CHERRY-PICK: UPSTREAM: ptrace: ensure arch_ptrace/ptrace_request can never race with...

putreg() assumes that the tracee is not running and pt_regs_access() can
safely play with its stack.  However a killed tracee can return from
ptrace_stop() to the low-level asm code and do RESTORE_REST, this means
that debugger can actually read/modify the kernel stack until the tracee
does SAVE_REST again.

set_task_blockstep() can race with SIGKILL too and in some sense this
race is even worse, the very fact the tracee can be woken up breaks the
logic.

As Linus suggested we can clear TASK_WAKEKILL around the arch_ptrace()
call, this ensures that nobody can ever wakeup the tracee while the
debugger looks at it.  Not only this fixes the mentioned problems, we
can do some cleanups/simplifications in arch_ptrace() paths.

Probably ptrace_unfreeze_traced() needs more callers, for example it
makes sense to make the tracee killable for oom-killer before
access_process_vm().

While at it, add the comment into may_ptrace_stop() to explain why
ptrace_stop() still can't rely on SIGKILL and signal_pending_state().

Reported-by: Salman Qazi <sqazi@google.com>
Reported-by: Suleiman Souhlal <suleiman@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
BUG=chromium-os:38610
TEST=link build

(cherry picked from upstream commit 9899d11f654474d2d54ea52ceaa2a1f4db3abd68)
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43144
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Change-Id: Id8100beee1789f5e678adc59c33e5935df31fba4
(cherry picked from ToT commit 10b55a2686c6c3dabf57d8f21e595422eb4ad391)
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43663
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 4c47fda2aa19aa37a4159abfbb8e6b0c1299006b..615a168ebdde410a43ae1cdbfb530adc0d3a5e26 100644 (file)
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -122,6 +122,40 @@ void __ptrace_unlink(struct task_struct *child)
        spin_unlock(&child->sighand->siglock);
 }
 
+/* Ensure that nothing can wake it up, even SIGKILL */
+static bool ptrace_freeze_traced(struct task_struct *task)
+{
+       bool ret = false;
+
+       /* Lockless, nobody but us can set this flag */
+       if (task->jobctl & JOBCTL_LISTENING)
+               return ret;
+
+       spin_lock_irq(&task->sighand->siglock);
+       if (task_is_traced(task) && !__fatal_signal_pending(task)) {
+               task->state = __TASK_TRACED;
+               ret = true;
+       }
+       spin_unlock_irq(&task->sighand->siglock);
+
+       return ret;
+}
+
+static void ptrace_unfreeze_traced(struct task_struct *task)
+{
+       if (task->state != __TASK_TRACED)
+               return;
+
+       WARN_ON(!task->ptrace || task->parent != current);
+
+       spin_lock_irq(&task->sighand->siglock);
+       if (__fatal_signal_pending(task))
+               wake_up_state(task, __TASK_TRACED);
+       else
+               task->state = TASK_TRACED;
+       spin_unlock_irq(&task->sighand->siglock);
+}
+
 /**
  * ptrace_check_attach - check whether ptracee is ready for ptrace operation
  * @child: ptracee to check for
@@ -151,24 +185,29 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state)
         * be changed by us so it's not changing right after this.
         */
        read_lock(&tasklist_lock);
-       if ((child->ptrace & PT_PTRACED) && child->parent == current) {
+       if (child->ptrace && child->parent == current) {
+               WARN_ON(child->state == __TASK_TRACED);
                /*
                 * child->sighand can't be NULL, release_task()
                 * does ptrace_unlink() before __exit_signal().
                 */
-               spin_lock_irq(&child->sighand->siglock);
-               WARN_ON_ONCE(task_is_stopped(child));
-               if (ignore_state || (task_is_traced(child) &&
-                                    !(child->jobctl & JOBCTL_LISTENING)))
+               if (ignore_state || ptrace_freeze_traced(child))
                        ret = 0;
-               spin_unlock_irq(&child->sighand->siglock);
        }
        read_unlock(&tasklist_lock);
 
-       if (!ret && !ignore_state)
-               ret = wait_task_inactive(child, TASK_TRACED) ? 0 : -ESRCH;
+       if (!ret && !ignore_state) {
+               if (!wait_task_inactive(child, __TASK_TRACED)) {
+                       /*
+                        * This can only happen if may_ptrace_stop() fails and
+                        * ptrace_stop() changes ->state back to TASK_RUNNING,
+                        * so we should not worry about leaking __TASK_TRACED.
+                        */
+                       WARN_ON(child->state == __TASK_TRACED);
+                       ret = -ESRCH;
+               }
+       }
 
-       /* All systems go.. */
        return ret;
 }
 
@@ -894,6 +933,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
                goto out_put_task_struct;
 
        ret = arch_ptrace(child, request, addr, data);
+       if (ret || request != PTRACE_DETACH)
+               ptrace_unfreeze_traced(child);
 
  out_put_task_struct:
        put_task_struct(child);
@@ -1033,8 +1074,11 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
 
        ret = ptrace_check_attach(child, request == PTRACE_KILL ||
                                  request == PTRACE_INTERRUPT);
-       if (!ret)
+       if (!ret) {
                ret = compat_arch_ptrace(child, request, addr, data);
+               if (ret || request != PTRACE_DETACH)
+                       ptrace_unfreeze_traced(child);
+       }
 
  out_put_task_struct:
        put_task_struct(child);

diff --git a/kernel/signal.c b/kernel/signal.c
index d48f457bb194a657d246ad869e392de8104d828e..89684c16e7f83c6f4c9107aacb62f56a6009d38f 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1802,6 +1802,10 @@ static inline int may_ptrace_stop(void)
         * If SIGKILL was already sent before the caller unlocked
         * ->siglock we must see ->core_state != NULL. Otherwise it
         * is safe to enter schedule().
+        *
+        * This is almost outdated, a task with the pending SIGKILL can't
+        * block in TASK_TRACED. But PTRACE_EVENT_EXIT can be reported
+        * after SIGKILL was already dequeued.
         */
        if (unlikely(current->mm->core_state) &&
            unlikely(current->mm == current->parent->mm))
@@ -1927,6 +1931,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
                if (gstop_done)
                        do_notify_parent_cldstop(current, false, why);
 
+               /* tasklist protects us from ptrace_freeze_traced() */
                __set_current_state(TASK_RUNNING);
                if (clear_code)
                        current->exit_code = 0;