv6: add `force_forwarding` sysctl to enable per-interface forwarding")
Signed-off-by: Fernando Fernandez Mancera <redacted>
---
 net/ipv6/addrconf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index cbe681de3818..8c0741e9dfcc 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c

@@ -6825,8 +6825,10 @@ static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int wri
 	ret = proc_douintvec_minmax(&tmp_ctl, write, buffer, lenp, ppos);
 
 	if (write && old_val != new_val) {
-		if (!rtnl_net_trylock(net))
+		if (!rtnl_net_trylock(net)) {
+			*ppos = pos;
 			return restart_syscall();
+		}
 
 		WRITE_ONCE(*valp, new_val);
 

@@ -6851,8 +6853,6 @@ static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int wri
 		rtnl_net_unlock(net);
 	}
 
-	if (ret)
-		*ppos = pos;
 	return ret;
 }
 

-- 
2.54.0