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

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