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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Lai Jiangshan <jiangshan.ljs@antgroup.com>,
        Borislav Petkov <bp@suse.de>,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Subject: [PATCH 5.15 02/89] x86/entry: Switch the stack after error_entry() returns
Date:   Fri, 22 Jul 2022 11:10:36 +0200
Message-Id: <20220722091133.500016168@linuxfoundation.org>
In-Reply-To: <20220722091133.320803732@linuxfoundation.org>
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From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

commit 520a7e80c96d655fbe4650d9cc985bd9d0443389 upstream.

error_entry() calls fixup_bad_iret() before sync_regs() if it is a fault
from a bad IRET, to copy pt_regs to the kernel stack. It switches to the
kernel stack directly after sync_regs().

But error_entry() itself is also a function call, so it has to stash
the address it is going to return to, in %r12 which is unnecessarily
complicated.

Move the stack switching after error_entry() and get rid of the need to
handle the return address.

  [ bp: Massage commit message. ]

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220503032107.680190-3-jiangshanlai@gmail.com
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/entry/entry_64.S |   16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -323,6 +323,8 @@ SYM_CODE_END(ret_from_fork)
 .macro idtentry_body cfunc has_error_code:req
 
 	call	error_entry
+	movq	%rax, %rsp			/* switch to the task stack if from userspace */
+	ENCODE_FRAME_POINTER
 	UNWIND_HINT_REGS
 
 	movq	%rsp, %rdi			/* pt_regs pointer into 1st argument*/
@@ -982,14 +984,10 @@ 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
 
 	/*
@@ -1021,6 +1019,7 @@ 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:
@@ -1041,12 +1040,9 @@ 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)