Re: [PATCH 3/3] [RFC] x86/devmem: remove low 1MB hack for x86-64

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Jun 3, 2025, at 20:18, Dan Williams wrote:
> [add Naveen]
>
> Arnd Bergmann wrote:
>> On Thu, May 22, 2025, at 00:14, Dan Williams wrote:
>> > Arnd Bergmann wrote:
>>
>> The third one maps the BIOS area at 0xf0000, and as far as I can tell
>> the hack explicitly allowed mapping that even though it is marked
>> busy on x86-64 since 5d94e81f69d4 ("x86: Introduce pci_map_biosrom()").
>> 
>> Is there any downside to marking this one non-busy and still allowing
>> the ROM to be mapped? Would that bring back the issue of conflicting
>> mapping flags between kernel and userspace?
>
> For the confidential VM case I expect the answer is "yes" per this patch
> attempt:
>
> http://lore.kernel.org/20250403120228.2344377-1-naveen@kernel.org

I thought the problem here was the read() on /dev/mem, not
the mmap(), are you sure it's both?

With this patch [3/3], the memremap() hack for mem_read() goes away on
64-bit, so there should be no way it gets mapped again using that,
and the generic devmem_is_allowed() just forbids it as well.

The mmap() access in turn goes through this function

pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
                                unsigned long size, pgprot_t vma_prot)
{
        if (!phys_mem_access_encrypted(pfn << PAGE_SHIFT, size))
                vma_prot = pgprot_decrypted(vma_prot);

        return vma_prot;
}

which I would expect to return the correct vma_prot value already.

      Arnd