108 lines
3.7 KiB
Diff
108 lines
3.7 KiB
Diff
commit c61a2810a2161986353705b44d9503e6bb079f4f
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Author: Eric W. Biederman <ebiederm@xmission.com>
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Date: Fri Dec 28 18:58:39 2012 -0800
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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
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put_user_ns on it's parent which may in turn call free_user_ns again.
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When -fno-optimize-sibling-calls is passed to gcc one stack frame per
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user namespace is left on the stack, potentially overflowing the
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kernel stack. CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
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so we can't count on gcc to optimize this code.
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Remove struct kref and use a plain atomic_t. Making the code more
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flexible and easier to comprehend. Make the loop in free_user_ns
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explict to guarantee that the stack does not overflow with
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CONFIG_FRAME_POINTER enabled.
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I have tested this fix with a simple program that uses unshare to
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create a deeply nested user namespace structure and then calls exit.
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With 1000 nesteuser namespaces before this change running my test
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program causes the kernel to die a horrible death. With 10,000,000
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nested user namespaces after this change my test program runs to
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completion and causes no harm.
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Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
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Pointed-out-by: Vasily Kulikov <segoon@openwall.com>
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Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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--- linux-3.7.9-105.fc17.noarch/include/linux/user_namespace.h 2013-02-14 11:29:49.757652513 -0600
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+++ linux-3.7.9-105.fc17.user_ns/include/linux/user_namespace.h 2013-02-26 15:19:40.696782035 -0600
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@@ -21,7 +21,7 @@ struct user_namespace {
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struct uid_gid_map uid_map;
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struct uid_gid_map gid_map;
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struct uid_gid_map projid_map;
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- struct kref kref;
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+ atomic_t count;
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struct user_namespace *parent;
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kuid_t owner;
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kgid_t group;
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@@ -34,17 +34,17 @@ extern struct user_namespace init_user_n
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static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
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{
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if (ns)
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- kref_get(&ns->kref);
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+ atomic_inc(&ns->count);
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return ns;
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}
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extern int create_user_ns(struct cred *new);
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-extern void free_user_ns(struct kref *kref);
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+extern void free_user_ns(struct user_namespace *ns);
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static inline void put_user_ns(struct user_namespace *ns)
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{
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- if (ns)
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- kref_put(&ns->kref, free_user_ns);
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+ if (ns && atomic_dec_and_test(&ns->count))
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+ free_user_ns(ns);
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}
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struct seq_operations;
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--- linux-3.7.9-105.fc17.noarch/kernel/user.c 2013-02-14 11:29:46.675652732 -0600
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+++ linux-3.7.9-105.fc17.user_ns/kernel/user.c 2013-02-26 15:16:12.347796824 -0600
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@@ -46,9 +46,7 @@ struct user_namespace init_user_ns = {
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.count = 4294967295U,
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},
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},
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- .kref = {
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- .refcount = ATOMIC_INIT(3),
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- },
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+ .count = ATOMIC_INIT(3),
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.owner = GLOBAL_ROOT_UID,
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.group = GLOBAL_ROOT_GID,
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};
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--- linux-3.7.9-105.fc17.noarch/kernel/user_namespace.c 2013-02-14 11:29:46.690652731 -0600
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+++ linux-3.7.9-105.fc17.user_ns/kernel/user_namespace.c 2013-02-26 15:24:47.984760224 -0600
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@@ -52,7 +52,7 @@ int create_user_ns(struct cred *new)
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if (!ns)
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return -ENOMEM;
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- kref_init(&ns->kref);
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+ atomic_set(&ns->count, 1);
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ns->parent = parent_ns;
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ns->owner = owner;
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ns->group = group;
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@@ -78,14 +78,15 @@ int create_user_ns(struct cred *new)
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return 0;
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}
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-void free_user_ns(struct kref *kref)
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+void free_user_ns(struct user_namespace *ns)
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{
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- struct user_namespace *parent, *ns =
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- container_of(kref, struct user_namespace, kref);
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+ struct user_namespace *parent;
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- parent = ns->parent;
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- kmem_cache_free(user_ns_cachep, ns);
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- put_user_ns(parent);
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+ do {
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+ parent = ns->parent;
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+ kmem_cache_free(user_ns_cachep, ns);
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+ ns = parent;
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+ } while (atomic_dec_and_test(&parent->count));
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}
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EXPORT_SYMBOL(free_user_ns);
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