[PATCH v4 30/75] x86/head/64: Install boot GDT

Joerg Roedel
2020-07-14T12:15:41Z

From: Joerg Roedel <jroedel@suse.de>

Handling exceptions during boot requires a working GDT. The kernel GDT
is not yet ready for use, so install a temporary boot GDT.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/kernel/head64.c | 19 +++++++++++++++++++
arch/x86/kernel/head_64.S | 20 ++++++++++++++++++++
2 files changed, 39 insertions(+)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index cbb71c1b574f..51059bfd4b99 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -61,6 +61,25 @@ unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
EXPORT_SYMBOL(vmemmap_base);
#endif

+/*
+ * GDT used before %gs is set up and the kernel can use gdt_page. Needed for
+ * early exception handling.
+ */
+struct desc_struct boot_gdt[GDT_ENTRIES] = {
+ [GDT_ENTRY_KERNEL32_CS] = GDT_ENTRY_INIT(0xc09b, 0, 0xfffff),
+ [GDT_ENTRY_KERNEL_CS] = GDT_ENTRY_INIT(0xa09b, 0, 0xfffff),
+ [GDT_ENTRY_KERNEL_DS] = GDT_ENTRY_INIT(0xc093, 0, 0xfffff),
+};
+
+/*
+ * Address needs to be set at runtime because it references the boot_gdt while
+ * the kernel still uses a direct mapping.
+ */
+struct desc_ptr boot_gdt_descr = {
+ .size = sizeof(boot_gdt),
+ .address = 0,
+};
+
#define __head __section(.head.text)

static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 16da4ac01597..473e60a12e2f 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -73,6 +73,26 @@ SYM_CODE_START_NOALIGN(startup_64)
/* Set up the stack for verify_cpu(), similar to initial_stack below */
leaq (__end_init_task - SIZEOF_PTREGS)(%rip), %rsp

+ /* Setup boot GDT descriptor and load boot GDT */
+ leaq boot_gdt(%rip), %rax
+ movq %rax, boot_gdt_descr+2(%rip)
+ lgdt boot_gdt_descr(%rip)
+
+ /* New GDT is live - reload data segment registers */
+ movl $__KERNEL_DS, %eax
+ movl %eax, %ds
+ movl %eax, %ss
+ movl %eax, %es
+
+ /* Now switch to __KERNEL_CS so IRET works reliably */
+ pushq $__KERNEL_CS
+ leaq .Lon_kernel_cs(%rip), %rax
+ pushq %rax
+ lretq
+
+.Lon_kernel_cs:
+ UNWIND_HINT_EMPTY
+
/* Sanitize CPU configuration */
call verify_cpu

--
2.27.0