[v2,18/20] x86: efistub: Avoid legacy decompressor when doing EFI boot

  The bare metal decompressor code was never really intended to run in a
hosted environment such as the EFI boot services, and does a few things
that are problematic in the context of EFI boot now that the logo
requirements are getting tighter.

In particular, the decompressor moves its own executable image around in
memory, and relies on demand paging to populate the identity mappings,
and these things are difficult to support in a context where memory is
not permitted to be mapped writable and executable at the same time or,
at the very least, is mapped non-executable by default, and needs
special treatment for this restriction to be lifted.

Since EFI already maps all of memory 1:1, we don't need to create new
page tables or handle page faults when decompressing the kernel. That
means there is also no need to replace the special exception handlers
for SEV. Generally, there is little need to do anything that the
decompressor does beyond

- initialize SEV encryption, if needed,
- perform the 4/5 level paging switch, if needed,
- decompress the kernel
- relocate the kernel

So let's do all of this from the EFI stub code, and avoid the bare metal
decompressor altogether.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/boot/compressed/Makefile       |   5 +
 arch/x86/boot/compressed/efi_mixed.S    |  58 +------
 arch/x86/boot/compressed/head_32.S      |  22 +--
 arch/x86/boot/compressed/head_64.S      |  34 ----
 arch/x86/include/asm/efi.h              |   7 +-
 drivers/firmware/efi/libstub/x86-stub.c | 176 +++++++++-----------
 6 files changed, 95 insertions(+), 207 deletions(-)
  

On 5/8/23 02:03, Ard Biesheuvel wrote:
> The bare metal decompressor code was never really intended to run in a
> hosted environment such as the EFI boot services, and does a few things
> that are problematic in the context of EFI boot now that the logo
> requirements are getting tighter.
> 
> In particular, the decompressor moves its own executable image around in
> memory, and relies on demand paging to populate the identity mappings,
> and these things are difficult to support in a context where memory is
> not permitted to be mapped writable and executable at the same time or,
> at the very least, is mapped non-executable by default, and needs
> special treatment for this restriction to be lifted.
> 
> Since EFI already maps all of memory 1:1, we don't need to create new
> page tables or handle page faults when decompressing the kernel. That
> means there is also no need to replace the special exception handlers
> for SEV. Generally, there is little need to do anything that the
> decompressor does beyond
> 
> - initialize SEV encryption, if needed,
> - perform the 4/5 level paging switch, if needed,
> - decompress the kernel
> - relocate the kernel
> 
> So let's do all of this from the EFI stub code, and avoid the bare metal
> decompressor altogether.

This patch crashes SEV guests, probably because of the BSS is accessed 
encrypted and results in ciphertext for what would be a zero for a non-SEV 
guest. After pushing patch #19 everything started working again. From a 
bisectability perspective, you probably want patch #19 before this one.

