linux-kernel - [PATCH] x86 efi: bugfix interrupt disabling sequence

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Date:	Wed, 18 Sep 2013 19:28:53 +0200
From:	Bart Kuivenhoven <bemk@...hat.com>
To:	matt.fleming@...el.com
Cc:	hpa@...or.com, tglx@...utronix.de, mingo@...hat.com,
	x86@...nel.org, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, jcm@...hat.com, msalter@...hat.com,
	Bart Kuivenhoven <bemk@...hat.com>
Subject: [PATCH] x86 efi: bugfix interrupt disabling sequence

The problem in efi_main was that the idt was cleared before the
interrupts were disabled.

The UEFI spec states that interrupts aren't used so this shouldn't be
too much of a problem. Peripherals however don't necessarily know about
this and thus might cause interrupts to happen anyway. Even if
ExitBootServices() has been called.

This means there is a risk of an interrupt being triggered while the IDT
register is nullified and the interrupt bit hasn't been cleared,
allowing for a triple fault.

This patch fixes this by clearing the interrupt bit before the lidt
instruction.

Signed-off-by: Bart Kuivenhoven <bemk@...hat.com>
---
 arch/x86/boot/compressed/eboot.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
index b7388a4..100b812 100644
--- a/arch/x86/boot/compressed/eboot.c
+++ b/arch/x86/boot/compressed/eboot.c
@@ -1267,11 +1267,11 @@ struct boot_params *efi_main(void *handle, efi_system_table_t *_table,
 	desc->base2 = 0x00;
 #endif /* CONFIG_X86_64 */

+	asm volatile("cli");
+
 	asm volatile ("lidt %0" : : "m" (*idt));
 	asm volatile ("lgdt %0" : : "m" (*gdt));

-	asm volatile("cli");
-
 	return boot_params;
 fail:
 	return NULL;
-- 
1.8.4

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