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Date:   Thu, 6 Apr 2023 23:01:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Lai Jiangshan <jiangshanlai@gmail.com>
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        Lai Jiangshan <jiangshan.ljs@antgroup.com>,
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Subject: Re: [RFC PATCH 3/7] x86/entry: Implement atomic-IST-entry
Message-ID: <20230406210116.GG405948@hirez.programming.kicks-ass.net>
References: <20230403140605.540512-1-jiangshanlai@gmail.com>
 <20230403140605.540512-4-jiangshanlai@gmail.com>
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On Mon, Apr 03, 2023 at 10:06:01PM +0800, Lai Jiangshan wrote:

> diff --git a/arch/x86/entry/Makefile b/arch/x86/entry/Makefile
> index ca2fe186994b..7cc1254ca519 100644
> --- a/arch/x86/entry/Makefile
> +++ b/arch/x86/entry/Makefile
> @@ -8,11 +8,14 @@ UBSAN_SANITIZE := n
>  KCOV_INSTRUMENT := n
>  
>  CFLAGS_REMOVE_common.o		= $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_ist_entry.o	= $(CC_FLAGS_FTRACE) $(RETHUNK_CFLAGS)

This ^^^


> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 49ddc4dd3117..50a24cc83581 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -443,6 +443,184 @@ SYM_CODE_END(\asmsym)

> +.macro idtentry_ist vector asmsym cfunc user_cfunc has_error_code:req, stack_offset:req
> +SYM_CODE_START(\asmsym)
> +	UNWIND_HINT_IRET_REGS offset=\has_error_code*8
> +	ENDBR
> +
> +	/*
> +	 * Clear X86_EFLAGS_AC, X86_EFLAGS_DF and set a default ORIG_RAX.
> +	 *
> +	 * The code setting ORIG_RAX will not be replicated if interrupted.
> +	 */
> +	ASM_CLAC
> +	cld
> +
> +	.if \has_error_code == 0
> +		pushq	$-1		/* ORIG_RAX: no syscall to restart */
> +	.endif
> +
> +	/*
> +	 * No register can be touched except %rsp,%rflags,%rip before
> +	 * pushing all the registers.  It is indispensable for nested
> +	 * atomic-IST-entry to replicate pushing the registers.
> +	 */
> +	PUSH_REGS
> +
> +	/*
> +	 * Finished pushing register, all registers can be touched by now.
> +	 *
> +	 * Clear registers for the C function ist_copy_regs_to_main_stack()
> +	 * and the handler to avoid any possible exploitation of any
> +	 * speculation attack.
> +	 */
> +	CLEAR_REGS
> +
> +	/*
> +	 * Copy the pt_regs to the IST main stack including the pt_regs of
> +	 * the interrupted atomic-IST-entris, if any, by replicating.
> +	 */
> +	movq	%rsp, %rdi				/* pt_regs pointer on its own IST stack */
> +	leaq	PTREGS_SIZE-\stack_offset(%rsp), %rsi	/* struct cea_exception_stacks pointer */
> +	call	ist_copy_regs_to_main_stack

IIUC you do a CALL+RET here, before you call paranoid_entry ...

> +
> +	/*
> +	 * Commit stage.
> +	 */
> +SYM_INNER_LABEL(start_commit_\asmsym, SYM_L_GLOBAL)
> +	/*
> +	 * Switches to the IST main stack.  Before the switching is done,
> +	 * %rax is the copied pt_regs pointer in IST main stack.
> +	 */
> +	movq	%rax, %rsp
> +
> +	/*
> +	 * The label should be immediate after the instruction that switches
> +	 * the stack since there is code assuming there is only one single
> +	 * instruction in the commit stage and the code assumes "%rsp in the
> +	 * IST main stack is also the sign of ending a atomic-IST-entry".
> +	 * (The code will be removed in future when %rip-based identifying
> +	 * is added.)
> +	 */
> +SYM_INNER_LABEL(commit_\asmsym, SYM_L_GLOBAL)
> +
> +	/*
> +	 * Now, it is on the IST main stack.  For the whole kernel, the entries
> +	 * of the IST exceptions can be seen from here because the inside
> +	 * of the atomic-IST-entry can not be seen from the whole kernel
> +	 * except in the atomic-IST-entry or #DF.
> +	 */
> +	UNWIND_HINT_REGS
> +	ENCODE_FRAME_POINTER
> +
> +	/*
> +	 * The code setting ORIG_RAX will not be replicated if interrupted.
> +	 * So redo it here.
> +	 */
> +	.if \has_error_code == 0
> +		movq	$-1, ORIG_RAX(%rsp)	/* ORIG_RAX: no syscall to restart */
> +	.endif
> +
> +	/*
> +	 * If the entry is from userspace, switch stacks and treat it as
> +	 * a normal entry.
> +	 */
> +	testb	$3, CS(%rsp)
> +	jnz	.Lfrom_usermode_switch_stack_\@
> +
> +	/*
> +	 * paranoid_entry returns GS/CR3/SPEC_CTL information for
> +	 * paranoid_exit in RBX/R14/R15.
> +	 */
> +	call	paranoid_entry

... all the way down here, which will do:

  IBRS_ENTER;
  UNTRAIN_RET_FROM_CALL;

Which thus breaks the whole RetBleed mess, since that must not do RET
before that happens.