Re: [PATCH v5 3/3] x86/tdx: Add Quote generation support

[Kai Huang]

On Thu, 2022-05-05 at 16:06 -0700, Dave Hansen wrote:
> On 5/5/22 15:15, Kai Huang wrote:
> > set_memory_xx()  is supposedly only for direct-mapping.  Please use my
> > suggestion above.
> 
> Kai, please take a look at some of the other users, especially
> set_memory_x().  See how long the "supposed" requirement holds up.
> 
> That said, I've forgotten by now if this _could_ have used vmalloc() or
> vmap() or vmap_pfn().  None of the logic about why or how the allocator
> and mapping design decisions were made.  Could that be be rectified for
> the next post?

Hi Dave,

(Sorry previous reply was too fast..)

I spent some time looking into how __change_page_attr_set_clr() is implemented,
which is called by all set_memory_xxx() variants.  If my understanding is
correct, __change_page_attr_set_clr() will work for vmap() variants, because it
internally uses lookup_address(), which walks the page table directly, to find
the actual PTE (and PFN).  So set_memory_decrypted() can be fixed to support
vmap() mapping for TDX.

However, looking at the code, set_memory_decrypted() calls
__change_page_attr_set_clr(&cpa, 1).  The second argument is 'checkalias', which
means even we call set_memory_decrypted() against vmap() address, the aliasing
mappings will be changed too.  And if I understand correctly, the aliasing
mapping includes direct-mapping too:

static int cpa_process_alias(struct cpa_data *cpa)                             
{                                                                              
        struct cpa_data alias_cpa;
        unsigned long laddr = (unsigned long)__va(cpa->pfn << PAGE_SHIFT);     
        unsigned long vaddr;
        int ret;                                                               
                                                                                                                                                   
        if (!pfn_range_is_mapped(cpa->pfn, cpa->pfn + 1))                      
                return 0;                                                      

        /*
         * No need to redo, when the primary call touched the direct           
         * mapping already:                                                    
         */
        vaddr = __cpa_addr(cpa, cpa->curpage);                                 
        if (!(within(vaddr, PAGE_OFFSET,
                    PAGE_OFFSET + (max_pfn_mapped << PAGE_SHIFT)))) {
                
                alias_cpa = *cpa; 
                alias_cpa.vaddr = &laddr;
                alias_cpa.flags &= ~(CPA_PAGES_ARRAY | CPA_ARRAY);             
                alias_cpa.curpage = 0;                                         
                
                cpa->force_flush_all = 1;                                      

                ret = __change_page_attr_set_clr(&alias_cpa, 0);               
                if (ret)                                                       
                        return ret;
        }                                                                      

#ifdef CONFIG_X86_64                                                           
        /*
         * If the primary call didn't touch the high mapping already           
         * and the physical address is inside the kernel map, we need          
         * to touch the high mapped kernel as well:
         */                                                                    
        if (!within(vaddr, (unsigned long)_text, _brk_end) &&                  
            __cpa_pfn_in_highmap(cpa->pfn)) {                                  
                unsigned long temp_cpa_vaddr = (cpa->pfn << PAGE_SHIFT) +      
                                               __START_KERNEL_map - phys_base; 
                alias_cpa = *cpa;                                              
                alias_cpa.vaddr = &temp_cpa_vaddr;                             
                alias_cpa.flags &= ~(CPA_PAGES_ARRAY | CPA_ARRAY);
                alias_cpa.curpage = 0;                                         

                cpa->force_flush_all = 1;
                /*
                 * The high mapping range is imprecise, so ignore the
                 * return value.
                 */
                __change_page_attr_set_clr(&alias_cpa, 0);
        }
#endif

        return 0;
}

As you can see, the first chunk checks whether the virtual address is direct-
mapping, and if it is not, direct mapping is changed too.

The second chunk even changes the high kernel mapping.

So, if we use set_memory_decrypted(), there's no difference whether the address
is vmap() or direct mapping address.  The direct mapping will be changed anyway.

(However, it seems if we call set_memory_decrypted() against direct mapping
address, the vmap() mapping won't be impacted, because it seems
cpa_process_alias() doesn't check vmap() area..).

However I may have missed something.  Kirill please help to confirm if you see
this.

-- 
Thanks,
-Kai