Re: [PATCH v3 31/75] x86/head/64: Install boot GDT

Joerg Roedel
2020-06-04T11:48:21Z

On Mon, May 18, 2020 at 10:23:13AM +0200, Borislav Petkov wrote:
> On Tue, Apr 28, 2020 at 05:16:41PM +0200, Joerg Roedel wrote:
> > @@ -480,6 +500,22 @@ SYM_DATA_LOCAL(early_gdt_descr_base, .quad INIT_PER_CPU_VAR(gdt_page))
> > SYM_DATA(phys_base, .quad 0x0)
> > EXPORT_SYMBOL(phys_base)
> >
> > +/* Boot GDT used when kernel addresses are not mapped yet */
> > +SYM_DATA_LOCAL(boot_gdt_descr, .word boot_gdt_end - boot_gdt)
> > +SYM_DATA_LOCAL(boot_gdt_base, .quad 0)
> > +SYM_DATA_START(boot_gdt)
> > + .quad 0
> > + .quad 0x00cf9a000000ffff /* __KERNEL32_CS */
> > + .quad 0x00af9a000000ffff /* __KERNEL_CS */
> > + .quad 0x00cf92000000ffff /* __KERNEL_DS */
> > + .quad 0 /* __USER32_CS - unused */
> > + .quad 0 /* __USER_DS - unused */
> > + .quad 0 /* __USER_CS - unused */
> > + .quad 0 /* unused */
> > + .quad 0x0080890000000000 /* TSS descriptor */
> > + .quad 0x0000000000000000 /* TSS continued */
>
> Any chance you could use macros ala GDT_ENTRY_INIT() for those instead
> of the naked values?

Yeah, seems to work. Updated patch attached.

From 1350746f9063147a33156c0b56a7b12b1794f555 Mon Sep 17 00:00:00 2001
From: Joerg Roedel <jroedel@suse.de>
Date: Fri, 15 Nov 2019 13:49:55 +0100
Subject: [PATCH 31/74] x86/head/64: Install boot GDT

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 | 25 +++++++++++++++++++++++++
2 files changed, 44 insertions(+)

diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 206a4b6144c2..900c3372cc6a 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 4bbc770af632..62513dd1e0e4 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -72,6 +72,26 @@ 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

@@ -143,6 +163,11 @@ SYM_CODE_START(secondary_startup_64)
1:
UNWIND_HINT_EMPTY

+ /* 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)
+
/* Check if nx is implemented */
movl $0x80000001, %eax
cpuid
--
2.26.2