linux-kernel - [PATCH 3/5] x86/entry: directly switch to kernel stack when .Lerror_bad

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Message-Id: <20200527073107.2127-4-laijs@linux.alibaba.com>
Date:   Wed, 27 May 2020 07:31:05 +0000
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [PATCH 3/5] x86/entry: directly switch to kernel stack when .Lerror_bad_iret

Directly copy pt_regs to kernel stack when .Lerror_bad_iret.
Directly switch to kernel stack when .Lerror_bad_iret.

We can see that entry_64.S do the following things back to back
when .Lerror_bad_iret:
	call fixup_bad_iret(), switch to sp0 stack with pt_regs copied
	call sync_regs(), switch to kernel stack with pt_regs copied

So we can do the all things together in fixup_bad_iret().

After this patch, fixup_bad_iret() is restored to the behavior before
7f2590a110b8("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 arch/x86/entry/entry_64.S | 13 ++-----------
 arch/x86/kernel/traps.c   |  9 ++++-----
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e8817ae31390..c5db048e5bed 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1329,16 +1329,6 @@ SYM_CODE_START_LOCAL(error_entry)
 	ENCODE_FRAME_POINTER 8
 	ret
 
-.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
-
 .Lerror_entry_done_lfence:
 	FENCE_SWAPGS_KERNEL_ENTRY
 .Lerror_entry_done:
@@ -1392,7 +1382,8 @@ SYM_CODE_START_LOCAL(error_entry)
 	mov	%rsp, %rdi
 	call	fixup_bad_iret
 	mov	%rax, %rsp
-	jmp	.Lerror_entry_from_usermode_after_swapgs
+	ENCODE_FRAME_POINTER 8
+	ret
 SYM_CODE_END(error_entry)
 
 SYM_CODE_START_LOCAL(error_exit)
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9e5d81cb94ba..3bef95934644 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -666,13 +666,12 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
 	/*
 	 * This is called from entry_64.S early in handling a fault
 	 * caused by a bad iret to user mode.  To handle the fault
-	 * correctly, we want to move our stack frame to where it would
-	 * be had we entered directly on the entry stack (rather than
-	 * just below the IRET frame) and we want to pretend that the
-	 * exception came from the IRET target.
+	 * correctly, we want to move our stack frame to kernel stack
+	 * (rather than just below the IRET frame) and we want to
+	 * pretend that the exception came from the IRET target.
 	 */
 	struct bad_iret_stack tmp, *new_stack =
-		(struct bad_iret_stack *)__this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1;
+		(struct bad_iret_stack *)__this_cpu_read(cpu_current_top_of_stack) - 1;
 
 	/* Copy the IRET target to the temporary storage. */
 	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
-- 
2.20.1