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From:   Lai Jiangshan <jiangshanlai@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     x86@kernel.org, Lai Jiangshan <laijs@linux.alibaba.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: [PATCH V4 19/50] x86/entry: Switch the stack after error_entry() returns
Date:   Tue, 26 Oct 2021 22:30:01 +0800
Message-Id: <20211026143032.18624-1-jiangshanlai@gmail.com>
In-Reply-To: <20211026141420.17138-1-jiangshanlai@gmail.com>
References: <20211026141420.17138-1-jiangshanlai@gmail.com>
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From: Lai Jiangshan <laijs@linux.alibaba.com>

error_entry() calls sync_regs() to settle/copy the pt_regs and switches
the stack directly after sync_regs().  But because error_entry() is also
called from entry, the switching has to handle the return address together,
which causes the behavior tangly.

Switching to the stack after error_entry() makes the code simpler and
intuitive.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry_64.S | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 7f2338c3b676..1af28027a138 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -324,6 +324,8 @@ SYM_CODE_END(ret_from_fork)
 .macro idtentry_body cfunc has_error_code:req
 
 	call	error_entry
+	movq	%rax, %rsp			/* switch stack settled by sync_regs() */
+	ENCODE_FRAME_POINTER
 	UNWIND_HINT_REGS
 
 	movq	%rsp, %rdi			/* pt_regs pointer into 1st argument*/
@@ -987,14 +989,10 @@ SYM_CODE_START_LOCAL(error_entry)
 	/* We have user CR3.  Change to kernel CR3. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
 
+	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
 .Lerror_entry_from_usermode_after_swapgs:
 	/* Put us onto the real thread stack. */
-	popq	%r12				/* save return addr in %12 */
-	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
 	call	sync_regs
-	movq	%rax, %rsp			/* switch stack */
-	ENCODE_FRAME_POINTER
-	pushq	%r12
 	ret
 
 	/*
@@ -1027,6 +1025,7 @@ SYM_CODE_START_LOCAL(error_entry)
 	 */
 .Lerror_entry_done_lfence:
 	FENCE_SWAPGS_KERNEL_ENTRY
+	leaq	8(%rsp), %rax			/* return pt_regs pointer */
 	ret
 
 .Lbstep_iret:
@@ -1047,12 +1046,9 @@ SYM_CODE_START_LOCAL(error_entry)
 	 * Pretend that the exception came from user mode: set up pt_regs
 	 * as if we faulted immediately after IRET.
 	 */
-	popq	%r12				/* save return addr in %12 */
-	movq	%rsp, %rdi			/* arg0 = pt_regs pointer */
+	leaq	8(%rsp), %rdi			/* arg0 = pt_regs pointer */
 	call	fixup_bad_iret
-	mov	%rax, %rsp
-	ENCODE_FRAME_POINTER
-	pushq	%r12
+	mov	%rax, %rdi
 	jmp	.Lerror_entry_from_usermode_after_swapgs
 SYM_CODE_END(error_entry)
 
-- 
2.19.1.6.gb485710b