linux-kernel - [RFC PATCH 2/9] x86/purgatory: Simplify stack handling

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Message-ID: <20240424155309.1719454-13-ardb+git@google.com>
Date: Wed, 24 Apr 2024 17:53:12 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Kees Cook <keescook@...omium.org>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Masahiro Yamada <masahiroy@...nel.org>
Subject: [RFC PATCH 2/9] x86/purgatory: Simplify stack handling

From: Ard Biesheuvel <ardb@...nel.org>

The x86 purgatory, which does little more than verify a SHA-256 hash of
the loaded segments, currently uses three different stacks:
- one in .bss that is used to call the purgatory C code
- one in .rodata that is only used to switch to an updated code segment
  descriptor in the GDT
- one in .data, which allows it to be prepopulated from the kexec loader
  in theory, but this is not actually being taken advantage of.

Simplify this, by dropping the latter two stacks, as well as the loader
logic that programs RSP.

Both the stacks in .bss and .data are 4k aligned, but 16 byte alignment
is more than sufficient.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/include/asm/kexec.h      |  1 -
 arch/x86/kernel/kexec-bzimage64.c |  8 --------
 arch/x86/purgatory/entry64.S      |  8 --------
 arch/x86/purgatory/setup-x86_64.S |  2 +-
 arch/x86/purgatory/stack.S        | 18 ------------------
 5 files changed, 1 insertion(+), 36 deletions(-)

diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
index 91ca9a9ee3a2..ee7b32565e5f 100644
--- a/arch/x86/include/asm/kexec.h
+++ b/arch/x86/include/asm/kexec.h
@@ -163,7 +163,6 @@ struct kexec_entry64_regs {
 	uint64_t rcx;
 	uint64_t rdx;
 	uint64_t rbx;
-	uint64_t rsp;
 	uint64_t rbp;
 	uint64_t rsi;
 	uint64_t rdi;
diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
index cde167b0ea92..f5bf1b7d01a6 100644
--- a/arch/x86/kernel/kexec-bzimage64.c
+++ b/arch/x86/kernel/kexec-bzimage64.c
@@ -400,7 +400,6 @@ static void *bzImage64_load(struct kimage *image, char *kernel,
 	unsigned long bootparam_load_addr, kernel_load_addr, initrd_load_addr;
 	struct bzimage64_data *ldata;
 	struct kexec_entry64_regs regs64;
-	void *stack;
 	unsigned int setup_hdr_offset = offsetof(struct boot_params, hdr);
 	unsigned int efi_map_offset, efi_map_sz, efi_setup_data_offset;
 	struct kexec_buf kbuf = { .image = image, .buf_max = ULONG_MAX,
@@ -550,14 +549,7 @@ static void *bzImage64_load(struct kimage *image, char *kernel,
 	regs64.rbx = 0; /* Bootstrap Processor */
 	regs64.rsi = bootparam_load_addr;
 	regs64.rip = kernel_load_addr + 0x200;
-	stack = kexec_purgatory_get_symbol_addr(image, "stack_end");
-	if (IS_ERR(stack)) {
-		pr_err("Could not find address of symbol stack_end\n");
-		ret = -EINVAL;
-		goto out_free_params;
-	}
 
-	regs64.rsp = (unsigned long)stack;
 	ret = kexec_purgatory_get_set_symbol(image, "entry64_regs", &regs64,
 					     sizeof(regs64), 0);
 	if (ret)
diff --git a/arch/x86/purgatory/entry64.S b/arch/x86/purgatory/entry64.S
index 0b4390ce586b..9913877b0dbe 100644
--- a/arch/x86/purgatory/entry64.S
+++ b/arch/x86/purgatory/entry64.S
@@ -26,8 +26,6 @@ SYM_CODE_START(entry64)
 	movl    %eax, %fs
 	movl    %eax, %gs
 
-	/* Setup new stack */
-	leaq    stack_init(%rip), %rsp
 	pushq   $0x10 /* CS */
 	leaq    new_cs_exit(%rip), %rax
 	pushq   %rax
@@ -41,7 +39,6 @@ new_cs_exit:
 	movq	rdx(%rip), %rdx
 	movq	rsi(%rip), %rsi
 	movq	rdi(%rip), %rdi
-	movq    rsp(%rip), %rsp
 	movq	rbp(%rip), %rbp
 	movq	r8(%rip), %r8
 	movq	r9(%rip), %r9
@@ -63,7 +60,6 @@ rax:	.quad 0x0
 rcx:	.quad 0x0
 rdx:	.quad 0x0
 rbx:	.quad 0x0
-rsp:	.quad 0x0
 rbp:	.quad 0x0
 rsi:	.quad 0x0
 rdi:	.quad 0x0
@@ -97,7 +93,3 @@ SYM_DATA_START_LOCAL(gdt)
 	/* 0x18 4GB flat data segment */
 	.word 0xFFFF, 0x0000, 0x9200, 0x00CF
 SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
-
-SYM_DATA_START_LOCAL(stack)
-	.quad   0, 0
-SYM_DATA_END_LABEL(stack, SYM_L_LOCAL, stack_init)
diff --git a/arch/x86/purgatory/setup-x86_64.S b/arch/x86/purgatory/setup-x86_64.S
index 89d9e9e53fcd..2d10ff88851d 100644
--- a/arch/x86/purgatory/setup-x86_64.S
+++ b/arch/x86/purgatory/setup-x86_64.S
@@ -53,7 +53,7 @@ SYM_DATA_START_LOCAL(gdt)
 SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
 
 	.bss
-	.balign 4096
+	.balign 16
 SYM_DATA_START_LOCAL(lstack)
 	.skip 4096
 SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)
diff --git a/arch/x86/purgatory/stack.S b/arch/x86/purgatory/stack.S
deleted file mode 100644
index 1ef507ca50a5..000000000000
--- a/arch/x86/purgatory/stack.S
+++ /dev/null
@@ -1,18 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0-only */
-/*
- * purgatory:  stack
- *
- * Copyright (C) 2014 Red Hat Inc.
- */
-
-#include <linux/linkage.h>
-
-	/* A stack for the loaded kernel.
-	 * Separate and in the data section so it can be prepopulated.
-	 */
-	.data
-	.balign 4096
-
-SYM_DATA_START(stack)
-	.skip 4096
-SYM_DATA_END_LABEL(stack, SYM_L_GLOBAL, stack_end)
-- 
2.44.0.769.g3c40516874-goog