linux-kernel - [PATCH] ptrace: clear TIF_SYSCALL

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Date:   Thu,  1 Sep 2016 21:51:33 +0200
From:   Ales Novak <alnovak@...e.cz>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>, Ales Novak <alnovak@...e.cz>
Subject: [PATCH] ptrace: clear TIF_SYSCALL_TRACE on ptrace detach

On __ptrace_detach(), called from do_exit()->exit_notify()->
forget_original_parent()->exit_ptrace(), the TIF_SYSCALL_TRACE in
thread->flags of the tracee is not cleared up. This results in the
tracehook_report_syscall_* being called (though there's no longer a
tracer listening to that) upon its further syscalls.

Example scenario - attach "strace" to a running process and kill it (the
strace) with SIGKILL. You'll see that the syscall trace hooks are still
being called.

The clearing of this flag should be moved from ptrace_detach() to
__ptrace_detach().

Signed-off-by: Ales Novak <alnovak@...e.cz>
Cc: Jiri Kosina <jkosina@...e.cz>
---
 kernel/ptrace.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 1d3b766..2a99027 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -73,6 +73,8 @@ void __ptrace_unlink(struct task_struct *child)
 {
 	BUG_ON(!child->ptrace);

+	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+
 	child->parent = child->real_parent;
 	list_del_init(&child->ptrace_entry);

@@ -489,7 +491,6 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)

 	/* Architecture-specific hardware disable .. */
 	ptrace_disable(child);
-	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);

 	write_lock_irq(&tasklist_lock);
 	/*
-- 
2.7.0