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

Le 09/10/2018 à 18:53, Jann Horn a écrit :

On Tue, Oct 9, 2018 at 6:45 PM Laurent Vivier [off-list ref] wrote:

quoted

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

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@@ -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.

Things don't work that way on alpha, but that's why READ_ONCE()
includes an smp_read_barrier_depends():

#define __READ_ONCE(x, check)                                           \
({                                                                      \
        union { typeof(x) __val; char __c[1]; } __u;                    \
        if (check)                                                      \
                __read_once_size(&(x), __u.__c, sizeof(x));             \
        else                                                            \
                __read_once_size_nocheck(&(x), __u.__c, sizeof(x));     \
        smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
        __u.__val;                                                      \
})
#define READ_ONCE(x) __READ_ONCE(x, 1)

So my questions are:

- do we need a smp_wmb() barrier if we use READ_ONCE() and WRITE_ONCE()?

- if we need an smp_wmb() barrier, do we need an smp_rmb() barrier as
the data we want to "protect" are behind an access to the pointer?

Thanks,
Laurent