linux-kernel - Re: [PATCH][RFC] fix PTRACE

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Date:   Tue, 17 Aug 2021 13:21:03 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][RFC] fix PTRACE_KILL

Al Viro <viro@...iv.linux.org.uk> writes:

> [Cc'd to security@k.o, *NOT* because I consider it a serious security hole;
> it's just that the odds of catching relevant reviewers there are higher
> than on l-k and there doesn't seem to be any lists where that would be
> on-topic.  My apologies for misuse of security@k.o ;-/]

Hmm.  I don't see security@...nel.org Cc'd.

> Current implementation is racy in quite a few ways - we check that
> the child is traced by us and use ptrace_resume() to feed it
> SIGKILL, provided that it's still alive.
>
> What we do not do is making sure that the victim is in ptrace stop;
> as the result, it can go and violate all kinds of assumptions,
> starting with "child->sighand won't change under ptrace_resume()",
> "child->ptrace won't get changed under user_disable_single_step()",
> etc.
>
> Note that ptrace(2) manpage has this to say:
>     
> PTRACE_KILL
>       Send  the  tracee a SIGKILL to terminate it.  (addr and data are
>       ignored.)
>     
>       This operation is deprecated; do not use it!   Instead,  send  a
>       SIGKILL  directly  using kill(2) or tgkill(2).  The problem with
>       PTRACE_KILL is that it requires the  tracee  to  be  in  signal-
>       delivery-stop,  otherwise  it  may  not work (i.e., may complete
>       successfully but won't kill the tracee).  By contrast, sending a
>       SIGKILL directly has no such limitation.
>     
> So let it check (under tasklist_lock) that the victim is traced by us
> and call sig_send_info() to feed it SIGKILL.  It's easier that trying
> to force ptrace_resume() into handling that mess and it's less brittle
> that way.

I took a quick look and despite being deprecated PTRACE_KILL appears
to still have some active users (like gcc-10).  So that seems to rule
out just removing PTRACE_KILL.

I looked at the bug that PTRACE_KILL only kills a process when it is
stopped and it is present in Linux 1.0.  Given that I expect userspace
applications are ok with the current semantics rather than the intended
semantics.

The current semantics also include the weirdness that PTRACE_KILL only
kills a process when it is stopped in ptrace_signal, and not at other
ptrace stops.

So rather than fix the code to do what was intended 27 years ago,
why don't we accept the fact that PTRACE_KILL is equivalent
to PTRACE_CONT with data = SIGKILL.

If there are regressions or we really care we can tweak the return value
to return 0 instead of -ESRCH when the process is not stopped.

Something like this:

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index f8589bf8d7dc..f40f0a0ff70a 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -1221,8 +1221,6 @@ int ptrace_request(struct task_struct *child, long request,
 		return ptrace_resume(child, request, data);
 
 	case PTRACE_KILL:
-		if (child->exit_state)	/* already dead */
-			return 0;
 		return ptrace_resume(child, request, SIGKILL);
 
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
@@ -1304,8 +1302,7 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
-				  request == PTRACE_INTERRUPT);
+	ret = ptrace_check_attach(child, request == PTRACE_INTERRUPT);
 	if (ret < 0)
 		goto out_put_task_struct;
 
@@ -1449,8 +1446,7 @@ COMPAT_SYSCALL_DEFINE4(ptrace, compat_long_t, request, compat_long_t, pid,
 		goto out_put_task_struct;
 	}
 
-	ret = ptrace_check_attach(child, request == PTRACE_KILL ||
-				  request == PTRACE_INTERRUPT);
+	ret = ptrace_check_attach(child, request == PTRACE_INTERRUPT);
 	if (!ret) {
 		ret = compat_arch_ptrace(child, request, addr, data);
 		if (ret || request != PTRACE_DETACH)

Eric