From 0b6f31a3c0b16066f812e94b52a704174a6f23cd Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Wed, 30 Sep 2026 15:32:08 +0200 Subject: [PATCH] unshare: don't drop active namespace references that were never taken Active references on the namespaces of an nsproxy are taken when the nsproxy is installed into a task in switch_task_namespaces() and copy_namespaces() and dropped again by put_nsproxy() through deactivate_nsproxy(). But ksys_unshare() calls put_nsproxy() on an nsproxy that was never installed when set_cred_ucounts() fails. The new namespaces of that nsproxy go from zero to minus one and the namespaces shared with the caller lose a reference that belongs to the nsproxy the caller keeps using: WARNING: kernel/nscommon.c:171 at __ns_ref_active_put+0x1cd/0x230 nsproxy_ns_active_put deactivate_nsproxy ksys_unshare Afterwards the namespaces the caller lives in aren't listed by listns() anymore. set_cred_ucounts() only fails when alloc_ucounts() can't allocate and it only allocates when the real uid of the caller differs from its effective uid. Commit cefd55bd2159 ("nsproxy: fix free_nsproxy() and simplify create_new_namespaces()") separated the two cases on purpose. nsproxy_free() frees an nsproxy that was prepared but never installed and that's what a failed setns() uses in put_nsset(). Export it and use it for a failed unshare() as well. Fixes: a98621a0f187 ("unshare: fix nsproxy leak in ksys_unshare() on set_cred_ucounts() failure") Cc: stable@vger.kernel.org # v7.1+ Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-16-be34c83956ae@kernel.org Signed-off-by: Christian Brauner (Amutable) --- include/linux/nsproxy.h | 1 + kernel/fork.c | 3 ++- kernel/nsproxy.c | 2 +- 3 files changed, 4 insertions(+), 2 deletions(-) diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h index 5a67648721c7..dc2447f5f092 100644 --- a/include/linux/nsproxy.h +++ b/include/linux/nsproxy.h @@ -100,6 +100,7 @@ void exit_cred_namespaces(struct task_struct *tsk); void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new); int exec_task_namespaces(void); void deactivate_nsproxy(struct nsproxy *ns); +void nsproxy_free(struct nsproxy *ns); int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **, struct cred *, struct fs_struct *); int __init nsproxy_cache_init(void); diff --git a/kernel/fork.c b/kernel/fork.c index da48168c504f..d442cd68a37a 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -3338,8 +3338,9 @@ int ksys_unshare(unsigned long unshare_flags) perf_event_namespaces(current); bad_unshare_cleanup_nsproxy: + /* never installed, so no active references to drop */ if (new_nsproxy) - put_nsproxy(new_nsproxy); + nsproxy_free(new_nsproxy); bad_unshare_cleanup_cred: if (new_cred) put_cred(new_cred); diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c index d9d3d5973bf5..3fb1595a4a59 100644 --- a/kernel/nsproxy.c +++ b/kernel/nsproxy.c @@ -61,7 +61,7 @@ static inline struct nsproxy *create_nsproxy(void) return nsproxy; } -static inline void nsproxy_free(struct nsproxy *ns) +void nsproxy_free(struct nsproxy *ns) { put_mnt_ns(ns->mnt_ns); put_uts_ns(ns->uts_ns);