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Message-ID: <20250716031814.2096113-43-ardb+git@google.com>
Date: Wed, 16 Jul 2025 05:18:34 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: linux-efi@...r.kernel.org, x86@...nel.org,
Ard Biesheuvel <ardb@...nel.org>, Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
Kevin Loughlin <kevinloughlin@...gle.com>, Tom Lendacky <thomas.lendacky@....com>,
Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
Nikunj A Dadhania <nikunj@....com>
Subject: [PATCH v5 19/22] x86/boot: Create a confined code area for startup code
From: Ard Biesheuvel <ardb@...nel.org>
In order to be able to have tight control over which code may execute
from the early 1:1 mapping of memory, but still link vmlinux as a single
executable, prefix all symbol references in startup code with __pi_, and
invoke it from outside using the __pi_ prefix.
Use objtool to check that no absolute symbol references are present in
the startup code, as these cannot be used from code running from the 1:1
mapping.
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
arch/x86/boot/startup/Makefile | 14 ++++++++++++++
arch/x86/boot/startup/sev-shared.c | 4 +---
arch/x86/boot/startup/sme.c | 1 -
arch/x86/coco/sev/core.c | 2 +-
arch/x86/include/asm/setup.h | 1 +
arch/x86/include/asm/sev.h | 1 +
arch/x86/kernel/head64.c | 2 +-
arch/x86/kernel/head_64.S | 8 ++++----
arch/x86/mm/mem_encrypt_boot.S | 6 +++---
tools/objtool/check.c | 3 ++-
10 files changed, 28 insertions(+), 14 deletions(-)
diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index 32737f4ab5a8..e8fdf020b422 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -4,6 +4,7 @@ KBUILD_AFLAGS += -D__DISABLE_EXPORTS
KBUILD_CFLAGS += -D__DISABLE_EXPORTS -mcmodel=small -fPIC \
-Os -DDISABLE_BRANCH_PROFILING \
$(DISABLE_STACKLEAK_PLUGIN) \
+ $(DISABLE_LATENT_ENTROPY_PLUGIN) \
-fno-stack-protector -D__NO_FORTIFY \
-fno-jump-tables \
-include $(srctree)/include/linux/hidden.h
@@ -36,3 +37,16 @@ $(patsubst %.o,$(obj)/%.o,$(lib-y)): OBJECT_FILES_NON_STANDARD := y
#
$(pi-objs): objtool-enabled = 1
$(pi-objs): objtool-args = $(if $(delay-objtool),,$(objtool-args-y)) --noabs
+
+#
+# Confine the startup code by prefixing all symbols with __pi_ (for position
+# independent). This ensures that startup code can only call other startup
+# code, or code that has explicitly been made accessible to it via a symbol
+# alias.
+#
+$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_
+$(obj)/%.pi.o: $(obj)/%.o FORCE
+ $(call if_changed,objcopy)
+
+targets += $(obj-y)
+obj-y := $(patsubst %.o,%.pi.o,$(obj-y))
diff --git a/arch/x86/boot/startup/sev-shared.c b/arch/x86/boot/startup/sev-shared.c
index cc14daf816e8..b60d546e74a7 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -11,9 +11,7 @@
#include <asm/setup_data.h>
-#ifndef __BOOT_COMPRESSED
-#define error(v) pr_err(v)
-#else
+#ifdef __BOOT_COMPRESSED
#undef WARN
#define WARN(condition, format...) (!!(condition))
#endif
diff --git a/arch/x86/boot/startup/sme.c b/arch/x86/boot/startup/sme.c
index 70ea1748c0a7..eb6a758ba660 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -567,7 +567,6 @@ void __head sme_enable(struct boot_params *bp)
#ifdef CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
/* Local version for startup code, which never operates on user page tables */
-__weak
pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd)
{
return pgd;
diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index be89f0a4a28f..fbed1651f6d8 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -272,7 +272,7 @@ static int svsm_perform_call_protocol(struct svsm_call *call)
