On Sun, Dec 15, 2024 at 10:09:57AM +0000, David Woodhouse wrote: > On Sat, 2024-12-14 at 16:08 -0700, Nathan Chancellor wrote: > > > > I guess this seems somewhat unavoidable because control_page is just a > > 'void *', perhaps machine_kexec() should just be marked as __nocfi? This > > diff resolves that issue for me. > > The patch below seems to work too. I already wanted to deal with the Can confirm, thanks for the quick fix. With your fix for the first issue I reported, the fix I sent for LTO, and this patch below, I can kexec on a CFI and LTO enabled kernel without any issues. > case where relocate_kernel isn't at the start of the page, so it forces > me to do that. > > For some reason it also started complaining > vmlinux.o: warning: objtool: relocate_kernel+0x6a: return with modified stack frame > ... which is easy to fix just by turning it into a jmp *%rsi; I have no > idea why it was done with a ret like that in the first place. > > I don't know why it puts 16 bytes of NOPs between __reloc_start and > __cfi_relocate_kernel (in addition to the 16 before relocate_kernel > itself), and space is *fairly* tight in the control page, but it's > tolerable. I think this is something to do with FineIBT IIRC? PeterZ might have more details. > To make the CFI check actually give useful output if it triggers, I'm > tempted to do the IDT/GDT invalidation relocate_kernel itself, instead > of before the call. > > > diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S > index ccb297765e08..e6befd11fee4 100644 > --- a/arch/x86/kernel/relocate_kernel_64.S > +++ b/arch/x86/kernel/relocate_kernel_64.S > @@ -6,6 +6,7 @@ > > #include <linux/linkage.h> > #include <linux/stringify.h> > +#include <linux/cfi_types.h> > #include <asm/alternative.h> > #include <asm/page_types.h> > #include <asm/kexec.h> > @@ -61,7 +62,10 @@ SYM_DATA_END(kexec_debug_idt) > > .section .text.relocate_kernel,"ax"; > .code64 > -SYM_CODE_START_NOALIGN(relocate_kernel) > +__reloc_start: > + ANNOTATE_NOENDBR > + > +SYM_TYPED_FUNC_START(relocate_kernel) > UNWIND_HINT_END_OF_STACK > ANNOTATE_NOENDBR > /* > @@ -115,10 +119,9 @@ SYM_CODE_START_NOALIGN(relocate_kernel) > lea PAGE_SIZE(%rsi), %rsp > > /* jump to identity mapped page */ > - addq $(identity_mapped - relocate_kernel), %rsi > - pushq %rsi > - ANNOTATE_UNRET_SAFE > - ret > + addq $(identity_mapped - __reloc_start), %rsi > + ANNOTATE_RETPOLINE_SAFE > + jmp *%rsi > int3 > SYM_CODE_END(relocate_kernel) > > @@ -263,7 +266,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped) > > /* get the re-entry point of the peer system */ > popq %rbp > - leaq relocate_kernel(%rip), %r8 > + leaq __reloc_start(%rip), %r8 > movq kexec_pa_swap_page(%rip), %r10 > movq pa_backup_pages_map(%rip), %rdi > movq kexec_pa_table_page(%rip), %rax > @@ -272,7 +275,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped) > movq $1, %r11 /* Ensure preserve_context flag is set */ > call swap_pages > movq kexec_va_control_page(%rip), %rax > - addq $(virtual_mapped - relocate_kernel), %rax > + addq $(virtual_mapped - __reloc_start), %rax > pushq %rax > ANNOTATE_UNRET_SAFE > ret >
