[PATCH 16/17] unshare: don't drop active namespace references that were never taken

[Christian Brauner <brauner@kernel.org>]

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+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 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);

-- 
2.53.0