[tip: x86/urgent] x86: Fix kthread unwind

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     2e7e5bbb1c3c8d502edeb5c0670eac4995134b6f
Gitweb:        https://git.kernel.org/tip/2e7e5bbb1c3c8d502edeb5c0670eac4995134b6f
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Wed, 19 Jul 2023 22:15:38 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 20 Jul 2023 23:03:50 +02:00

x86: Fix kthread unwind

The rewrite of ret_from_form() misplaced an unwind hint which caused
all kthread stack unwinds to be marked unreliable, breaking
livepatching.

Restore the annotation and add a comment to explain the how and why of
things.

Fixes: 3aec4ecb3d1f ("x86: Rewrite ret_from_fork() in C")
Reported-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lkml.kernel.org/r/20230719201538.GA3553016@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 arch/x86/entry/entry_64.S | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 91f6818..43606de 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -285,7 +285,15 @@ SYM_FUNC_END(__switch_to_asm)
  */
 .pushsection .text, "ax"
 SYM_CODE_START(ret_from_fork_asm)
-	UNWIND_HINT_REGS
+	/*
+	 * This is the start of the kernel stack; even through there's a
+	 * register set at the top, the regset isn't necessarily coherent
+	 * (consider kthreads) and one cannot unwind further.
+	 *
+	 * This ensures stack unwinds of kernel threads terminate in a known
+	 * good state.
+	 */
+	UNWIND_HINT_END_OF_STACK
 	ANNOTATE_NOENDBR // copy_thread
 	CALL_DEPTH_ACCOUNT
 
@@ -295,6 +303,12 @@ SYM_CODE_START(ret_from_fork_asm)
 	movq	%r12, %rcx		/* fn_arg */
 	call	ret_from_fork
 
+	/*
+	 * Set the stack state to what is expected for the target function
+	 * -- at this point the register set should be a valid user set
+	 * and unwind should work normally.
+	 */
+	UNWIND_HINT_REGS
 	jmp	swapgs_restore_regs_and_return_to_usermode
 SYM_CODE_END(ret_from_fork_asm)
 .popsection