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Thu, 22 Oct 2026 11:36:40 -0700 (PDT) Received: from toxicpanda.com ([153.61.196.244]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-536ab6f5c9csm33898881cf.11.2026.10.22.11.36.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Oct 2026 11:36:39 -0700 (PDT) From: Josef Bacik Date: Thu, 22 Oct 2026 18:36:20 +0000 Subject: [PATCH v4 2/5] userns: don't let setns() lend CAP_SETFCAP X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261022-b4-setfcap-userns-v4-2-88aa0ffaa08f@toxicpanda.com> References: <20261022-b4-setfcap-userns-v4-0-88aa0ffaa08f@toxicpanda.com> In-Reply-To: <20261022-b4-setfcap-userns-v4-0-88aa0ffaa08f@toxicpanda.com> To: Serge Hallyn , Paul Moore , Christian Brauner Cc: James Morris , David Howells , Jarkko Sakkinen , "Andrew G. Morgan" , Serge Hallyn , linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, keyrings@vger.kernel.org, linux-fsdevel@vger.kernel.org, Josef Bacik X-Mailer: b4 0.15.2 X-Developer-Signature: v=1; a=openssh-sha256; t=1792694192; l=6171; i=josef@toxicpanda.com; h=from:subject:message-id; bh=n4GgDASw93W0W4hVR9BfgOUvZQHADy8KOEbGO3+EfSU=; b=U1NIU0lHAAAAAQAAADMAAAALc3NoLWVkMjU1MTkAAAAgUBr36M/n0nWN0DNbnxwzIiCZez6MG JiruuNaSCI/zXsAAAAGcGF0YXR0AAAAAAAAAAZzaGE1MTIAAABTAAAAC3NzaC1lZDI1NTE5AAAA QM/ahjV20208az5IZRy82VEqyZ0IuWu0b9yg2I1rzhGFwGftatKCP3w0WLLSTlI160zk3va7lOg pPLWLMlVn9Aw= X-Developer-Key: i=josef@toxicpanda.com; a=openssh; fpr=SHA256:C8kOX2QUJCMqnCX+KEeoqRAjLo9L+ELOSH2NSAJHqGA 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/ Fixes: db2e718a4798 ("capabilities: require CAP_SETFCAP to map uid 0") Assisted-by: LLM Signed-off-by: Josef Bacik --- 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