Re: [PATCH v2] x86, efi: never relocate kernel below lowest acceptable address

From: Ard Biesheuvel
Date: Wed Sep 25 2019 - 11:25:27 EST

On Thu, 19 Sep 2019 at 18:06, Kairui Song <kasong@xxxxxxxxxx> wrote:
>
> Currently, kernel fails to boot on some HyperV VMs when using EFI.
> And it's a potential issue on all platforms.
>
> It's caused a broken kernel relocation on EFI systems, when below three
> conditions are met:
>
> 1. Kernel image is not loaded to the default address (LOAD_PHYSICAL_ADDR)
> by the loader.
> 2. There isn't enough room to contain the kernel, starting from the
> default load address (eg. something else occupied part the region).
> 3. In the memmap provided by EFI firmware, there is a memory region
> starts below LOAD_PHYSICAL_ADDR, and suitable for containing the
> kernel.
>
> Efi stub will perform a kernel relocation when condition 1 is met. But
> due to condition 2, efi stub can't relocate kernel to the preferred
> address, so it fallback to query and alloc from EFI firmware for lowest
> usable memory region.
>
> It's incorrect to use the lowest memory address. In later stage, kernel
> will assume LOAD_PHYSICAL_ADDR as the minimal acceptable relocate address,
> but efi stub will end up relocating kernel below it.
>
> Then before the kernel decompressing. Kernel will do another relocation
> to address not lower than LOAD_PHYSICAL_ADDR, this time the relocate will
> over write the blockage at the default load address, which efi stub tried
> to avoid, and lead to unexpected behavior. Beside, the memory region it
> writes to is not allocated from EFI firmware, which is also wrong.
>
> To fix it, just don't let efi stub relocate the kernel to any address
> lower than lowest acceptable address.
>
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>
>

Hello Kairui,

This patch looks correct to me, but it needs an ack from the x86
maintainers, since the rules around LOAD_PHYSICAL_ADDR are specific to
the x86 architecture.


