Commit 77508cd7 authored Apr 29, 2022 by Eric W. Biederman Committed by Greg Kroah-Hartman Jun 14, 2022

ptrace: Reimplement PTRACE_KILL by always sending SIGKILL



commit 6a2d90ba027adba528509ffa27097cffd3879257 upstream.

The current implementation of PTRACE_KILL is buggy and has been for
many years as it assumes it's target has stopped in ptrace_stop.  At a
quick skim it looks like this assumption has existed since ptrace
support was added in linux v1.0.

While PTRACE_KILL has been deprecated we can not remove it as
a quick search with google code search reveals many existing
programs calling it.

When the ptracee is not stopped at ptrace_stop some fields would be
set that are ignored except in ptrace_stop.  Making the userspace
visible behavior of PTRACE_KILL a noop in those case.

As the usual rules are not obeyed it is not clear what the
consequences are of calling PTRACE_KILL on a running process.
Presumably userspace does not do this as it achieves nothing.

Replace the implementation of PTRACE_KILL with a simple
send_sig_info(SIGKILL) followed by a return 0.  This changes the
observable user space behavior only in that PTRACE_KILL on a process
not stopped in ptrace_stop will also kill it.  As that has always
been the intent of the code this seems like a reasonable change.

Cc: stable@vger.kernel.org
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20220505182645.497868-7-ebiederm@xmission.com


Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

parent c35ff848

arch/x86/kernel/step.c

+1 −2

Original line number	Original line	Diff line number	Diff line
	@@ -175,8 +175,7 @@ void set_task_blockstep(struct task_struct *task, bool on)
	*		*
	* NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if		* NOTE: this means that set/clear TIF_BLOCKSTEP is only safe if
	* task is current or it can't be running, otherwise we can race		* task is current or it can't be running, otherwise we can race
	* with __switch_to_xtra(). We rely on ptrace_freeze_traced() but		* with __switch_to_xtra(). We rely on ptrace_freeze_traced().
	* PTRACE_KILL is not safe.
	*/		*/
	local_irq_disable();		local_irq_disable();
	debugctl = get_debugctlmsr();		debugctl = get_debugctlmsr();

kernel/ptrace.c

+2 −3

Original line number	Original line	Diff line number	Diff line
	@@ -1121,9 +1121,8 @@ int ptrace_request(struct task_struct *child, long request,
	return ptrace_resume(child, request, data);		return ptrace_resume(child, request, data);

	case PTRACE_KILL:		case PTRACE_KILL:
	if (child->exit_state) /* already dead */		send_sig_info(SIGKILL, SEND_SIG_NOINFO, child);
	return 0;		return 0;
	return ptrace_resume(child, request, SIGKILL);

	#ifdef CONFIG_HAVE_ARCH_TRACEHOOK		#ifdef CONFIG_HAVE_ARCH_TRACEHOOK
	case PTRACE_GETREGSET:		case PTRACE_GETREGSET: