[PATCH 4.14 005/218] ptrace: Reimplement PTRACE_KILL by always sending SIGKILL

From: Greg Kroah-Hartman
Date: Mon Jun 13 2022 - 06:35:54 EST

From: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx
Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20220505182645.497868-7-ebiederm@xxxxxxxxxxxx
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
arch/x86/kernel/step.c | 3 +--
kernel/ptrace.c | 5 ++---
2 files changed, 3 insertions(+), 5 deletions(-)

--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -175,8 +175,7 @@ void set_task_blockstep(struct task_stru
* 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
- * with __switch_to_xtra(). We rely on ptrace_freeze_traced() but
- * PTRACE_KILL is not safe.
+ * with __switch_to_xtra(). We rely on ptrace_freeze_traced().
debugctl = get_debugctlmsr();
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1127,9 +1127,8 @@ int ptrace_request(struct task_struct *c
return ptrace_resume(child, request, data);

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