Thanks,
Tom

> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>   arch/x86/boot/compressed/Makefile       |   5 +
>   arch/x86/boot/compressed/efi_mixed.S    |  58 +------
>   arch/x86/boot/compressed/head_32.S      |  22 +--
>   arch/x86/boot/compressed/head_64.S      |  34 ----
>   arch/x86/include/asm/efi.h              |   7 +-
>   drivers/firmware/efi/libstub/x86-stub.c | 176 +++++++++-----------
>   6 files changed, 95 insertions(+), 207 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 6b6cfe607bdbdfaa..f3289f17f820f982 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -74,6 +74,11 @@ LDFLAGS_vmlinux += -z noexecstack
>   ifeq ($(CONFIG_LD_IS_BFD),y)
>   LDFLAGS_vmlinux += $(call ld-option,--no-warn-rwx-segments)
>   endif
> +ifeq ($(CONFIG_EFI_STUB),y)
> +# ensure that we'll pull in the static EFI stub library even if we never
> +# reference it from the startup code
> +LDFLAGS_vmlinux += -u efi_pe_entry
> +endif
>   LDFLAGS_vmlinux += -T
>   
>   hostprogs	:= mkpiggy
> diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
> index 4ca70bf93dc0bdcd..dec579eb1caa16d5 100644
> --- a/arch/x86/boot/compressed/efi_mixed.S
> +++ b/arch/x86/boot/compressed/efi_mixed.S
> @@ -49,9 +49,12 @@ SYM_FUNC_START(startup_64_mixed_mode)
>   	lea	efi32_boot_args(%rip), %rdx
>   	mov	0(%rdx), %edi
>   	mov	4(%rdx), %esi
> +#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
>   	mov	8(%rdx), %edx		// saved bootparams pointer
>   	test	%edx, %edx
>   	jnz	efi64_stub_entry
> +#endif
> +
>   	/*
>   	 * efi_pe_entry uses MS calling convention, which requires 32 bytes of
>   	 * shadow space on the stack even if all arguments are passed in
> @@ -245,10 +248,6 @@ SYM_FUNC_START(efi32_entry)
>   	jmp	startup_32
>   SYM_FUNC_END(efi32_entry)
>   
> -#define ST32_boottime		60 // offsetof(efi_system_table_32_t, boottime)
> -#define BS32_handle_protocol	88 // offsetof(efi_boot_services_32_t, handle_protocol)
> -#define LI32_image_base		32 // offsetof(efi_loaded_image_32_t, image_base)
> -
>   /*
>    * efi_status_t efi32_pe_entry(efi_handle_t image_handle,
>    *			       efi_system_table_32_t *sys_table)
> @@ -256,8 +255,6 @@ SYM_FUNC_END(efi32_entry)
>   SYM_FUNC_START(efi32_pe_entry)
>   	pushl	%ebp
>   	movl	%esp, %ebp
> -	pushl	%eax				// dummy push to allocate loaded_image
> -
>   	pushl	%ebx				// save callee-save registers
>   	pushl	%edi
>   
> @@ -266,48 +263,8 @@ SYM_FUNC_START(efi32_pe_entry)
>   	movl	$0x80000003, %eax		// EFI_UNSUPPORTED
>   	jnz	2f
>   
> -	call	1f
> -1:	pop	%ebx
> -
> -	/* Get the loaded image protocol pointer from the image handle */
> -	leal	-4(%ebp), %eax
> -	pushl	%eax				// &loaded_image
> -	leal	(loaded_image_proto - 1b)(%ebx), %eax
> -	pushl	%eax				// pass the GUID address
> -	pushl	8(%ebp)				// pass the image handle
> -
> -	/*
> -	 * Note the alignment of the stack frame.
> -	 *   sys_table
> -	 *   handle             <-- 16-byte aligned on entry by ABI
> -	 *   return address
> -	 *   frame pointer
> -	 *   loaded_image       <-- local variable
> -	 *   saved %ebx		<-- 16-byte aligned here
> -	 *   saved %edi
> -	 *   &loaded_image
> -	 *   &loaded_image_proto
> -	 *   handle             <-- 16-byte aligned for call to handle_protocol
> -	 */
> -
> -	movl	12(%ebp), %eax			// sys_table
> -	movl	ST32_boottime(%eax), %eax	// sys_table->boottime
> -	call	*BS32_handle_protocol(%eax)	// sys_table->boottime->handle_protocol
> -	addl	$12, %esp			// restore argument space
> -	testl	%eax, %eax
> -	jnz	2f
> -
>   	movl	8(%ebp), %ecx			// image_handle
>   	movl	12(%ebp), %edx			// sys_table
> -	movl	-4(%ebp), %esi			// loaded_image
> -	movl	LI32_image_base(%esi), %esi	// loaded_image->image_base
> -	leal	(startup_32 - 1b)(%ebx), %ebp	// runtime address of startup_32
> -	/*
> -	 * We need to set the image_offset variable here since startup_32() will
> -	 * use it before we get to the 64-bit efi_pe_entry() in C code.