do {
ret = ghcb ? svsm_perform_ghcb_protocol(ghcb, call)
- : svsm_perform_msr_protocol(call);
+ : __pi_svsm_perform_msr_protocol(call);
} while (ret == -EAGAIN);
if (sev_cfg.ghcbs_initialized)
diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 692af46603a1..914eb32581c7 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -53,6 +53,7 @@ extern void i386_reserve_resources(void);
extern unsigned long __startup_64(unsigned long p2v_offset, struct boot_params *bp);
extern void startup_64_setup_gdt_idt(void);
extern void startup_64_load_idt(void *vc_handler);
+extern void __pi_startup_64_load_idt(void *vc_handler);
extern void early_setup_idt(void);
extern void __init do_early_exception(struct pt_regs *regs, int trapnr);
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index f3acbfcdca9a..2d61b13e1810 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -551,6 +551,7 @@ struct cpuid_leaf {
};
int svsm_perform_msr_protocol(struct svsm_call *call);
+int __pi_svsm_perform_msr_protocol(struct svsm_call *call);
int snp_cpuid(void (*cpuid_hv)(void *ctx, struct cpuid_leaf *),
void *ctx, struct cpuid_leaf *leaf);
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 1bc40d0785ee..fd28b53dbac5 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -319,5 +319,5 @@ void early_setup_idt(void)
handler = vc_boot_ghcb;
}
- startup_64_load_idt(handler);
+ __pi_startup_64_load_idt(handler);
}
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3e9b3a3bd039..d219963ecb60 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -71,7 +71,7 @@ SYM_CODE_START_NOALIGN(startup_64)
xorl %edx, %edx
wrmsr
- call startup_64_setup_gdt_idt
+ call __pi_startup_64_setup_gdt_idt
/* Now switch to __KERNEL_CS so IRET works reliably */
pushq $__KERNEL_CS
@@ -91,7 +91,7 @@ SYM_CODE_START_NOALIGN(startup_64)
* subsequent code. Pass the boot_params pointer as the first argument.
*/
movq %r15, %rdi
- call sme_enable
+ call __pi_sme_enable
#endif
/* Sanitize CPU configuration */
@@ -111,7 +111,7 @@ SYM_CODE_START_NOALIGN(startup_64)
* programmed into CR3.
*/
movq %r15, %rsi
- call __startup_64
+ call __pi___startup_64
/* Form the CR3 value being sure to include the CR3 modifier */
leaq early_top_pgt(%rip), %rcx
@@ -562,7 +562,7 @@ SYM_CODE_START_NOALIGN(vc_no_ghcb)
/* Call C handler */
movq %rsp, %rdi
movq ORIG_RAX(%rsp), %rsi
- call do_vc_no_ghcb
+ call __pi_do_vc_no_ghcb
/* Unwind pt_regs */
POP_REGS
diff --git a/arch/x86/mm/mem_encrypt_boot.S b/arch/x86/mm/mem_encrypt_boot.S
index f8a33b25ae86..edbf9c998848 100644
--- a/arch/x86/mm/mem_encrypt_boot.S
+++ b/arch/x86/mm/mem_encrypt_boot.S
@@ -16,7 +16,7 @@
.text
.code64
-SYM_FUNC_START(sme_encrypt_execute)
+SYM_FUNC_START(__pi_sme_encrypt_execute)
/*
* Entry parameters:
@@ -69,9 +69,9 @@ SYM_FUNC_START(sme_encrypt_execute)
ANNOTATE_UNRET_SAFE
ret
int3
-SYM_FUNC_END(sme_encrypt_execute)
+SYM_FUNC_END(__pi_sme_encrypt_execute)
-SYM_FUNC_START(__enc_copy)
+SYM_FUNC_START_LOCAL(__enc_copy)
ANNOTATE_NOENDBR
/*
* Routine used to encrypt memory in place.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 5d1d38404892..f43bd598d928 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3563,7 +3563,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
if (func && insn_func(insn) && func != insn_func(insn)->pfunc) {
/* Ignore KCFI type preambles, which always fall through */
if (!strncmp(func->name, "__cfi_", 6) ||
- !strncmp(func->name, "__pfx_", 6))
+ !strncmp(func->name, "__pfx_", 6) ||
+ !strncmp(func->name, "__pi___pfx_", 11))
return 0;
if (file->ignore_unreachables)
--
2.50.0.727.gbf7dc18ff4-goog
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