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Date: Wed, 5 Jun 2013 17:49:51 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Luis Henriques <luis.henriques@canonical.com>
Cc: Willy Tarreau <w@1wt.eu>, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>,
        Colin King <colin.king@canonical.com>,
        Tim Gardner <tim.gardner@canonical.com>,
        John Johansen <john.johansen@canonical.com>
Subject: Re: [ 020/184] ptrace: ensure arch_ptrace/ptrace_request can never
Message-ID: <20130605154950.GA23647@redhat.com>
References: <20130604172131.136042108@1wt.eu> <87sj0wucft.fsf@canonical.com> <20130605154051.GA23025@redhat.com>
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On 06/05, Oleg Nesterov wrote:
>
> Note: I can make a _much_ simpler patch for 2.6.32, please let me know
> if you need it.
>
> We can rely on sys_ptrace()->lock_kernel() and simply do lock/unlock
> if fatal_signal_pending() in ptrace_stop/do_signal_stop. This is not
> the same, this doesn't prevent wakeup(), but this should be enough.

Something like below. Untested/uncompiled. I think it should close the
security problems.

Oleg.


--- x/kernel/signal.c
+++ x/kernel/signal.c
@@ -1545,6 +1545,14 @@ static int sigkill_pending(struct task_s
 		sigismember(&tsk->signal->shared_pending.signal, SIGKILL);
 }
 
+static void ptrace_sync(void)
+{
+	if (fatal_signal_pending(current)) {
+		lock_kernel();
+		unlock_kernel();
+	}
+}
+
 /*
  * This must be called with current->sighand->siglock held.
  *
@@ -1603,6 +1611,7 @@ static void ptrace_stop(int exit_code, i
 		read_unlock(&tasklist_lock);
 		preempt_enable_no_resched();
 		schedule();
+		ptrace_sync();
 	} else {
 		/*
 		 * By the time we got the lock, our tracer went away.
@@ -1722,6 +1731,9 @@ static int do_signal_stop(int signr)
 		schedule();
 	} while (try_to_freeze());
 
+	if (current->ptrace)
+		ptrace_sync();
+
 	tracehook_finish_jctl();
 	current->exit_code = 0;