> -	 */
> -	subl	%esi, %ebp			// calculate image_offset
> -	movl	%ebp, (image_offset - 1b)(%ebx)	// save image_offset
>   	xorl	%esi, %esi
>   	jmp	efi32_entry			// pass %ecx, %edx, %esi
>   						// no other registers remain live
> @@ -318,15 +275,6 @@ SYM_FUNC_START(efi32_pe_entry)
>   	RET
>   SYM_FUNC_END(efi32_pe_entry)
>   
> -	.section ".rodata"
> -	/* EFI loaded image protocol GUID */
> -	.balign 4
> -SYM_DATA_START_LOCAL(loaded_image_proto)
> -	.long	0x5b1b31a1
> -	.word	0x9562, 0x11d2
> -	.byte	0x8e, 0x3f, 0x00, 0xa0, 0xc9, 0x69, 0x72, 0x3b
> -SYM_DATA_END(loaded_image_proto)
> -
>   	.data
>   	.balign	8
>   SYM_DATA_START_LOCAL(efi32_boot_gdt)
> diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> index 8ee8749007595fcc..3f9b80726070a8e7 100644
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@ -84,19 +84,6 @@ SYM_FUNC_START(startup_32)
>   
>   #ifdef CONFIG_RELOCATABLE
>   	leal	startup_32@GOTOFF(%edx), %ebx
> -
> -#ifdef CONFIG_EFI_STUB
> -/*
> - * If we were loaded via the EFI LoadImage service, startup_32() will be at an
> - * offset to the start of the space allocated for the image. efi_pe_entry() will
> - * set up image_offset to tell us where the image actually starts, so that we
> - * can use the full available buffer.
> - *	image_offset = startup_32 - image_base
> - * Otherwise image_offset will be zero and has no effect on the calculations.
> - */
> -	subl    image_offset@GOTOFF(%edx), %ebx
> -#endif
> -
>   	movl	BP_kernel_alignment(%esi), %eax
>   	decl	%eax
>   	addl    %eax, %ebx
> @@ -150,15 +137,10 @@ SYM_FUNC_START(startup_32)
>   	jmp	*%eax
>   SYM_FUNC_END(startup_32)
>   
> -#ifdef CONFIG_EFI_STUB
> +#ifdef CONFIG_EFI_HANDOVER_PROTOCOL
>   SYM_FUNC_START(efi32_stub_entry)
> -	add	$0x4, %esp
> -	movl	8(%esp), %esi	/* save boot_params pointer */
> -	call	efi_main
> -	/* efi_main returns the possibly relocated address of startup_32 */
> -	jmp	*%eax
> +	jmp	efi_main
>   SYM_FUNC_END(efi32_stub_entry)
> -SYM_FUNC_ALIAS(efi_stub_entry, efi32_stub_entry)
>   #endif
>   
>   	.text
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index bcf678aed81468e3..320e2825ff0b32da 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -146,19 +146,6 @@ SYM_FUNC_START(startup_32)
>   
>   #ifdef CONFIG_RELOCATABLE
>   	movl	%ebp, %ebx
> -
> -#ifdef CONFIG_EFI_STUB
> -/*
> - * If we were loaded via the EFI LoadImage service, startup_32 will be at an
> - * offset to the start of the space allocated for the image. efi_pe_entry will
> - * set up image_offset to tell us where the image actually starts, so that we
> - * can use the full available buffer.
> - *	image_offset = startup_32 - image_base
> - * Otherwise image_offset will be zero and has no effect on the calculations.
> - */
> -	subl    rva(image_offset)(%ebp), %ebx
> -#endif
> -
>   	movl	BP_kernel_alignment(%esi), %eax
>   	decl	%eax
>   	addl	%eax, %ebx
> @@ -346,20 +333,6 @@ SYM_CODE_START(startup_64)
>   	/* Start with the delta to where the kernel will run at. */
>   #ifdef CONFIG_RELOCATABLE
>   	leaq	startup_32(%rip) /* - $startup_32 */, %rbp
> -
> -#ifdef CONFIG_EFI_STUB
> -/*
> - * If we were loaded via the EFI LoadImage service, startup_32 will be at an
> - * offset to the start of the space allocated for the image. efi_pe_entry will
> - * set up image_offset to tell us where the image actually starts, so that we
> - * can use the full available buffer.
