linux-kernel - Re: [PATCH RFC 0/5] Handle UEFI NX-restricted page tables

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Date:   Wed, 10 Nov 2021 12:11:48 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Baskov Evgeniy <baskov@...ras.ru>
Cc:     Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 0/5] Handle UEFI NX-restricted page tables

On Wed, 10 Nov 2021 at 11:56, Baskov Evgeniy <baskov@...ras.ru> wrote:
>
> Note, that this patch series is RFC, since it is yet untested
> and possibly incompatible with AMD SEV and related extensions.
>
> The UEFI specification states that certain memory regions may
> not have every permission, i.e. may not be writable or executable.
>
> Furthermore there exist some implementations (at least on i386/x86_64)
> that restrict execution of memory regions expected by the kernel to
> be executable. E.g. first megabyte of address space, where trampoline
> for switching between 4/5 level paging is placed and memory regions,
> allocated as loader data.
>
> This patch series allows Linux kernel to boot on such UEFI
> implementations on i386 and x86_64.
>
> The simplest way to achieve that on i386 is to disable paging
> before jumping to potentially relocated code.
>
> x86_64, on the other hand, does not allow disabling paging so it
> is required to build temporary page tables containing memory regions
> required for Linux kernel to boot with appropriate access permissions.
>

Hello Baskov,

To be honest, I am truly not a fan of this approach.

Which systems is this issue occurring on? Did you try something like
the below to allocate executable memory explicitly?


diff --git a/drivers/firmware/efi/libstub/relocate.c
b/drivers/firmware/efi/libstub/relocate.c
index 8ee9eb2b9039..b73012a7bcdc 100644
--- a/drivers/firmware/efi/libstub/relocate.c
+++ b/drivers/firmware/efi/libstub/relocate.c
@@ -80,7 +80,7 @@ efi_status_t efi_low_alloc_above(unsigned long size,
unsigned long align,
                        continue;

                status = efi_bs_call(allocate_pages, EFI_ALLOCATE_ADDRESS,
-                                    EFI_LOADER_DATA, nr_pages, &start);
+                                    EFI_LOADER_CODE, nr_pages, &start);
                if (status == EFI_SUCCESS) {
                        *addr = start;
                        break;
@@ -146,7 +146,7 @@ efi_status_t efi_relocate_kernel(unsigned long *image_addr,
         */
        nr_pages = round_up(alloc_size, EFI_ALLOC_ALIGN) / EFI_PAGE_SIZE;
        status = efi_bs_call(allocate_pages, EFI_ALLOCATE_ADDRESS,
-                            EFI_LOADER_DATA, nr_pages, &efi_addr);
+                            EFI_LOADER_CODE, nr_pages, &efi_addr);
        new_addr = efi_addr;
        /*
         * If preferred address allocation failed allocate as low as



-- 
Ard.