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![Cyrill Gorcunov](/assets/img/avatar_default.png)
While doing the checkpoint-restore in the user space one need to determine whether various kernel objects (like mm_struct-s of file_struct-s) are shared between tasks and restore this state. The 2nd step can be solved by using appropriate CLONE_ flags and the unshare syscall, while there's currently no ways for solving the 1st one. One of the ways for checking whether two tasks share e.g. mm_struct is to provide some mm_struct ID of a task to its proc file, but showing such info considered to be not that good for security reasons. Thus after some debates we end up in conclusion that using that named 'comparison' syscall might be the best candidate. So here is it -- __NR_kcmp. It takes up to 5 arguments - the pids of the two tasks (which characteristics should be compared), the comparison type and (in case of comparison of files) two file descriptors. Lookups for pids are done in the caller's PID namespace only. At moment only x86 is supported and tested. [akpm@linux-foundation.org: fix up selftests, warnings] [akpm@linux-foundation.org: include errno.h] [akpm@linux-foundation.org: tweak comment text] Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org> Acked-by: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Pavel Emelyanov <xemul@parallels.com> Cc: Andrey Vagin <avagin@openvz.org> Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Glauber Costa <glommer@parallels.com> Cc: Andi Kleen <andi@firstfloor.org> Cc: Tejun Heo <tj@kernel.org> Cc: Matt Helsley <matthltc@us.ibm.com> Cc: Pekka Enberg <penberg@kernel.org> Cc: Eric Dumazet <eric.dumazet@gmail.com> Cc: Vasiliy Kulikov <segoon@openwall.com> Cc: Alexey Dobriyan <adobriyan@gmail.com> Cc: Valdis.Kletnieks@vt.edu Cc: Michal Marek <mmarek@suse.cz> Cc: Frederic Weisbecker <fweisbec@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
196 lines
4.3 KiB
C
196 lines
4.3 KiB
C
#include <linux/kernel.h>
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#include <linux/syscalls.h>
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#include <linux/fdtable.h>
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#include <linux/string.h>
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#include <linux/random.h>
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#include <linux/module.h>
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#include <linux/init.h>
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#include <linux/errno.h>
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#include <linux/cache.h>
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#include <linux/bug.h>
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#include <linux/err.h>
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#include <linux/kcmp.h>
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#include <asm/unistd.h>
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/*
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* We don't expose the real in-memory order of objects for security reasons.
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* But still the comparison results should be suitable for sorting. So we
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* obfuscate kernel pointers values and compare the production instead.
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*
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* The obfuscation is done in two steps. First we xor the kernel pointer with
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* a random value, which puts pointer into a new position in a reordered space.
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* Secondly we multiply the xor production with a large odd random number to
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* permute its bits even more (the odd multiplier guarantees that the product
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* is unique ever after the high bits are truncated, since any odd number is
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* relative prime to 2^n).
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*
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* Note also that the obfuscation itself is invisible to userspace and if needed
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* it can be changed to an alternate scheme.
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*/
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static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
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static long kptr_obfuscate(long v, int type)
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{
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return (v ^ cookies[type][0]) * cookies[type][1];
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}
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/*
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* 0 - equal, i.e. v1 = v2
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* 1 - less than, i.e. v1 < v2
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* 2 - greater than, i.e. v1 > v2
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* 3 - not equal but ordering unavailable (reserved for future)
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*/
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static int kcmp_ptr(void *v1, void *v2, enum kcmp_type type)
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{
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long ret;
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ret = kptr_obfuscate((long)v1, type) - kptr_obfuscate((long)v2, type);
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return (ret < 0) | ((ret > 0) << 1);
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}
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/* The caller must have pinned the task */
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static struct file *
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get_file_raw_ptr(struct task_struct *task, unsigned int idx)
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{
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struct file *file = NULL;
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task_lock(task);
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rcu_read_lock();
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if (task->files)
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file = fcheck_files(task->files, idx);
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rcu_read_unlock();
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task_unlock(task);
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return file;
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}
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static void kcmp_unlock(struct mutex *m1, struct mutex *m2)
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{
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if (likely(m2 != m1))
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mutex_unlock(m2);
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mutex_unlock(m1);
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}
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static int kcmp_lock(struct mutex *m1, struct mutex *m2)
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{
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int err;
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if (m2 > m1)
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swap(m1, m2);
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err = mutex_lock_killable(m1);
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if (!err && likely(m1 != m2)) {
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err = mutex_lock_killable_nested(m2, SINGLE_DEPTH_NESTING);
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if (err)
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mutex_unlock(m1);
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}
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return err;
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}
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SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
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unsigned long, idx1, unsigned long, idx2)
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{
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struct task_struct *task1, *task2;
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int ret;
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rcu_read_lock();
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/*
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* Tasks are looked up in caller's PID namespace only.
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*/
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task1 = find_task_by_vpid(pid1);
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task2 = find_task_by_vpid(pid2);
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if (!task1 || !task2)
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goto err_no_task;
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get_task_struct(task1);
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get_task_struct(task2);
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rcu_read_unlock();
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/*
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* One should have enough rights to inspect task details.
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*/
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ret = kcmp_lock(&task1->signal->cred_guard_mutex,
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&task2->signal->cred_guard_mutex);
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if (ret)
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goto err;
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if (!ptrace_may_access(task1, PTRACE_MODE_READ) ||
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!ptrace_may_access(task2, PTRACE_MODE_READ)) {
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ret = -EPERM;
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goto err_unlock;
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}
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switch (type) {
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case KCMP_FILE: {
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struct file *filp1, *filp2;
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filp1 = get_file_raw_ptr(task1, idx1);
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filp2 = get_file_raw_ptr(task2, idx2);
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if (filp1 && filp2)
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ret = kcmp_ptr(filp1, filp2, KCMP_FILE);
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else
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ret = -EBADF;
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break;
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}
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case KCMP_VM:
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ret = kcmp_ptr(task1->mm, task2->mm, KCMP_VM);
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break;
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case KCMP_FILES:
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ret = kcmp_ptr(task1->files, task2->files, KCMP_FILES);
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break;
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case KCMP_FS:
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ret = kcmp_ptr(task1->fs, task2->fs, KCMP_FS);
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break;
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case KCMP_SIGHAND:
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ret = kcmp_ptr(task1->sighand, task2->sighand, KCMP_SIGHAND);
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break;
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case KCMP_IO:
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ret = kcmp_ptr(task1->io_context, task2->io_context, KCMP_IO);
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break;
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case KCMP_SYSVSEM:
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#ifdef CONFIG_SYSVIPC
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ret = kcmp_ptr(task1->sysvsem.undo_list,
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task2->sysvsem.undo_list,
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KCMP_SYSVSEM);
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#else
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ret = -EOPNOTSUPP;
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#endif
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break;
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default:
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ret = -EINVAL;
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break;
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}
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err_unlock:
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kcmp_unlock(&task1->signal->cred_guard_mutex,
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&task2->signal->cred_guard_mutex);
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err:
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put_task_struct(task1);
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put_task_struct(task2);
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return ret;
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err_no_task:
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rcu_read_unlock();
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return -ESRCH;
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}
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static __init int kcmp_cookies_init(void)
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{
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int i;
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get_random_bytes(cookies, sizeof(cookies));
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for (i = 0; i < KCMP_TYPES; i++)
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cookies[i][1] |= (~(~0UL >> 1) | 1);
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return 0;
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}
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arch_initcall(kcmp_cookies_init);
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