Hi @ljf,
I tried all the commands you suggested without success but thank you for your leads.
After laboriously deploying the fallback_ynh app (eventually it was not needed), I used the procedure defined in the ovh documentation (updating-kernel-dedicated-server):
$ uname -r
3.14.32-xxxx-grs-ipv6-64
$ sudo wget ftp://ftp.ovh.net/made-in-ovh/bzImage/4.9.78/bzImage-4.9.78-xxxx-std-ipv6-64
$ sudo update-grub
$ sudo reboot
$ uname -r
4.9.78-xxxx-std-ipv6-64
and the server is now protected from Meltdown \o/
Note that I experienced a Yunohost internal error when trying to run diagnosis, inviting me to report it: https://paste.yunohost.org/bidajayuvu.rb. I cannot reproduce it.
Note2: I don’t know if it’s related but Yunohost versions listed in monitor are all “[object Object]”