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Date:   Thu, 20 Jul 2023 11:28:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     Petr Mladek <pmladek@suse.com>, Brian Gerst <brgerst@gmail.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        alyssa.milburn@linux.intel.com, keescook@chromium.org,
        joao@overdrivepizza.com, tim.c.chen@linux.intel.com,
        live-patching@vger.kernel.org
Subject: Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C
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On Thu, Jul 20, 2023 at 07:22:08AM +0200, Peter Zijlstra wrote:

> > I'm not sure that would be worth the trouble / cleverness.  The hint is
> > straightforward IMO.
> 
> I tried, it doesn't work, clearly I missed something.

FWIW, I tried the below. That should make user_mode() true for the
kernel thread regset, and while the kernel did boot, it still fails the
livepatch self-test.

The difference seems to be that END_OF_STACK terminates it right there,
while REGS thinks its a valid frame and only terminates on user_mode()
when unwinding one more frame. The frame at REGS clearly isn't very
sane.


diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 72015dba72ab..45a400b16b80 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -232,6 +232,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		 * It does the same kernel frame setup to return to a kernel
 		 * function that a kernel thread does.
 		 */
+		childregs->cs = 3;
 		childregs->sp = 0;
 		childregs->ip = 0;
 		kthread_frame_init(frame, args->fn, args->fn_arg);