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.

Eric W. Biederman committed 13 years ago

Commit e66eded1 parent 6c23cbb2 files · +8 −1Browse files

Showing the changes to kernel (2 of 2 files). Show all

Unified · Split

2 files

kernel/fork.c +4 −1
kernel/user_namespace.c +4 −0

kernel/fork.c+4−1

@@ -1141,6 +1141,9 @@ static struct task_struct *copy_process(unsigned long clone_flags, · 1140 unmodified lines
1141	1141	if ((clone_flags & (CLONE_NEWNS\|CLONE_FS)) == (CLONE_NEWNS\|CLONE_FS))
1142	1142	return ERR_PTR(-EINVAL);
1143	1143
	1144	+ if ((clone_flags & (CLONE_NEWUSER\|CLONE_FS)) == (CLONE_NEWUSER\|CLONE_FS))
	1145	+ return ERR_PTR(-EINVAL);
	1146	+
1144	1147	/*
1145	1148	* Thread groups must share signals as well, and detached threads
1146	1149	* can only be started up within the thread group.
@@ -1807,7 +1810,7 @@ SYSCALL_DEFINE1(unshare, unsigned long, unshare_flags) · 660 unmodified lines
1807	1810	* If unsharing a user namespace must also unshare the thread.
1808	1811	*/
1809	1812	if (unshare_flags & CLONE_NEWUSER)
1810		− unshare_flags \|= CLONE_THREAD;
	1813	+ unshare_flags \|= CLONE_THREAD \| CLONE_FS;
1811	1814	/*
1812	1815	* If unsharing a pid namespace must also unshare the thread.
1813	1816	*/

kernel/user_namespace.c+4−0

@@ -21,6 +21,7 @@ · 20 unmodified lines
21	21	#include <linux/uaccess.h>
22	22	#include <linux/ctype.h>
23	23	#include <linux/projid.h>
	24	+#include <linux/fs_struct.h>
24	25
25	26	static struct kmem_cache *user_ns_cachep __read_mostly;
26	27
@@ -837,6 +838,9 @@ static int userns_install(struct nsproxy nsproxy, void ns) · 810 unmodified lines
837	838	if (atomic_read(&current->mm->mm_users) > 1)
838	839	return -EINVAL;
839	840
	841	+ if (current->fs->users != 1)
	842	+ return -EINVAL;
	843	+
840	844	if (!ns_capable(user_ns, CAP_SYS_ADMIN))
841	845	return -EPERM;
842	846

userns: Don't allow CLONE_NEWUSER | CLONE_FS

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