Commit b25eb5f5 authored by David Woodhouse's avatar David Woodhouse Committed by Ingo Molnar
Browse files

x86/kexec: Add relocate_kernel() debugging support: Load a GDT 



There are some failure modes which lead to triple-faults in the
relocate_kernel() function, which is fairly much undebuggable
for normal mortals.

Adding a GDT in the relocate_kernel() environment is step 1 towards
being able to catch faults and do something more useful.

Signed-off-by: default avatarDavid Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250312144257.2348250-2-dwmw2@infradead.org
parent e27dffba

arch/x86/kernel/relocate_kernel_64.S

+23 −0
Original line number Diff line number Diff line
@@ -40,6 +40,16 @@ SYM_DATA(kexec_pa_table_page, .quad 0) 
SYM_DATA(kexec_pa_swap_page, .quad 0)

SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)



	.balign 16

SYM_DATA_START_LOCAL(kexec_debug_gdt)

	.word   kexec_debug_gdt_end - kexec_debug_gdt - 1

	.long   0

	.word   0

	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */

	.quad   0x00af9a000000ffff      /* __KERNEL_CS */

	.quad   0x00cf92000000ffff      /* __KERNEL_DS */

SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)



	.section .text..relocate_kernel,"ax";

	.code64

SYM_CODE_START_NOALIGN(relocate_kernel)
@@ -116,6 +126,19 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped) 
	/* store the start address on the stack */

	pushq   %rdx



	/* Create a GDTR (16 bits limit, 64 bits addr) on stack */

	leaq	kexec_debug_gdt(%rip), %rax

	pushq	%rax

	pushw	(%rax)



	/* Load the GDT, put the stack back */

	lgdt	(%rsp)

	addq	$10, %rsp



	/* Test that we can load segments */

	movq	%ds, %rax

	movq	%rax, %ds



	/*

	 * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP

	 * below.