Re: [PATCH v2 2/2] x86: Rewrite ret_from_fork() in C

Petr Mladek
2023-07-19T15:21:22Z

On Fri 2023-06-23 18:55:29, Brian Gerst wrote:
> When kCFI is enabled, special handling is needed for the indirect call
> to the kernel thread function. Rewrite the ret_from_fork() function in
> C so that the compiler can properly handle the indirect call.

This patch broke livepatching. Kthreads never have a reliable stack.
It works when I revert it.

See also below.

> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -284,36 +284,19 @@ SYM_FUNC_END(__switch_to_asm)
> * r12: kernel thread arg
> */
> .pushsection .text, "ax"
> - __FUNC_ALIGN
> -SYM_CODE_START_NOALIGN(ret_from_fork)
> - UNWIND_HINT_END_OF_STACK
> +SYM_CODE_START(ret_from_fork_asm)
> + UNWIND_HINT_REGS
> ANNOTATE_NOENDBR // copy_thread
> CALL_DEPTH_ACCOUNT
> - movq %rax, %rdi
> - call schedule_tail /* rdi: 'prev' task parameter */
>
> - testq %rbx, %rbx /* from kernel_thread? */
> - jnz 1f /* kernel threads are uncommon */
> + movq %rax, %rdi /* prev */
> + movq %rsp, %rsi /* regs */
> + movq %rbx, %rdx /* fn */
> + movq %r12, %rcx /* fn_arg */
> + call ret_from_fork
>
> -2:
> - UNWIND_HINT_REGS
> - movq %rsp, %rdi
> - call syscall_exit_to_user_mode /* returns with IRQs disabled */
> jmp swapgs_restore_regs_and_return_to_usermode
> -
> -1:
> - /* kernel thread */
> - UNWIND_HINT_END_OF_STACK

I think that it might be related to removal of this line.
The following intructions are going to call fn(fn_arg).
See below.

> - movq %r12, %rdi
> - CALL_NOSPEC rbx
> - /*
> - * A kernel thread is allowed to return here after successfully
> - * calling kernel_execve(). Exit to userspace to complete the execve()
> - * syscall.
> - */
> - movq $0, RAX(%rsp)
> - jmp 2b
> -SYM_CODE_END(ret_from_fork)
> +SYM_CODE_END(ret_from_fork_asm)
> .popsection
>
> .macro DEBUG_ENTRY_ASSERT_IRQS_OFF
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 5c91305d09d2..f42dbf17f52b 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -12,7 +12,9 @@ struct task_struct *__switch_to_asm(struct task_struct *prev,
> __visible struct task_struct *__switch_to(struct task_struct *prev,
> struct task_struct *next);
>
> -asmlinkage void ret_from_fork(void);
> +asmlinkage void ret_from_fork_asm(void);
> +__visible void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
> + int (*fn)(void *), void *fn_arg);
>
> /*
> * This is the structure pointed to by thread.sp for an inactive task. The
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index cc7a642f8c9d..001e6dad9a48 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -136,6 +137,25 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
> return do_set_thread_area_64(p, ARCH_SET_FS, tls);
> }
>
> +__visible noinstr void ret_from_fork(struct task_struct *prev, struct pt_regs *regs,
> + int (*fn)(void *), void *fn_arg)
> +{
> + schedule_tail(prev);
> +
> + /* Is this a kernel thread? */
> + if (unlikely(fn)) {
> + fn(fn_arg);

This is the related code but it does not include the annotation
about the end of the stack.

Honestly, I am not familiar with the stack unwinder and how this is
supposed to work.

I hope that Josh or anyone else might know better.

> + /*
> + * A kernel thread is allowed to return here after successfully
> + * calling kernel_execve(). Exit to userspace to complete the
> + * execve() syscall.
> + */
> + regs->ax = 0;
> + }
> +
> + syscall_exit_to_user_mode(regs);
> +}
> +
> int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> {
> unsigned long clone_flags = args->flags;

Best Regards,
Petr