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Message-ID: <CALCETrW6FiQP0KKYVSRV=zBY9PcU3xx_qnFW9ead2kYscB4Hvg@mail.gmail.com>
Date: Sat, 29 Nov 2014 08:24:28 -0800
From: Andy Lutomirski <luto@...capital.net>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Linux Containers <containers@...ts.linux-foundation.org>,
Josh Triplett <josh@...htriplett.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook <keescook@...omium.org>,
Michael Kerrisk-manpages <mtk.manpages@...il.com>,
Linux API <linux-api@...r.kernel.org>,
linux-man <linux-man@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
LSM <linux-security-module@...r.kernel.org>,
Casey Schaufler <casey@...aufler-ca.com>,
"Serge E. Hallyn" <serge@...lyn.com>,
Richard Weinberger <richard@....at>,
Kenton Varda <kenton@...dstorm.io>,
stable <stable@...r.kernel.org>
Subject: Re: [RFC PATCH] userns: Disallow setgroups unless the gid_map writer
is privileged
On Sat, Nov 29, 2014 at 8:16 AM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
> The patch is buggy.
>
> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
>> ---
>>
>> Eric, this is an alternative to your patch. I think it will cause
>> less breakage, and it will keep unprivileged user namespaces
>> more or less fully functional.
>>
>> Kenton, I think that neither run-bundle nor supervisor-main will be
>> broken by this patch.
>>
>> include/linux/user_namespace.h | 3 +++
>> kernel/groups.c | 3 +++
>> kernel/user.c | 1 +
>> kernel/user_namespace.c | 36 ++++++++++++++++++++++++++++++++++++
>> 4 files changed, 43 insertions(+)
>>
>> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
>> index e95372654f09..a74c1f3d44fe 100644
>> --- a/include/linux/user_namespace.h
>> +++ b/include/linux/user_namespace.h
>> @@ -17,6 +17,8 @@ struct uid_gid_map { /* 64 bytes -- 1 cache line */
>> } extent[UID_GID_MAP_MAX_EXTENTS];
>> };
>>
>> +#define USERNS_SETGROUPS_ALLOWED 1
>> +
>> struct user_namespace {
>> struct uid_gid_map uid_map;
>> struct uid_gid_map gid_map;
>> @@ -27,6 +29,7 @@ struct user_namespace {
>> kuid_t owner;
>> kgid_t group;
>> unsigned int proc_inum;
>> + unsigned int flags;
>
> If you are going to add a flags field it needs to be atomic as otherwise
> changing or reading individual flags won't be safe without a lock.
Will fix.
>
>> /* Register of per-UID persistent keyrings for this namespace */
>> #ifdef CONFIG_PERSISTENT_KEYRINGS
>> diff --git a/kernel/groups.c b/kernel/groups.c
>> index 451698f86cfa..e27433809978 100644
>> --- a/kernel/groups.c
>> +++ b/kernel/groups.c
>> @@ -6,6 +6,7 @@
>> #include <linux/slab.h>
>> #include <linux/security.h>
>> #include <linux/syscalls.h>
>> +#include <linux/user_namespace.h>
>> #include <asm/uaccess.h>
>>
>> /* init to 2 - one for init_task, one to ensure it is never freed */
>> @@ -223,6 +224,8 @@ SYSCALL_DEFINE2(setgroups, int, gidsetsize, gid_t __user *, grouplist)
>> struct group_info *group_info;
>> int retval;
>>
>> + if (!(current_user_ns()->flags & USERNS_SETGROUPS_ALLOWED))
>> + return -EPERM;
>> if (!ns_capable(current_user_ns(), CAP_SETGID))
>> return -EPERM;
>> if ((unsigned)gidsetsize > NGROUPS_MAX)
>> diff --git a/kernel/user.c b/kernel/user.c
>> index 4efa39350e44..f8cdb1ec6049 100644
>> --- a/kernel/user.c
>> +++ b/kernel/user.c
>> @@ -51,6 +51,7 @@ struct user_namespace init_user_ns = {
>> .owner = GLOBAL_ROOT_UID,
>> .group = GLOBAL_ROOT_GID,
>> .proc_inum = PROC_USER_INIT_INO,
>> + .flags = USERNS_SETGROUPS_ALLOWED,
>> #ifdef CONFIG_PERSISTENT_KEYRINGS
>> .persistent_keyring_register_sem =
>> __RWSEM_INITIALIZER(init_user_ns.persistent_keyring_register_sem),
>> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
>> index aa312b0dc3ec..6e7b9ee5bddc 100644
>> --- a/kernel/user_namespace.c
>> +++ b/kernel/user_namespace.c
>> @@ -600,6 +600,8 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>> unsigned long page = 0;
>> char *kbuf, *pos, *next_line;
>> ssize_t ret = -EINVAL;
>> + unsigned int gid_flags = 0;
>> + bool seen_explicit_gid_flag = false;
>>
>> /*
>> * The id_map_mutex serializes all writes to any given map.
