userns: Don't allow CLONE_NEWUSER | CLONE_FS
Don't allowing sharing the root directory with processes in a different user namespace. There doesn't seem to be any point, and to allow it would require the overhead of putting a user namespace reference in fs_struct (for permission checks) and incrementing that reference count on practically every call to fork. So just perform the inexpensive test of forbidding sharing fs_struct acrosss processes in different user namespaces. We already disallow other forms of threading when unsharing a user namespace so this should be no real burden in practice. This updates setns, clone, and unshare to disallow multiple user namespaces sharing an fs_struct. Cc: stable@vger.kernel.org Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1141,6 +1141,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
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if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
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return ERR_PTR(-EINVAL);
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if ((clone_flags & (CLONE_NEWUSER|CLONE_FS)) == (CLONE_NEWUSER|CLONE_FS))
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return ERR_PTR(-EINVAL);
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/*
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* Thread groups must share signals as well, and detached threads
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* can only be started up within the thread group.
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@ -1807,7 +1810,7 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags)
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* If unsharing a user namespace must also unshare the thread.
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*/
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if (unshare_flags & CLONE_NEWUSER)
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unshare_flags |= CLONE_THREAD;
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unshare_flags |= CLONE_THREAD | CLONE_FS;
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/*
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* If unsharing a pid namespace must also unshare the thread.
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*/
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@ -21,6 +21,7 @@
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#include <linux/uaccess.h>
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#include <linux/ctype.h>
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#include <linux/projid.h>
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#include <linux/fs_struct.h>
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static struct kmem_cache *user_ns_cachep __read_mostly;
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@ -837,6 +838,9 @@ static int userns_install(struct nsproxy *nsproxy, void *ns)
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if (atomic_read(¤t->mm->mm_users) > 1)
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return -EINVAL;
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if (current->fs->users != 1)
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return -EINVAL;
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if (!ns_capable(user_ns, CAP_SYS_ADMIN))
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return -EPERM;
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