[RFT PATCH v2 19/23] x86/boot: Created a confined code area for startup code

Ard Biesheuvel
2025-05-04T09:53:45Z

From: Ard Biesheuvel <ardb@kernel.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.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/startup/Makefile | 13 +++++++++++++
arch/x86/boot/startup/sev-shared.c | 2 +-
arch/x86/boot/startup/sme.c | 1 -
arch/x86/coco/sev/core.c | 2 +-
arch/x86/coco/sev/vc-handle.c | 1 -
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 +++---
10 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/arch/x86/boot/startup/Makefile b/arch/x86/boot/startup/Makefile
index b514f7e81332..c7e690091f81 100644
--- a/arch/x86/boot/startup/Makefile
+++ b/arch/x86/boot/startup/Makefile
@@ -28,3 +28,16 @@ lib-$(CONFIG_EFI_MIXED) += efi-mixed.o
# to be linked into the decompressor or the EFI stub but not vmlinux
#
$(patsubst %.o,$(obj)/%.o,$(lib-y)): OBJECT_FILES_NON_STANDARD := y
+
+#
+# Confine the startup code by prefixing all symbols with __pi_ (for position
+# independent). This ensures that startup code can only call other startup
+# code, or code that has explicitly been made accessible to it via a symbol
+# alias.
+#
+$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_
+$(obj)/%.pi.o: $(obj)/%.o FORCE
+ $(call if_changed,objcopy)
+
+extra-y := $(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 9ab132bed77b..0215f2355dfa 100644
--- a/arch/x86/boot/startup/sev-shared.c
+++ b/arch/x86/boot/startup/sev-shared.c
@@ -12,7 +12,7 @@
#include <asm/setup_data.h>

#ifndef __BOOT_COMPRESSED
-#define error(v) pr_err(v)
+#define error(v)
#define has_cpuflag(f) boot_cpu_has(f)
#else
#undef WARN
diff --git a/arch/x86/boot/startup/sme.c b/arch/x86/boot/startup/sme.c
index 1e9f1f5a753c..19dce2304636 100644
--- a/arch/x86/boot/startup/sme.c
+++ b/arch/x86/boot/startup/sme.c
@@ -560,7 +560,6 @@ 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 33332c4299b9..98921cde217e 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -276,7 +276,7 @@ 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/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
index b4895c648024..0820accb9e80 100644
--- a/arch/x86/coco/sev/vc-handle.c
+++ b/arch/x86/coco/sev/vc-handle.c
@@ -1058,4 +1058,3 @@ bool __init handle_vc_boot_ghcb(struct pt_regs *regs)

sev_es_terminate(SEV_TERM_SET_GEN, GHCB_SEV_ES_GEN_REQ);
}
-
diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 6324f4c6c545..895d09faaf83 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 ca914f7384c6..cfec803a9b6c 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -535,6 +535,7 @@ 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 b7da8b45b6d8..23dc505bf609 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -306,5 +306,5 @@ 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 069420853304..88cbf932f7c2 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.
--
2.49.0.906.g1f30a19c02-goog