diff options
| author | Eric W. Biederman <ebiederm@xmission.com> | 2012-12-28 18:58:39 -0800 |
|---|---|---|
| committer | Eric W. Biederman <ebiederm@xmission.com> | 2013-01-26 22:11:41 -0800 |
| commit | c61a2810a2161986353705b44d9503e6bb079f4f (patch) | |
| tree | 1f949f359d0344c2a47c8d0c597815aceb8091c0 /kernel/user.c | |
| parent | 923c7538236564c46ee80c253a416705321f13e3 (diff) | |
userns: Avoid recursion in put_user_ns
When freeing a deeply nested user namespace free_user_ns calls
put_user_ns on it's parent which may in turn call free_user_ns again.
When -fno-optimize-sibling-calls is passed to gcc one stack frame per
user namespace is left on the stack, potentially overflowing the
kernel stack. CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
so we can't count on gcc to optimize this code.
Remove struct kref and use a plain atomic_t. Making the code more
flexible and easier to comprehend. Make the loop in free_user_ns
explict to guarantee that the stack does not overflow with
CONFIG_FRAME_POINTER enabled.
I have tested this fix with a simple program that uses unshare to
create a deeply nested user namespace structure and then calls exit.
With 1000 nesteuser namespaces before this change running my test
program causes the kernel to die a horrible death. With 10,000,000
nested user namespaces after this change my test program runs to
completion and causes no harm.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Pointed-out-by: Vasily Kulikov <segoon@openwall.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Diffstat (limited to 'kernel/user.c')
| -rw-r--r-- | kernel/user.c | 4 |
1 files changed, 1 insertions, 3 deletions
diff --git a/kernel/user.c b/kernel/user.c index 33acb5e53a5..57ebfd42023 100644 --- a/kernel/user.c +++ b/kernel/user.c @@ -47,9 +47,7 @@ struct user_namespace init_user_ns = { .count = 4294967295U, }, }, - .kref = { - .refcount = ATOMIC_INIT(3), - }, + .count = ATOMIC_INIT(3), .owner = GLOBAL_ROOT_UID, .group = GLOBAL_ROOT_GID, .proc_inum = PROC_USER_INIT_INO, |