The following commit has been merged into the x86/boot branch of tip:
Commit-ID: e7af930606442f838c7efeaa1bcd46020337dda8
Gitweb: https://git.kernel.org/tip/e7af930606442f838c7efeaa1bcd46020337dda8
Author: David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate: Wed, 12 Mar 2025 14:34:14
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 13 Mar 2025 11:23:32 +01:00
x86/kexec: Add relocate_kernel() debugging support: Load an IDT and basic exception entry points
Further extend relocate_kernel() debuggability by loading an
IDT and adding exception entry points.
[ mingo: Minor beautifying of prepare_debug_idt() ]
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250312144257.2348250-3-dwmw2@xxxxxxxxxxxxx
---
arch/x86/include/asm/kexec.h | 5 ++-
arch/x86/kernel/machine_kexec_64.c | 21 ++++++++-
arch/x86/kernel/relocate_kernel_64.S | 74 +++++++++++++++++++++++++++-
3 files changed, 100 insertions(+)
diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index 8ad1874..ec7636f 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -8,6 +8,9 @@
# define PA_PGD 2
# define PA_SWAP_PAGE 3
# define PAGES_NR 4
+#else
+/* Size of each exception handler referenced by the IDT */
+# define KEXEC_DEBUG_EXC_HANDLER_SIZE 6 /* pushi, pushi, 2-byte jmp */
#endif
# define KEXEC_CONTROL_PAGE_SIZE 4096
@@ -58,6 +61,8 @@ struct kimage;
extern unsigned long kexec_va_control_page;
extern unsigned long kexec_pa_table_page;
extern unsigned long kexec_pa_swap_page;
+extern gate_desc kexec_debug_idt[];
+extern unsigned char kexec_debug_exc_vectors[];
#endif
/*
diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index a68f5a0..cc73f97 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -304,6 +304,24 @@ static void load_segments(void)
);
}
+static void prepare_debug_idt(unsigned long control_page, unsigned long vec_ofs)
+{
+ gate_desc idtentry = { 0 };
+ int i;
+
+ idtentry.bits.p = 1;
+ idtentry.bits.type = GATE_TRAP;
+ idtentry.segment = __KERNEL_CS;
+ idtentry.offset_low = (control_page & 0xFFFF) + vec_ofs;
+ idtentry.offset_middle = (control_page >> 16) & 0xFFFF;
+ idtentry.offset_high = control_page >> 32;
+
+ for (i = 0; i < 16; i++) {
+ kexec_debug_idt[i] = idtentry;
+ idtentry.offset_low += KEXEC_DEBUG_EXC_HANDLER_SIZE;
+ }
+}
+
int machine_kexec_prepare(struct kimage *image)
{
void *control_page = page_address(image->control_code_page);
@@ -321,6 +339,9 @@ int machine_kexec_prepare(struct kimage *image)
if (image->type == KEXEC_TYPE_DEFAULT)
kexec_pa_swap_page = page_to_pfn(image->swap_page) << PAGE_SHIFT;
+ prepare_debug_idt((unsigned long)__pa(control_page),
+ (unsigned long)kexec_debug_exc_vectors - reloc_start);
+
__memcpy(control_page, __relocate_kernel_start, reloc_end - reloc_start);
set_memory_rox((unsigned long)control_page, 1);
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index ac05897..bce0cb7 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -50,6 +50,11 @@ SYM_DATA_START_LOCAL(kexec_debug_gdt)
.quad 0x00cf92000000ffff /* __KERNEL_DS */
SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
+ .balign 8
+SYM_DATA_START(kexec_debug_idt)
+ .skip 0x100, 0x00
+SYM_DATA_END(kexec_debug_idt)
+
.section .text..relocate_kernel,"ax";
.code64
SYM_CODE_START_NOALIGN(relocate_kernel)
@@ -139,6 +144,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
movq %ds, %rax
movq %rax, %ds
+ /* Now an IDTR on the stack to load the IDT the kernel created */
+ leaq kexec_debug_idt(%rip), %rsi
+ pushq %rsi
+ pushw $0xff
+ lidt (%rsp)
+ addq $10, %rsp
+
+ // int3
+
/*
* Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
* below.
@@ -364,3 +378,63 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
ret
int3
SYM_CODE_END(swap_pages)
+
+SYM_CODE_START_NOALIGN(kexec_debug_exc_vectors)
+ /* Each of these is 6 bytes. */
+.macro vec_err exc
+ UNWIND_HINT_ENTRY
+ . = kexec_debug_exc_vectors + (\exc * KEXEC_DEBUG_EXC_HANDLER_SIZE)
+ nop
+ nop
+ pushq $\exc
+ jmp exc_handler
+.endm
+
+.macro vec_noerr exc
+ UNWIND_HINT_ENTRY
+ . = kexec_debug_exc_vectors + (\exc * KEXEC_DEBUG_EXC_HANDLER_SIZE)
+ pushq $0
+ pushq $\exc
+ jmp exc_handler
+.endm
+
+ ANNOTATE_NOENDBR
+ vec_noerr 0 // #DE
+ vec_noerr 1 // #DB
+ vec_noerr 2 // #NMI
+ vec_noerr 3 // #BP
+ vec_noerr 4 // #OF
+ vec_noerr 5 // #BR
+ vec_noerr 6 // #UD
+ vec_noerr 7 // #NM
+ vec_err 8 // #DF
+ vec_noerr 9
+ vec_err 10 // #TS
+ vec_err 11 // #NP
+ vec_err 12 // #SS
+ vec_err 13 // #GP
+ vec_err 14 // #PF
+ vec_noerr 15
+SYM_CODE_END(kexec_debug_exc_vectors)
+
+SYM_CODE_START_LOCAL_NOALIGN(exc_handler)
+ pushq %rax
+ pushq %rdx
+ movw $0x3f8, %dx
+ movb $'A', %al
+ outb %al, %dx
+ popq %rdx
+ popq %rax
+
+ /* Only return from int3 */
+ cmpq $3, (%rsp)
+ jne .Ldie
+
+ addq $16, %rsp
+ iretq
+
+.Ldie:
+ hlt
+ jmp .Ldie
+
+SYM_CODE_END(exc_handler)
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