>> @@ -633,6 +635,19 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>> if (cap_valid(cap_setid) && !file_ns_capable(file, ns, CAP_SYS_ADMIN))
>> goto out;
>>
>> + /* Deal with supplementary groups. */
>> + if (map == &ns->gid_map) {
>> + /*
>> + * By default, setgroups is allowed inside the userns
>> + * if the writer has no supplementary groups (making
>> + * it useless) or if the writer is privileged.
>> + */
>> + if ((ns->parent->flags & USERNS_SETGROUPS_ALLOWED) &&
>> + file_ns_capable(file, ns->parent, CAP_SETGID) &&
>> + ns_capable(ns->parent, CAP_SETGID))
>> + gid_flags = USERNS_SETGROUPS_ALLOWED;
>
> We can't do this.
>
> It is wrong to mix permissions and flags to request functionality.
>
> That way lies madness, and impossible maintenance, and it will silently
> break every application that expects setgroups to work if they have
> CAP_SETGID after a mapping has been established.
We can make writing gid_map fail unless you either have permissions or
add the "setgroups disallow" option. Would that be better? It avoids
silent creation of a strangely behaving userns. This would be an easy
adjustment to this patch.
I don't like the "allow setgroups if there are no supplementary
groups" because it's weird and unexpected (and will therefore break
things, I expect) and because it may interact insecurely with setns
and other such things.
--Andy
>
>> + }
>> +
>> /* Get a buffer */
>> ret = -ENOMEM;
>> page = __get_free_page(GFP_TEMPORARY);
>> @@ -667,6 +682,25 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>> next_line = NULL;
>> }
>>
>> + /* Is this line a gid_map option? */
>> + if (map == &ns->gid_map) {
>> + if (!strcmp(pos, "setgroups deny")) {
>> + if (seen_explicit_gid_flag)
>> + goto out;
>> + seen_explicit_gid_flag = 1;
>> + gid_flags = 0;
>> + continue;
>> + } else if (!strcmp(pos, "setgroups allow")) {
>> + if (seen_explicit_gid_flag)
>> + goto out;
>> + if (!(gid_flags & USERNS_SETGROUPS_ALLOWED)) {
>> + ret = -EPERM;
>> + goto out;
>> + }
>> + continue;
>> + }
>> + }
>> +
>> pos = skip_spaces(pos);
>> extent->first = simple_strtoul(pos, &pos, 10);
>> if (!isspace(*pos))
>> @@ -746,6 +780,8 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>> new_map.nr_extents*sizeof(new_map.extent[0]));
>> smp_wmb();
>> map->nr_extents = new_map.nr_extents;
>> + if (map == &ns->gid_map)
>> + ns->flags |= gid_flags;
>>
>> *ppos = count;
>> ret = count;
>
> Eric
--
Andy Lutomirski
AMA Capital Management, LLC
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