> - *	image_offset = startup_32 - image_base
> - * Otherwise image_offset will be zero and has no effect on the calculations.
> - */
> -	movl    image_offset(%rip), %eax
> -	subq	%rax, %rbp
> -#endif
> -
>   	movl	BP_kernel_alignment(%rsi), %eax
>   	decl	%eax
>   	addq	%rax, %rbp
> @@ -481,19 +454,12 @@ SYM_CODE_START(startup_64)
>   	jmp	*%rax
>   SYM_CODE_END(startup_64)
>   
> -#ifdef CONFIG_EFI_STUB
>   #ifdef CONFIG_EFI_HANDOVER_PROTOCOL
>   	.org 0x390
> -#endif
>   SYM_FUNC_START(efi64_stub_entry)
>   	and	$~0xf, %rsp			/* realign the stack */
> -	movq	%rdx, %rbx			/* save boot_params pointer */
>   	call	efi_main
> -	movq	%rbx,%rsi
> -	leaq	rva(startup_64)(%rax), %rax
> -	jmp	*%rax
>   SYM_FUNC_END(efi64_stub_entry)
> -SYM_FUNC_ALIAS(efi_stub_entry, efi64_stub_entry)
>   #endif
>   
>   	.text
> diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
> index 419280d263d2e3f2..24fa828eeef7d9dd 100644
> --- a/arch/x86/include/asm/efi.h
> +++ b/arch/x86/include/asm/efi.h
> @@ -88,6 +88,8 @@ static inline void efi_fpu_end(void)
>   }
>   
>   #ifdef CONFIG_X86_32
> +#define EFI_X86_KERNEL_ALLOC_LIMIT		(SZ_512M - 1)
> +
>   #define arch_efi_call_virt_setup()					\
>   ({									\
>   	efi_fpu_begin();						\
> @@ -101,8 +103,7 @@ static inline void efi_fpu_end(void)
>   })
>   
>   #else /* !CONFIG_X86_32 */
> -
> -#define EFI_LOADER_SIGNATURE	"EL64"
> +#define EFI_X86_KERNEL_ALLOC_LIMIT		EFI_ALLOC_LIMIT
>   
>   extern asmlinkage u64 __efi_call(void *fp, ...);
>   
> @@ -216,6 +217,8 @@ efi_status_t efi_set_virtual_address_map(unsigned long memory_map_size,
>   
>   #ifdef CONFIG_EFI_MIXED
>   
> +#define EFI_ALLOC_LIMIT		(efi_is_64bit() ? ULONG_MAX : U32_MAX)
> +
>   #define ARCH_HAS_EFISTUB_WRAPPERS
>   
>   static inline bool efi_is_64bit(void)
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index 33d11ba78f1d8c4f..59076e16c1ac11ee 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -15,16 +15,13 @@
>   #include <asm/setup.h>
>   #include <asm/desc.h>
>   #include <asm/boot.h>
> +#include <asm/kaslr.h>
>   #include <asm/sev.h>
>   
>   #include "efistub.h"
>   
> -/* Maximum physical address for 64-bit kernel with 4-level paging */
> -#define MAXMEM_X86_64_4LEVEL (1ull << 46)
> -
>   const efi_system_table_t *efi_system_table;
>   const efi_dxe_services_table_t *efi_dxe_table;
> -u32 image_offset __section(".data");
>   static efi_loaded_image_t *image = NULL;
>   static efi_memory_attribute_protocol_t *memattr;
>   
> @@ -275,33 +272,9 @@ adjust_memory_range_protection(unsigned long start, unsigned long size)
>   	}
>   }
>   
> -void startup_32(struct boot_params *boot_params);
> -
> -static void
> -setup_memory_protection(unsigned long image_base, unsigned long image_size)
> -{
> -#ifdef CONFIG_64BIT
> -	if (image_base != (unsigned long)startup_32)
> -		adjust_memory_range_protection(image_base, image_size);
> -#else
> -	/*
> -	 * Clear protection flags on a whole range of possible
> -	 * addresses used for KASLR. We don't need to do that
> -	 * on x86_64, since KASLR/extraction is performed after
> -	 * dedicated identity page tables are built and we only
> -	 * need to remove possible protection on relocated image
> -	 * itself disregarding further relocations.
