Re: [PATCH v4 08/26] x86/boot: Map memory explicitly

[Evgeniy Baskov <baskov@ispras.ru>]

On 2023-03-08 13:28, Ard Biesheuvel wrote:
> On Wed, 8 Mar 2023 at 10:38, Ard Biesheuvel <ardb@kernel.org> wrote:
>> 
>> On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@ispras.ru> wrote:
>> >
>> > Implicit mappings hide possible memory errors, e.g. allocations for
>> > ACPI tables were not included in boot page table size.
>> >
>> > Replace all implicit mappings from page fault handler with
>> > explicit mappings.
>> >
>> 
>> I agree with the motivation but this patch seems to break the boot
>> under SeaBIOS/QEMU, and I imagine other legacy BIOS boot scenarios as
>> well.
>> 
>> Naively, I would assume that there is simply a legacy BIOS region that
>> we fail to map here, but I am fairly clueless when it comes to non-EFI
>> x86 boot so take this with a grain of salt.
>> 
> 
> The below seems to help - not sure why exactly, but apparently legacy
> BIOS needs the bootparams struct to be mapped writable?

I think I got too eager adding mappings to everything.
In the process_efi_entries() bootparams should already be mapped, so
I will just remove the call. And AFAIK bootparams is indeed gets
written to.

> 
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -31,6 +31,7 @@
>  #include <linux/ctype.h>
>  #include <generated/utsversion.h>
>  #include <generated/utsrelease.h>
> +#include <asm/shared/pgtable.h>
> 
>  #define _SETUP
>  #include <asm/setup.h> /* For COMMAND_LINE_SIZE */
> @@ -688,7 +689,7 @@ process_efi_entries(unsigned long minimum,
> unsigned long image_size)
>         u32 nr_desc;
>         int i;
> 
> -       kernel_add_identity_map((unsigned long)e, (unsigned long)(e + 
> 1), 0);
> +       kernel_add_identity_map((unsigned long)e, (unsigned long)(e +
> 1), MAP_WRITE);
> 
>         signature = (char *)&e->efi_loader_signature;
>         if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&

Thanks,
Evgeniy Baskov