linux-kernel - [PATCH v2 01/18] x86/entry/64: Fix entry_SYSCALL_64_after

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Date:   Tue, 21 Nov 2017 20:43:56 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>
Subject: [PATCH v2 01/18] x86/entry/64: Fix entry_SYSCALL_64_after_hwframe IRQ tracing

When I added entry_SYSCALL_64_after_hwframe, I left TRACE_IRQS_OFF
before it.  This means that users of entry_SYSCALL_64_after_hwframe
were responsible for invoking TRACE_IRQS_OFF, and the one and only
user (added in the same commit) got it wrong.

I think this would manifest as a warning if a Xen PV guest with
CONFIG_DEBUG_LOCKDEP=y were used with context tracking.  (The
context tracking bit is to cause lockdep to get invoked before we
turn IRQs back on.)  I haven't tested that for real yet because I
can't get a kernel configured like that to boot at all on Xen PV.
I've reported it upstream.  The problem seems to be that Xen PV is
missing early #UD handling, is hitting some WARN, and we rely on

Move TRACE_IRQS_OFF below the label.

Cc: stable@...r.kernel.org
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Juergen Gross <jgross@...e.com>
Fixes: 8a9949bc71a7 ("x86/xen/64: Rearrange the SYSCALL entries")
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/entry_64.S | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a2b30ec69497..5063ed1214dd 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -148,8 +148,6 @@ ENTRY(entry_SYSCALL_64)
 	movq	%rsp, PER_CPU_VAR(rsp_scratch)
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
-	TRACE_IRQS_OFF
-
 	/* Construct struct pt_regs on stack */
 	pushq	$__USER_DS			/* pt_regs->ss */
 	pushq	PER_CPU_VAR(rsp_scratch)	/* pt_regs->sp */
@@ -170,6 +168,8 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
 	sub	$(6*8), %rsp			/* pt_regs->bp, bx, r12-15 not saved */
 	UNWIND_HINT_REGS extra=0
 
+	TRACE_IRQS_OFF
+
 	/*
 	 * If we need to do entry work or if we guess we'll need to do
 	 * exit work, go straight to the slow path.
-- 
2.13.6