> -	 */
> -	adjust_memory_range_protection(LOAD_PHYSICAL_ADDR,
> -				       KERNEL_IMAGE_SIZE - LOAD_PHYSICAL_ADDR);
> -#endif
> -}
> -
>   static const efi_char16_t apple[] = L"Apple";
>   
> -static void setup_quirks(struct boot_params *boot_params,
> -			 unsigned long image_base,
> -			 unsigned long image_size)
> +static void setup_quirks(struct boot_params *boot_params)
>   {
>   	efi_char16_t *fw_vendor = (efi_char16_t *)(unsigned long)
>   		efi_table_attr(efi_system_table, fw_vendor);
> @@ -310,9 +283,6 @@ static void setup_quirks(struct boot_params *boot_params,
>   		if (IS_ENABLED(CONFIG_APPLE_PROPERTIES))
>   			retrieve_apple_device_properties(boot_params);
>   	}
> -
> -	if (IS_ENABLED(CONFIG_EFI_DXE_MEM_ATTRIBUTES))
> -		setup_memory_protection(image_base, image_size);
>   }
>   
>   /*
> @@ -432,6 +402,7 @@ static void __noreturn efi_exit(efi_handle_t handle, efi_status_t status)
>   		asm("hlt");
>   }
>   
> +static __alias(efi_main)
>   void __noreturn efi_stub_entry(efi_handle_t handle,
>   			       efi_system_table_t *sys_table_arg,
>   			       struct boot_params *boot_params);
> @@ -465,7 +436,6 @@ efi_status_t __efiapi efi_pe_entry(efi_handle_t handle,
>   	}
>   
>   	image_base = efi_table_attr(image, image_base);
> -	image_offset = (void *)startup_32 - image_base;
>   
>   	status = efi_allocate_pages(sizeof(struct boot_params),
>   				    (unsigned long *)&boot_params, ULONG_MAX);
> @@ -853,20 +823,82 @@ static void efi_5level_switch(void)
>   #endif
>   }
>   
> +static void efi_get_seed(void *seed, int size)
> +{
> +	efi_get_random_bytes(size, seed);
> +
> +	/*
> +	 * This only updates seed[0] when running on 32-bit, but in that case,
> +	 * we don't use seed[1] anyway, as there is no virtual KASLR on 32-bit.
