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Subject[PATCH] x86/idt: Make sure idt_table takes a whole page
From: Joerg Roedel <>

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

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 <>
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[] = {

-/* 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)

+ BUILD_BUG_ON(sizeof(idt_table) != PAGE_SIZE);
/* Make the IDT table read only */
set_memory_ro((unsigned long)&idt_table, 1);

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