Re: [PATCHv3 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition_pgtable()

[Huang, Kai]

On Mon, 2024-08-19 at 10:08 +0300, Kirill A. Shutemov wrote:
> The init_transition_pgtable() function sets up transitional page tables.
> It ensures that the relocate_kernel() function is present in the
> identity mapping at the same location as in the kernel page tables.
> relocate_kernel() switches to the identity mapping, and the function
> must be present at the same location in the virtual address space before
> and after switching page tables.
> 
> init_transition_pgtable() maps a copy of relocate_kernel() in
> image->control_code_page at the relocate_kernel() virtual address, but
> the original physical address of relocate_kernel() would also work.
> 
> It is safe to use original relocate_kernel() physical address cannot be
> overwritten until swap_pages() is called, and the relocate_kernel()
> virtual address will not be used by then.
> 
> Map the original relocate_kernel() at the relocate_kernel() virtual
> address in the identity mapping. It is preparation to replace the
> init_transition_pgtable() implementation with a call to
> kernel_ident_mapping_init().
> 
> Note that while relocate_kernel() switches to the identity mapping, it
> does not flush global TLB entries (CR4.PGE is not cleared). This means
> that in most cases, the kernel still runs relocate_kernel() from the
> original physical address before the change.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/machine_kexec_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 9c9ac606893e..645690e81c2d 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -157,7 +157,7 @@ static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
>  	pte_t *pte;
>  
>  	vaddr = (unsigned long)relocate_kernel;
> -	paddr = __pa(page_address(image->control_code_page)+PAGE_SIZE);
> +	paddr = __pa(relocate_kernel);
>  	pgd += pgd_index(vaddr);
>  	if (!pgd_present(*pgd)) {
>  		p4d = (p4d_t *)get_zeroed_page(GFP_KERNEL);


IIUC, this breaks KEXEC_JUMP (image->preserve_context is true).

The relocate_kernel() first saves couple of regs and some other data like PA
of swap page to the control page.  Note here the VA_CONTROL_PAGE is used to
access the control page, so those data are saved to the control page.

SYM_CODE_START_NOALIGN(relocate_kernel)
        UNWIND_HINT_END_OF_STACK
        ANNOTATE_NOENDBR
        /*      
         * %rdi indirection_page
         * %rsi page_list
         * %rdx start address
         * %rcx preserve_context
         * %r8  bare_metal
         */

	...

        movq    PTR(VA_CONTROL_PAGE)(%rsi), %r11                             
        movq    %rsp, RSP(%r11)                                              
        movq    %cr0, %rax
        movq    %rax, CR0(%r11)
        movq    %cr3, %rax
        movq    %rax, CR3(%r11)
        movq    %cr4, %rax
        movq    %rax, CR4(%r11)

	...

	/*
         * get physical address of control page now
         * this is impossible after page table switch
         */
        movq    PTR(PA_CONTROL_PAGE)(%rsi), %r8

        /* get physical address of page table now too */
        movq    PTR(PA_TABLE_PAGE)(%rsi), %r9

        /* get physical address of swap page now */
        movq    PTR(PA_SWAP_PAGE)(%rsi), %r10

        /* save some information for jumping back */
        movq    %r9, CP_PA_TABLE_PAGE(%r11)
        movq    %r10, CP_PA_SWAP_PAGE(%r11)
        movq    %rdi, CP_PA_BACKUP_PAGES_MAP(%r11)

	...

And after jumping back from the second kernel, relocate_kernel() tries to
restore the saved data:

	...

        /* get the re-entry point of the peer system */
        movq    0(%rsp), %rbp
        leaq    relocate_kernel(%rip), %r8		<---------  (*) 
        movq    CP_PA_SWAP_PAGE(%r8), %r10
        movq    CP_PA_BACKUP_PAGES_MAP(%r8), %rdi
        movq    CP_PA_TABLE_PAGE(%r8), %rax
        movq    %rax, %cr3
        lea     PAGE_SIZE(%r8), %rsp
        call    swap_pages
        movq    $virtual_mapped, %rax
        pushq   %rax
        ANNOTATE_UNRET_SAFE
        ret
        int3
SYM_CODE_END(identity_mapped)

Note the above code (*) uses the VA of relocate_kernel() to access the control
page.  IIUC, that means if we map VA of relocate_kernel() to the original PA
where the code relocate_kernel() resides, then the above code will never be
able to read those data back since they were saved to the control page.

Did I miss anything?