> +	 */
> +	*(unsigned long *)seed ^= kaslr_get_random_long("EFI");
> +}
> +
> +static void error(char *str)
> +{
> +	efi_warn("Decompression failed: %s\n", str);
> +}
> +
> +static efi_status_t efi_decompress_kernel(unsigned long *kernel_entry)
> +{
> +	unsigned long virt_addr = LOAD_PHYSICAL_ADDR;
> +	unsigned long addr, alloc_size, entry;
> +	efi_status_t status;
> +	u32 seed[2] = {};
> +
> +	/* determine the required size of the allocation */
> +	alloc_size = ALIGN(max((unsigned long)output_len, kernel_total_size),
> +			   MIN_KERNEL_ALIGN);
> +
> +	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && !efi_nokaslr) {
> +		u64 range = KERNEL_IMAGE_SIZE - LOAD_PHYSICAL_ADDR - kernel_total_size;
> +
> +		efi_get_seed(seed, sizeof(seed));
> +
> +		virt_addr += (range * seed[1]) >> 32;
> +		virt_addr &= ~(CONFIG_PHYSICAL_ALIGN - 1);
> +	}
> +
> +	status = efi_random_alloc(alloc_size, CONFIG_PHYSICAL_ALIGN, &addr,
> +				  seed[0], EFI_LOADER_CODE,
> +				  EFI_X86_KERNEL_ALLOC_LIMIT);
> +	if (status != EFI_SUCCESS)
> +		return status;
> +
> +	entry = decompress_kernel((void *)addr, virt_addr, error);
> +	if (entry == ULONG_MAX) {
> +		efi_free(alloc_size, addr);
> +		return EFI_LOAD_ERROR;
> +	}
> +
> +	*kernel_entry = addr + entry;
> +
> +	adjust_memory_range_protection(addr, kernel_total_size);
> +
> +	return EFI_SUCCESS;
> +}
> +
> +static void __noreturn enter_kernel(unsigned long kernel_addr,
> +				    struct boot_params *boot_params)
> +{
> +	/* enter decompressed kernel with boot_params pointer in RSI/ESI */
> +	asm("jmp *%0"::"r"(kernel_addr), "S"(boot_params));
> +
> +	unreachable();
> +}
> +
>   /*
> - * On success, we return the address of startup_32, which has potentially been
> - * relocated by efi_relocate_kernel.
> + * On success, we jump to the relocated kernel directly and never return.
>    * On failure, we exit to the firmware via efi_exit instead of returning.
>    */
> -asmlinkage unsigned long efi_main(efi_handle_t handle,
> -				  efi_system_table_t *sys_table_arg,
> -				  struct boot_params *boot_params)
> +asmlinkage void __noreturn efi_main(efi_handle_t handle,
> +				    efi_system_table_t *sys_table_arg,
> +				    struct boot_params *boot_params)
>   {
>   	efi_guid_t guid = EFI_MEMORY_ATTRIBUTE_PROTOCOL_GUID;
> -	unsigned long bzimage_addr = (unsigned long)startup_32;
> -	unsigned long buffer_start, buffer_end;
>   	struct setup_header *hdr = &boot_params->hdr;
>   	const struct linux_efi_initrd *initrd = NULL;
> +	unsigned long kernel_entry;
>   	efi_status_t status;
>   
>   	efi_system_table = sys_table_arg;
> @@ -892,60 +924,6 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>   		goto fail;
>   	}
>   
> -	/*
> -	 * If the kernel isn't already loaded at a suitable address,
> -	 * relocate it.
> -	 *
> -	 * It must be loaded above LOAD_PHYSICAL_ADDR.
> -	 *
> -	 * The maximum address for 64-bit is 1 << 46 for 4-level paging. This
> -	 * is defined as the macro MAXMEM, but unfortunately that is not a
> -	 * compile-time constant if 5-level paging is configured, so we instead
> -	 * define our own macro for use here.
> -	 *
> -	 * For 32-bit, the maximum address is complicated to figure out, for
> -	 * now use KERNEL_IMAGE_SIZE, which will be 512MiB, the same as what
> -	 * KASLR uses.
> -	 *
> -	 * Also relocate it if image_offset is zero, i.e. the kernel wasn't
> -	 * loaded by LoadImage, but rather by a bootloader that called the
> -	 * handover entry. The reason we must always relocate in this case is
> -	 * to handle the case of systemd-boot booting a unified kernel image,
> -	 * which is a PE executable that contains the bzImage and an initrd as
> -	 * COFF sections. The initrd section is placed after the bzImage
> -	 * without ensuring that there are at least init_size bytes available
> -	 * for the bzImage, and thus the compressed kernel's startup code may
> -	 * overwrite the initrd unless it is moved out of the way.
