[PATCH] user_ns: use correct check for single-threadedness

[Kees Cook <keescook@chromium.org>]

From: Ricky Zhou <rickyz@chromium.org>

Checking mm_users > 1 does not mean a process is multithreaded. For
example, reading /proc/PID/maps temporarily increments mm_users, allowing
other processes to (accidentally) interfere with unshare() calls.

This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
returning EINVAL if another processes happened to be simultaneously
reading the maps file.

Signed-off-by: Ricky Zhou <rickyz@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
---
 kernel/fork.c           | 4 ++--
 kernel/user_namespace.c | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index dbd9b8d7b7cc..7d138f152dcd 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -75,6 +75,7 @@
 #include <linux/aio.h>
 #include <linux/compiler.h>
 #include <linux/sysctl.h>
+#include <linux/sched.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -1876,8 +1877,7 @@ static int check_unshare_flags(unsigned long unshare_flags)
 	 * needs to unshare vm.
 	 */
 	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
-		/* FIXME: get_task_mm() increments ->mm_users */
-		if (atomic_read(&current->mm->mm_users) > 1)
+		if (!current_is_single_threaded())
 			return -EINVAL;
 	}
 
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 4109f8320684..7e0e021e4304 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -22,6 +22,7 @@
 #include <linux/ctype.h>
 #include <linux/projid.h>
 #include <linux/fs_struct.h>
+#include <linux/sched.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 static DEFINE_MUTEX(userns_state_mutex);
@@ -977,7 +978,7 @@ static int userns_install(struct nsproxy *nsproxy, struct ns_common *ns)
 		return -EINVAL;
 
 	/* Threaded processes may not enter a different user namespace */
-	if (atomic_read(&current->mm->mm_users) > 1)
+	if (!current_is_single_threaded())
 		return -EINVAL;
 
 	if (current->fs->users != 1)
-- 
1.9.1


-- 
Kees Cook
Chrome OS Security