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

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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: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo 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
This commit is contained in:
David Woodhouse 2025-03-12 14:34:13 +00:00 committed by Ingo Molnar
parent e27dffba1b
commit b25eb5f5e4
1 changed files with 23 additions and 0 deletions
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23
arch/x86/kernel/relocate_kernel_64.S
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@ -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.