> -	 */
> -
> -	buffer_start = ALIGN(bzimage_addr - image_offset,
> -			     hdr->kernel_alignment);
> -	buffer_end = buffer_start + hdr->init_size;
> -
> -	if ((buffer_start < LOAD_PHYSICAL_ADDR)				     ||
> -	    (IS_ENABLED(CONFIG_X86_32) && buffer_end > KERNEL_IMAGE_SIZE)    ||
> -	    (IS_ENABLED(CONFIG_X86_64) && buffer_end > MAXMEM_X86_64_4LEVEL) ||
> -	    (image_offset == 0)) {
> -		extern char _bss[];
> -
> -		status = efi_relocate_kernel(&bzimage_addr,
> -					     (unsigned long)_bss - bzimage_addr,
> -					     hdr->init_size,
> -					     hdr->pref_address,
> -					     hdr->kernel_alignment,
> -					     LOAD_PHYSICAL_ADDR);
> -		if (status != EFI_SUCCESS) {
> -			efi_err("efi_relocate_kernel() failed!\n");
> -			goto fail;
> -		}
> -		/*
> -		 * Now that we've copied the kernel elsewhere, we no longer
> -		 * have a set up block before startup_32(), so reset image_offset
> -		 * to zero in case it was set earlier.
> -		 */
> -		image_offset = 0;
> -	}
> -
>   #ifdef CONFIG_CMDLINE_BOOL
>   	status = efi_parse_options(CONFIG_CMDLINE);
>   	if (status != EFI_SUCCESS) {
> @@ -963,6 +941,12 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>   		}
>   	}
>   
> +	status = efi_decompress_kernel(&kernel_entry);
> +	if (status != EFI_SUCCESS) {
> +		efi_err("Failed to decompress kernel\n");
> +		goto fail;
> +	}
> +
>   	/*
>   	 * At this point, an initrd may already have been loaded by the
>   	 * bootloader and passed via bootparams. We permit an initrd loaded
> @@ -1002,7 +986,7 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>   
>   	setup_efi_pci(boot_params);
>   
> -	setup_quirks(boot_params, bzimage_addr, buffer_end - buffer_start);
> +	setup_quirks(boot_params);
>   
>   	status = exit_boot(boot_params, handle);
>   	if (status != EFI_SUCCESS) {
> @@ -1012,7 +996,7 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>   
>   	efi_5level_switch();
>   
> -	return bzimage_addr;
> +	enter_kernel(kernel_entry, boot_params);
>   fail:
>   	efi_err("efi_main() failed!\n");
>
  

On Thu, 18 May 2023 at 22:48, Tom Lendacky <thomas.lendacky@amd.com> wrote:
>
> On 5/8/23 02:03, Ard Biesheuvel wrote:
> > The bare metal decompressor code was never really intended to run in a
> > hosted environment such as the EFI boot services, and does a few things
> > that are problematic in the context of EFI boot now that the logo
> > requirements are getting tighter.
> >
> > In particular, the decompressor moves its own executable image around in
> > memory, and relies on demand paging to populate the identity mappings,
> > and these things are difficult to support in a context where memory is
> > not permitted to be mapped writable and executable at the same time or,
> > at the very least, is mapped non-executable by default, and needs
> > special treatment for this restriction to be lifted.
> >
> > Since EFI already maps all of memory 1:1, we don't need to create new
> > page tables or handle page faults when decompressing the kernel. That
> > means there is also no need to replace the special exception handlers
> > for SEV. Generally, there is little need to do anything that the
> > decompressor does beyond
> >
> > - initialize SEV encryption, if needed,
> > - perform the 4/5 level paging switch, if needed,
> > - decompress the kernel
> > - relocate the kernel
> >
> > So let's do all of this from the EFI stub code, and avoid the bare metal
> > decompressor altogether.
>
> This patch crashes SEV guests, probably because of the BSS is accessed
> encrypted and results in ciphertext for what would be a zero for a non-SEV
> guest. After pushing patch #19 everything started working again. From a
> bisectability perspective, you probably want patch #19 before this one.
>

Noted, thanks.