From: Eric W. Biederman <ebiederm@xmission.com>
Date: Mon, 22 May 2017 20:40:12 +0000 (-0500)
Subject: ptrace: Properly initialize ptracer_cred on fork
X-Git-Tag: Ubuntu-snapdragon-4.4.0-1064.69~763
X-Git-Url: https://git.proxmox.com/?a=commitdiff_plain;h=0394889d585fe142bf7730f69819145cef9ac633;p=mirror_ubuntu-artful-kernel.git

ptrace: Properly initialize ptracer_cred on fork

BugLink: http://bugs.launchpad.net/bugs/1698799

commit c70d9d809fdeecedb96972457ee45c49a232d97f upstream.

When I introduced ptracer_cred I failed to consider the weirdness of
fork where the task_struct copies the old value by default.  This
winds up leaving ptracer_cred set even when a process forks and
the child process does not wind up being ptraced.

Because ptracer_cred is not set on non-ptraced processes whose
parents were ptraced this has broken the ability of the enlightenment
window manager to start setuid children.

Fix this by properly initializing ptracer_cred in ptrace_init_task

This must be done with a little bit of care to preserve the current value
of ptracer_cred when ptrace carries through fork.  Re-reading the
ptracer_cred from the ptracing process at this point is inconsistent
with how PT_PTRACE_CAP has been maintained all of these years.

Tested-by: Takashi Iwai <tiwai@suse.de>
Fixes: 64b875f7ac8a ("ptrace: Capture the ptracer's creds not PT_PTRACE_CAP")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index e13bfdf7f314..81fdf4b8aba4 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -50,7 +50,8 @@ extern int ptrace_request(struct task_struct *child, long request,
 			  unsigned long addr, unsigned long data);
 extern void ptrace_notify(int exit_code);
 extern void __ptrace_link(struct task_struct *child,
-			  struct task_struct *new_parent);
+			  struct task_struct *new_parent,
+			  const struct cred *ptracer_cred);
 extern void __ptrace_unlink(struct task_struct *child);
 extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
 #define PTRACE_MODE_READ	0x01
@@ -202,7 +203,7 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
 
 	if (unlikely(ptrace) && current->ptrace) {
 		child->ptrace = current->ptrace;
-		__ptrace_link(child, current->parent);
+		__ptrace_link(child, current->parent, current->ptracer_cred);
 
 		if (child->ptrace & PT_SEIZED)
 			task_set_jobctl_pending(child, JOBCTL_TRAP_STOP);
@@ -211,6 +212,8 @@ static inline void ptrace_init_task(struct task_struct *child, bool ptrace)
 
 		set_tsk_thread_flag(child, TIF_SIGPENDING);
 	}
+	else
+		child->ptracer_cred = NULL;
 }
 
 /**
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index c7e8ed99c953..5e2cd1030702 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -28,19 +28,25 @@
 #include <linux/compat.h>
 
 
+void __ptrace_link(struct task_struct *child, struct task_struct *new_parent,
+		   const struct cred *ptracer_cred)
+{
+	BUG_ON(!list_empty(&child->ptrace_entry));
+	list_add(&child->ptrace_entry, &new_parent->ptraced);
+	child->parent = new_parent;
+	child->ptracer_cred = get_cred(ptracer_cred);
+}
+
 /*
  * ptrace a task: make the debugger its new parent and
  * move it to the ptrace list.
  *
  * Must be called with the tasklist lock write-held.
  */
-void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
+static void ptrace_link(struct task_struct *child, struct task_struct *new_parent)
 {
-	BUG_ON(!list_empty(&child->ptrace_entry));
-	list_add(&child->ptrace_entry, &new_parent->ptraced);
-	child->parent = new_parent;
 	rcu_read_lock();
-	child->ptracer_cred = get_cred(__task_cred(new_parent));
+	__ptrace_link(child, new_parent, __task_cred(new_parent));
 	rcu_read_unlock();
 }
 
@@ -353,7 +359,7 @@ static int ptrace_attach(struct task_struct *task, long request,
 		flags |= PT_SEIZED;
 	task->ptrace = flags;
 
-	__ptrace_link(task, current);
+	ptrace_link(task, current);
 
 	/* SEIZE doesn't trap tracee on attach */
 	if (!seize)
@@ -420,7 +426,7 @@ static int ptrace_traceme(void)
 		 */
 		if (!ret && !(current->real_parent->flags & PF_EXITING)) {
 			current->ptrace = PT_PTRACED;
-			__ptrace_link(current, current->real_parent);
+			ptrace_link(current, current->real_parent);
 		}
 	}
 	write_unlock_irq(&tasklist_lock);