Re: [RFC v5 1/1] ns: add binfmt_misc to the user namespace

On 09.10.2018 19:45, Laurent Vivier wrote:

Le 09/10/2018 à 18:15, Kirill Tkhai a écrit :

quoted

On 09.10.2018 13:37, Laurent Vivier wrote:

quoted

This patch allows to have a different binfmt_misc configuration
for each new user namespace. By default, the binfmt_misc configuration
is the one of the previous level, but if the binfmt_misc filesystem is
mounted in the new namespace a new empty binfmt instance is created and
used in this namespace.

For instance, using "unshare" we can start a chroot of an another
architecture and configure the binfmt_misc interpreter without being root
to run the binaries in this chroot.

Signed-off-by: Laurent Vivier <redacted>
---
 fs/binfmt_misc.c               | 106 ++++++++++++++++++++++++---------
 include/linux/user_namespace.h |  13 ++++
 kernel/user.c                  |  13 ++++
 kernel/user_namespace.c        |   3 +
 4 files changed, 107 insertions(+), 28 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index aa4a7a23ff99..1e0029d097d9 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c

...

quoted

quoted

@@ -80,18 +74,32 @@ static int entry_count;
  */
 #define MAX_REGISTER_LENGTH 1920
 
+static struct binfmt_namespace *binfmt_ns(struct user_namespace *ns)
+{
+	struct binfmt_namespace *b_ns;
+
+	while (ns) {
+		b_ns = READ_ONCE(ns->binfmt_ns);
+		if (b_ns)
+			return b_ns;
+		ns = ns->parent;
+	}
+	WARN_ON_ONCE(1);
+	return NULL;
+}
+

...

quoted

quoted

@@ -823,12 +847,34 @@ static const struct super_operations s_ops = {
 static int bm_fill_super(struct super_block *sb, void *data, int silent)
 {
 	int err;
+	struct user_namespace *ns = sb->s_user_ns;
 	static const struct tree_descr bm_files[] = {
 		[2] = {"status", &bm_status_operations, S_IWUSR|S_IRUGO},
 		[3] = {"register", &bm_register_operations, S_IWUSR},
 		/* last one */ {""}
 	};
 
+	/* create a new binfmt namespace
+	 * if we are not in the first user namespace
+	 * but the binfmt namespace is the first one
+	 */
+	if (READ_ONCE(ns->binfmt_ns) == NULL) {
+		struct binfmt_namespace *new_ns;
+
+		new_ns = kmalloc(sizeof(struct binfmt_namespace),
+				 GFP_KERNEL);
+		if (new_ns == NULL)
+			return -ENOMEM;
+		INIT_LIST_HEAD(&new_ns->entries);
+		new_ns->enabled = 1;
+		rwlock_init(&new_ns->entries_lock);
+		new_ns->bm_mnt = NULL;
+		new_ns->entry_count = 0;
+		/* ensure new_ns is completely initialized before sharing it */
+		smp_wmb();

(I haven't dived into patch logic, here just small barrier remark from quick sight).
smp_wmb() has no sense without paired smp_rmb() on the read side. Possible,
you want something like below in read hunk:

+		b_ns = READ_ONCE(ns->binfmt_ns);
+		if (b_ns) {
+			smp_rmb();
+			return b_ns;
+		}

The write barrier is here to ensure the structure is fully written
before we set the pointer.

I don't understand how read barrier can change something at this level,
IMHO the couple WRITE_ONCE()/READ_ONCE() should be enough to ensure we
have correctly initialized the pointer and the structure when we read
the pointer back.

I think the pointer itself is the "barrier" to access the memory
modified before.

smp_rmb() guarantees you see stores in the order you want. If you have:

[cpu0]					[cpu1]
new_ns->entry_count = 0; 
smp_wmb();
WRITE_ONCE(ns->binfmt_ns, new_ns); 	b_ns = READ_ONCE(ns->binfmt_ns);
					smp_rmb();
					<access b_ns->entry_count>

smp_rmb() guarantees you see true entry_count on the cpu1. Without
smp_rmb() you may see old value of new_ns->entry_count.
					
See Documentation/memory-barriers.txt