[PATCH v3 2/5] userns: don't let setns() lend CAP_SETFCAP
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From: Josef Bacik <josef@toxicpanda.com>
Date: 2026-10-21 20:37:44
Also in:
keyrings, linux-fsdevel, lkml
Subsystem:
capabilities, security subsystem, the rest · Maintainers:
Serge Hallyn, Paul Moore, James Morris, Linus Torvalds
Revision v3 of 4 in this series.
Revisions (4)
- v1 [diff vs current]
- v2 [diff vs current]
- v3 current
- v4 [diff vs current]
Mapping uid 0 of the parent into a user namespace requires CAP_SETFCAP,
because a task in such a namespace can write file capabilities that are
honoured in the parent. A task that is already in the namespace has a
full capability set there and can't be asked any more, so
verify_root_map() goes by what the creator had, now ns->setfcap_level.
But setns() lets other tasks in, and they get the full set as well:
task A: uid 0, full caps task B: uid 0, no CAP_SETFCAP
unshare(CLONE_NEWUSER)
setns(A's user ns)
allowed, euid == ns->owner
write "0 0 1" to /proc/self/uid_map
ns->setfcap_level is A's -> allowed
or, if A's namespace maps uid 0 already, B sets security.capability on a
file. Either way B ends up with file capabilities that are honoured in
the initial namespace, which it could not have written there, nor in a
namespace it created itself.
Refuse setns() into a user namespace whose tasks can write file
capabilities for the root user of an ancestor that the joiner's
CAP_SETFCAP doesn't reach. That is the case when the namespace maps
such a root user, or has no uid map yet and its parent maps one.
cred_may_setfcap_userns() checks it, and cred_setfcap_reaches() is the
comparison it and the later checks use. Then every task in such a
namespace is entitled to what its setfcap_level says, and the checks on
the way to file capabilities can go by the namespace.
An idmapped mount can store a root user that the namespace doesn't map,
for example a "0 100000" container whose rootfs maps 100000 to host uid
0. That is a deliberate setup and up to whoever creates the mount, so
it is not considered here.
The change in behaviour is that a task without CAP_SETFCAP gets -EPERM
when it tries to setns() into a user namespace that maps the root user
of an ancestor, and was created by a task whose CAP_SETFCAP reached that
far. Containers that don't map host uid 0, tasks with CAP_SETFCAP, and
unprivileged users joining their own namespaces are not affected.
Link: https://lore.kernel.org/all/20261021-bioenergie-allheilmittel-undurchdringlichen-35666eb62cff@brauner/ (local)
Fixes: db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
include/linux/capability.h | 3 +++
kernel/user_namespace.c | 6 +++++
security/commoncap.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 67 insertions(+)
diff --git a/include/linux/capability.h b/include/linux/capability.h
index 026974a5111b..e4199b03668b 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h@@ -224,5 +224,8 @@ int cap_convert_nscap(const struct mnt_idmap *idmap, struct dentry *dentry, const void **ivalue, size_t size); int cap_setfcap_level(const struct cred *cred, struct user_namespace *ns); +bool cred_setfcap_reaches(const struct cred *cred, struct user_namespace *ns, + int level); +bool cred_may_setfcap_userns(const struct cred *cred, struct user_namespace *ns); #endif /* !_LINUX_CAPABILITY_H */
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index a5a94367600d..7bd3692c2f3e 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c@@ -1390,6 +1390,12 @@ static int userns_install(struct nsset *nsset, struct ns_common *ns) if (!ns_capable(user_ns, CAP_SYS_ADMIN)) return -EPERM; + /* Tasks in the namespace may have CAP_SETFCAP that counts outside of + * it, so only let in tasks whose own counts as far. + */ + if (!cred_may_setfcap_userns(current_cred(), user_ns)) + return -EPERM; + cred = nsset_cred(nsset); if (!cred) return -EINVAL;
diff --git a/security/commoncap.c b/security/commoncap.c
index 4f49dba65426..8a9d998b1897 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c@@ -158,6 +158,64 @@ int cap_setfcap_level(const struct cred *cred, struct user_namespace *ns) return cred->user_ns->level + 1; } +/** + * cred_setfcap_reaches - Check how far up CAP_SETFCAP of a cred reaches + * @cred: The credentials to use + * @ns: The user namespace to look from + * @level: The ->level of the ancestor of @ns it has to count in + * + * Return: true if CAP_SETFCAP of @cred counts in @ns and in its ancestors + * up to the one at @level. + */ +bool cred_setfcap_reaches(const struct cred *cred, struct user_namespace *ns, + int level) +{ + return cap_setfcap_level(cred, ns) <= level; +} + +/* + * The topmost ancestor of @ns whose root user tasks in @ns can write file + * capabilities for: within @ns->setfcap_level, and mapped into @ns, or into + * its parent while @ns has no uid_map yet. NULL if there is none. An + * idmapped mount can store other root users, but that is up to whoever sets + * up the mount. + */ +static struct user_namespace *setfcap_top(struct user_namespace *ns) +{ + struct user_namespace *seen, *p, *top = NULL; + + if (ns->setfcap_level >= ns->level) + return NULL; + + seen = READ_ONCE(ns->uid_map.nr_extents) ? ns : ns->parent; + for (p = ns->parent; p; p = p->parent) { + if (p->level < ns->setfcap_level) + break; + if (kuid_has_mapping(seen, make_kuid(p, 0))) + top = p; + } + + return top; +} + +/** + * cred_may_setfcap_userns - Check whether a cred covers what a userns can do + * @cred: The credentials to use + * @ns: The user namespace + * + * Tasks in @ns can have CAP_SETFCAP that counts in ancestors of @ns, see + * cap_setfcap_level(). Check that CAP_SETFCAP of @cred reaches as far up as + * tasks in @ns can make use of theirs. + * + * Return: true if it does. + */ +bool cred_may_setfcap_userns(const struct cred *cred, struct user_namespace *ns) +{ + struct user_namespace *top = setfcap_top(ns); + + return !top || cred_setfcap_reaches(cred, ns->parent, top->level); +} + /** * cap_settime - Determine whether the current process may set the system clock * @ts: The time to set
--
2.55.0