linux-kernel - [PATCH] x86/alternatives: Let __text_poke() acquire the pte lock with enabled interrupts

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Message-Id: <20200706164215.2502730-1-bigeasy@linutronix.de>
Date:   Mon,  6 Jul 2020 18:42:15 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH] x86/alternatives: Let __text_poke() acquire the pte lock with enabled interrupts

The pte lock is never acquired from an IRQ-off region so it does not
require the interrupts to be disabled.
RT complains here because the spinlock_t must not be acquired with
disabled interrupts.

use_temporary_mm() expects interrupts to be off because it invokes
switch_mm_irqs_off() and uses per-CPU (current active mm) data.

Move local_irq_save() after the the pte lock has been acquired. Move
local_irq_restore() after the pte lock has been released.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/kernel/alternative.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8fd39ff74a499..7c59a87ebbde8 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -872,8 +872,6 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
 	 */
 	BUG_ON(!pages[0] || (cross_page_boundary && !pages[1]));
 
-	local_irq_save(flags);
-
 	/*
 	 * Map the page without the global bit, as TLB flushing is done with
 	 * flush_tlb_mm_range(), which is intended for non-global PTEs.
@@ -890,6 +888,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
 	 */
 	VM_BUG_ON(!ptep);
 
+	local_irq_save(flags);
+
 	pte = mk_pte(pages[0], pgprot);
 	set_pte_at(poking_mm, poking_addr, ptep, pte);
 
@@ -939,8 +939,8 @@ static void *__text_poke(void *addr, const void *opcode, size_t len)
 	 */
 	BUG_ON(memcmp(addr, opcode, len));
 
-	pte_unmap_unlock(ptep, ptl);
 	local_irq_restore(flags);
+	pte_unmap_unlock(ptep, ptl);
 	return addr;
 }
 
-- 
2.27.0