Le 09/10/2018 à 19:01, Kirill Tkhai a écrit :
On 09.10.2018 19:45, Laurent Vivier wrote:
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Le 09/10/2018 à 18:15, Kirill Tkhai a écrit :
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On 09.10.2018 13:37, Laurent Vivier wrote:
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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
...
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@@ -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;
+}
+
...
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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.
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
Yes, I tried to read this document several times...
What I understand from example line 1077 (7696f9910a9a
Documentation/memory-barriers.txt) is we only need a data dependency
barrier, and as explained by Jann it comes with the READ_ONCE() (and is
only needed for alpha).
Thanks,
Laurent