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

["Huang, Kai" <kai.huang@intel.com>]

On Mon, 2024-08-19 at 14:57 +0300, kirill.shutemov@linux.intel.com wrote:
> On Mon, Aug 19, 2024 at 11:16:52AM +0000, Huang, Kai wrote:
> > 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@linux.intel.com>
> > > ---
> > >  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?
> 
> Note that relocate_kernel() usage at (*) is inside identity_mapped(). We
> run from identity mapping there. Nothing changed to identity mapping
> around relocate_kernel(), only top mapping (at __START_KERNEL_map) is
> affected.

Yes, but before this patch the VA of relocate_kernel() is mapped to the copied
one, which resides in the control page:

        control_page = page_address(image->control_code_page) + PAGE_SIZE;
        __memcpy(control_page, relocate_kernel, KEXEC_CONTROL_CODE_MAX_SIZE);
        
        page_list[PA_CONTROL_PAGE] = virt_to_phys(control_page);
        page_list[VA_CONTROL_PAGE] = (unsigned long)control_page;    

So the (*) can actually access to the control page IIUC.

Now if we change to map VA of relocate_kernel() to the original one, then (*)
won't be able to access the control page.

> 
> But I didn't test kexec jump thing. Do you (or anybody else) have setup to
> test it?
> 

No I don't know how to test either, just my understanding on the code :-(

Git blame says Ying is the original author, so +Ying here hoping he can
provide some insight.

Anyway, my opinion is we should do patch 4 first but still map VA of
relocate_kernel() to control page so there will be no functional change.  This
patchset is about to reduce duplicated code anyway.