linux-kernel - [PATCH] x86/idt: Make sure idt

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Date:   Thu,  9 Jul 2020 12:33:55 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, hpa@...or.com, Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        joro@...tes.org
Subject: [PATCH] x86/idt: Make sure idt_table takes a whole page

From: Joerg Roedel <jroedel@...e.de>

On x86-32 the idt_table with 256 entries needs only 2048 bytes. It is
page-aligned, but the end of the .bss..page_aligned section is not
guaranteed to be page-aligned.

As a result, symbols from other .bss sections may end up on the same
4k page as the idt_table, and will accidentially get mapped read-only
during boot, causing unexpected page-faults when the kernel writes to
them.

Avoid this by making the idt_table 4kb in size even on x86-32. On
x86-64 the idt_table is already 4kb large, so nothing changes there.

Fixes: 3e77abda65b1c ("x86/idt: Consolidate idt functionality")
Signed-off-by: Joerg Roedel <jroedel@...e.de>
---
 arch/x86/kernel/idt.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 0db21206f2f3..83e24f837127 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -157,8 +157,13 @@ static const __initconst struct idt_data apic_idts[] = {
 #endif
 };
 
-/* Must be page-aligned because the real IDT is used in the cpu entry area */
-static gate_desc idt_table[IDT_ENTRIES] __page_aligned_bss;
+/*
+ * Must be page-aligned because the real IDT is used in the cpu entry area.
+ * Allocate more entries than needed so that idt_table takes a whole page, so it
+ * is safe to map the idt_table read-only and into the user-space page-table.
+ */
+#define IDT_ENTRIES_ALLOCATED	(PAGE_SIZE / sizeof(gate_desc))
+static gate_desc idt_table[IDT_ENTRIES_ALLOCATED] __page_aligned_bss;
 
 struct desc_ptr idt_descr __ro_after_init = {
 	.size		= IDT_TABLE_SIZE - 1,
@@ -335,6 +340,9 @@ void __init idt_setup_apic_and_irq_gates(void)
 	idt_map_in_cea();
 	load_idt(&idt_descr);
 
+	BUILD_BUG_ON(IDT_ENTRIES_ALLOCATED < IDT_ENTRIES);
+	BUILD_BUG_ON(sizeof(idt_table) != PAGE_SIZE);
+
 	/* Make the IDT table read only */
 	set_memory_ro((unsigned long)&idt_table, 1);
 
-- 
2.27.0