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userns: Avoid recursion in put_user_ns

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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>
This commit is contained in:
Eric W. Biederman 2012-12-28 18:58:39 -08:00
parent 923c753823
commit c61a2810a2
3 changed files with 15 additions and 16 deletions
Download patch file Download diff file Expand all files Collapse all files

17
kernel/user_namespace.c
View file

@ -78,7 +78,7 @@ int create_user_ns(struct cred *new)
return ret;
}
kref_init(&ns->kref);
atomic_set(&ns->count, 1);
/* Leave the new->user_ns reference with the new user namespace. */
ns->parent = parent_ns;
ns->owner = owner;
@ -104,15 +104,16 @@ int unshare_userns(unsigned long unshare_flags, struct cred **new_cred)
return create_user_ns(cred);
}
void free_user_ns(struct kref *kref)
void free_user_ns(struct user_namespace *ns)
{
struct user_namespace *parent, *ns =
container_of(kref, struct user_namespace, kref);
struct user_namespace *parent;
parent = ns->parent;
proc_free_inum(ns->proc_inum);
kmem_cache_free(user_ns_cachep, ns);
put_user_ns(parent);
do {
parent = ns->parent;
proc_free_inum(ns->proc_inum);
kmem_cache_free(user_ns_cachep, ns);
ns = parent;
} while (atomic_dec_and_test(&parent->count));
}
EXPORT_SYMBOL(free_user_ns);