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

tip-bot2 for Peter Zijlstra
2023-07-21T09:21:02Z

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@infradead.org>
AuthorDate: Wed, 19 Jul 2023 22:15:38 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
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@suse.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Petr Mladek <pmladek@suse.com>
Link: https://lkml.kernel.org/r/20230719201538.GA3553016@hirez.programming.kicks-ass.net
---
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