> ---
>
> Update from V1:
> - Redo the commit message.
>
> arch/x86/boot/compressed/eboot.c | 8 +++++---
> drivers/firmware/efi/libstub/arm32-stub.c | 2 +-
> drivers/firmware/efi/libstub/arm64-stub.c | 2 +-
> drivers/firmware/efi/libstub/efi-stub-helper.c | 12 ++++++++----
> include/linux/efi.h | 5 +++--
> 5 files changed, 18 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/eboot.c b/arch/x86/boot/compressed/eboot.c
> index 936bdb924ec2..8207e8aa297e 100644
> --- a/arch/x86/boot/compressed/eboot.c
> +++ b/arch/x86/boot/compressed/eboot.c
> @@ -13,6 +13,7 @@
> #include <asm/e820/types.h>
> #include <asm/setup.h>
> #include <asm/desc.h>
> +#include <asm/boot.h>
>
> #include "../string.h"
> #include "eboot.h"
> @@ -432,7 +433,7 @@ struct boot_params *make_boot_params(struct efi_config *c)
> }
>
> status = efi_low_alloc(sys_table, 0x4000, 1,
> - (unsigned long *)&boot_params);
> + (unsigned long *)&boot_params, 0);
> if (status != EFI_SUCCESS) {
> efi_printk(sys_table, "Failed to allocate lowmem for boot params\n");
> return NULL;
> @@ -817,7 +818,7 @@ efi_main(struct efi_config *c, struct boot_params *boot_params)
>
> gdt->size = 0x800;
> status = efi_low_alloc(sys_table, gdt->size, 8,
> - (unsigned long *)&gdt->address);
> + (unsigned long *)&gdt->address, 0);
> if (status != EFI_SUCCESS) {
> efi_printk(sys_table, "Failed to allocate memory for 'gdt'\n");
> goto fail;
> @@ -842,7 +843,8 @@ efi_main(struct efi_config *c, struct boot_params *boot_params)
> status = efi_relocate_kernel(sys_table, &bzimage_addr,
> hdr->init_size, hdr->init_size,
> hdr->pref_address,
> - hdr->kernel_alignment);
> + hdr->kernel_alignment,
> + LOAD_PHYSICAL_ADDR);
> if (status != EFI_SUCCESS) {
> efi_printk(sys_table, "efi_relocate_kernel() failed!\n");
> goto fail;
> diff --git a/drivers/firmware/efi/libstub/arm32-stub.c b/drivers/firmware/efi/libstub/arm32-stub.c
> index e8f7aefb6813..bf6f954d6afe 100644
> --- a/drivers/firmware/efi/libstub/arm32-stub.c
> +++ b/drivers/firmware/efi/libstub/arm32-stub.c
> @@ -220,7 +220,7 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table,
> *image_size = image->image_size;
> status = efi_relocate_kernel(sys_table, image_addr, *image_size,
> *image_size,
> - dram_base + MAX_UNCOMP_KERNEL_SIZE, 0);
> + dram_base + MAX_UNCOMP_KERNEL_SIZE, 0, 0);
> if (status != EFI_SUCCESS) {
> pr_efi_err(sys_table, "Failed to relocate kernel.\n");
> efi_free(sys_table, *reserve_size, *reserve_addr);
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index 1550d244e996..3d2e517e10f4 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -140,7 +140,7 @@ efi_status_t handle_kernel_image(efi_system_table_t *sys_table_arg,
> if (status != EFI_SUCCESS) {
> *reserve_size = kernel_memsize + TEXT_OFFSET;
> status = efi_low_alloc(sys_table_arg, *reserve_size,
> - MIN_KIMG_ALIGN, reserve_addr);
> + MIN_KIMG_ALIGN, reserve_addr, 0);
>
> if (status != EFI_SUCCESS) {
> pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index 3caae7f2cf56..00b00a2562aa 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -260,11 +260,11 @@ efi_status_t efi_high_alloc(efi_system_table_t *sys_table_arg,
> }
>
> /*
> - * Allocate at the lowest possible address.
> + * Allocate at the lowest possible address that is not below 'min'.
> */
> efi_status_t efi_low_alloc(efi_system_table_t *sys_table_arg,
> unsigned long size, unsigned long align,
> - unsigned long *addr)
> + unsigned long *addr, unsigned long min)
> {
> unsigned long map_size, desc_size, buff_size;
> efi_memory_desc_t *map;
> @@ -311,6 +311,9 @@ efi_status_t efi_low_alloc(efi_system_table_t *sys_table_arg,
> start = desc->phys_addr;
> end = start + desc->num_pages * EFI_PAGE_SIZE;
>
> + if (start < min)
> + start = min;
> +
> /*
> * Don't allocate at 0x0. It will confuse code that
> * checks pointers against NULL. Skip the first 8
> @@ -698,7 +701,8 @@ efi_status_t efi_relocate_kernel(efi_system_table_t *sys_table_arg,
> unsigned long image_size,
> unsigned long alloc_size,
> unsigned long preferred_addr,
> - unsigned long alignment)
> + unsigned long alignment,
> + unsigned long min_addr)
> {
> unsigned long cur_image_addr;
> unsigned long new_addr = 0;
> @@ -732,7 +736,7 @@ efi_status_t efi_relocate_kernel(efi_system_table_t *sys_table_arg,
> */
> if (status != EFI_SUCCESS) {
> status = efi_low_alloc(sys_table_arg, alloc_size, alignment,
> - &new_addr);
> + &new_addr, min_addr);
> }
> if (status != EFI_SUCCESS) {
> pr_efi_err(sys_table_arg, "Failed to allocate usable memory for kernel.\n");
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index f87fabea4a85..cc947c0f3e06 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -1587,7 +1587,7 @@ efi_status_t efi_get_memory_map(efi_system_table_t *sys_table_arg,
>
> efi_status_t efi_low_alloc(efi_system_table_t *sys_table_arg,
> unsigned long size, unsigned long align,
> - unsigned long *addr);
> + unsigned long *addr, unsigned long min);
>
> efi_status_t efi_high_alloc(efi_system_table_t *sys_table_arg,
> unsigned long size, unsigned long align,
> @@ -1598,7 +1598,8 @@ efi_status_t efi_relocate_kernel(efi_system_table_t *sys_table_arg,
> unsigned long image_size,
> unsigned long alloc_size,
> unsigned long preferred_addr,
> - unsigned long alignment);
> + unsigned long alignment,
> + unsigned long min_addr);
>
> efi_status_t handle_cmdline_files(efi_system_table_t *sys_table_arg,
> efi_loaded_image_t *image,
> --
> 2.21.0
>