Messages in this thread | | | From | (Eric W. Biederman) | Date | Fri, 12 May 2017 09:56:36 -0500 | Subject | Re: [PATCH] prctl: Allow local CAP_SYS_ADMIN changing exe_file |
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Kirill Tkhai <ktkhai@virtuozzo.com> writes:
> During checkpointing and restore of userspace tasks > we bumped into the situation, that it's not possible > to restore the tasks, which user namespace does not > have uid 0 or gid 0 mapped. > > People create user namespace mappings like they want, > and there is no a limitation on obligatory uid and gid > "must be mapped". So, if there is no uid 0 or gid 0 > in the mapping, it's impossible to restore mm->exe_file > of the processes belonging to this user namespace. > > Also, there is no a workaround. It's impossible > to create a temporary uid/gid mapping, because > only one write to /proc/[pid]/uid_map and gid_map > is allowed during a namespace lifetime. > If there is an entry, then no more mapings can't be > written. If there isn't an entry, we can't write > there too, otherwise user task won't be able > to do that in the future. > > The patch changes the check, and looks for CAP_SYS_ADMIN > instead of zero uid and gid. This allows to restore > a task independently of its user namespace mappings.
Applied thanks. Testing against 0 in the modern kernel is perhaps the most bizarre permisssion check I have seen lately.
Eric > > Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com> > CC: Andrew Morton <akpm@linux-foundation.org> > CC: Serge Hallyn <serge@hallyn.com> > CC: "Eric W. Biederman" <ebiederm@xmission.com> > CC: Oleg Nesterov <oleg@redhat.com> > CC: Michal Hocko <mhocko@suse.com> > CC: Andrei Vagin <avagin@openvz.org> > CC: Cyrill Gorcunov <gorcunov@openvz.org> > CC: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com> > CC: Pavel Tikhomirov <ptikhomirov@virtuozzo.com> > --- > kernel/sys.c | 8 ++------ > 1 file changed, 2 insertions(+), 6 deletions(-) > > diff --git a/kernel/sys.c b/kernel/sys.c > index 8a94b4eabcaa..7c6d78148fa0 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -1802,15 +1802,11 @@ static int validate_prctl_map(struct prctl_mm_map *prctl_map) > > /* > * Finally, make sure the caller has the rights to > - * change /proc/pid/exe link: only local root should > + * change /proc/pid/exe link: only local sys admin should > * be allowed to. > */ > if (prctl_map->exe_fd != (u32)-1) { > - struct user_namespace *ns = current_user_ns(); > - const struct cred *cred = current_cred(); > - > - if (!uid_eq(cred->uid, make_kuid(ns, 0)) || > - !gid_eq(cred->gid, make_kgid(ns, 0))) > + if (!ns_capable(current_user_ns(), CAP_SYS_ADMIN)) > goto out; > } >
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