linux-kernel - [PATCH entry v2 1/6] x86/entry/compat: Clear RAX high bits on Xen PV SYSENTER

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Date:   Fri,  3 Jul 2020 10:02:53 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH entry v2 1/6] x86/entry/compat: Clear RAX high bits on Xen PV SYSENTER

Move the cleari the high bits of RAX after Xen PV joins the SYSENTER
path so that Xen PV doesn't skip it.

Arguably we should delete this code instead, but that would belong
in the merge window.

Fixes: ffae641f5747 ("x86/entry/64/compat: Fix Xen PV SYSENTER frame setup")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/entry_64_compat.S | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 381a6de7de9c..541fdaf64045 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -57,15 +57,6 @@ SYM_CODE_START(entry_SYSENTER_compat)
 
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
-	/*
-	 * User tracing code (ptrace or signal handlers) might assume that
-	 * the saved RAX contains a 32-bit number when we're invoking a 32-bit
-	 * syscall.  Just in case the high bits are nonzero, zero-extend
-	 * the syscall number.  (This could almost certainly be deleted
-	 * with no ill effects.)
-	 */
-	movl	%eax, %eax
-
 	/* Construct struct pt_regs on stack */
 	pushq	$__USER32_DS		/* pt_regs->ss */
 	pushq	$0			/* pt_regs->sp = 0 (placeholder) */
@@ -80,6 +71,16 @@ SYM_CODE_START(entry_SYSENTER_compat)
 	pushq	$__USER32_CS		/* pt_regs->cs */
 	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
 SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
+
+	/*
+	 * User tracing code (ptrace or signal handlers) might assume that
+	 * the saved RAX contains a 32-bit number when we're invoking a 32-bit
+	 * syscall.  Just in case the high bits are nonzero, zero-extend
+	 * the syscall number.  (This could almost certainly be deleted
+	 * with no ill effects.)
+	 */
+	movl	%eax, %eax
+
 	pushq	%rax			/* pt_regs->orig_ax */
 	pushq	%rdi			/* pt_regs->di */
 	pushq	%rsi			/* pt_regs->si */
-- 
2.25.4