[PATCH v4 2/7] x86/boot/64: Determine VA/PA offset before entering C code

Ard Biesheuvel
2024-12-05T11:28:26Z

From: Ard Biesheuvel <ardb@kernel.org>

Implicit absolute symbol references (e.g., taking the address of a
global variable) must be avoided in the C code that runs from the early
1:1 mapping of the kernel, given that this is a practice that violates
assumptions on the part of the toolchain. I.e., RIP-relative and
absolute references are expected to produce the same values, and so the
compiler is free to choose either. However, the code currently assumes
that RIP-relative references are never emitted here.

So an explicit virtual-to-physical offset needs to be used instead to
derive the kernel virtual addresses of _text and _end, instead of simply
taking the addresses and assuming that the compiler will not choose to
use a RIP-relative references in this particular case.

Currently, phys_base is already used to perform such calculations, but
it is derived from the kernel virtual address of _text, which is taken
using an implicit absolute symbol reference. So instead, derive this
VA-to-PA offset in asm code, and pass it to the C startup code.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/include/asm/setup.h | 2 +-
arch/x86/kernel/head64.c | 8 +++++---
arch/x86/kernel/head_64.S | 12 +++++++++---
3 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 0667b2a88614..85f4fde3515c 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -49,7 +49,7 @@ extern unsigned long saved_video_mode;

extern void reserve_standard_io_resources(void);
extern void i386_reserve_resources(void);
-extern unsigned long __startup_64(unsigned long physaddr, struct boot_params *bp);
+extern unsigned long __startup_64(unsigned long p2v_offset, struct boot_params *bp);
extern void startup_64_setup_gdt_idt(void);
extern void early_setup_idt(void);
extern void __init do_early_exception(struct pt_regs *regs, int trapnr);
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 4b9d4557fc94..a7cd4053eeb3 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -138,12 +138,14 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
* doesn't have to generate PC-relative relocations when accessing globals from
* that function. Clang actually does not generate them, which leads to
* boot-time crashes. To work around this problem, every global pointer must
- * be accessed using RIP_REL_REF().
+ * be accessed using RIP_REL_REF(). Kernel virtual addresses can be determined
+ * by subtracting p2v_offset from the RIP-relative address.
*/
-unsigned long __head __startup_64(unsigned long physaddr,
+unsigned long __head __startup_64(unsigned long p2v_offset,
struct boot_params *bp)
{
pmd_t (*early_pgts)[PTRS_PER_PMD] = RIP_REL_REF(early_dynamic_pgts);
+ unsigned long physaddr = (unsigned long)&RIP_REL_REF(_text);
unsigned long pgtable_flags;
unsigned long load_delta;
pgdval_t *pgd;
@@ -163,7 +165,7 @@ unsigned long __head __startup_64(unsigned long physaddr,
* Compute the delta between the address I am compiled to run at
* and the address I am actually running at.
*/
- load_delta = physaddr - (unsigned long)(_text - __START_KERNEL_map);
+ load_delta = __START_KERNEL_map + p2v_offset;
RIP_REL_REF(phys_base) = load_delta;

/* Is the address not 2M aligned? */
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 56163e2124cf..31345e0ba006 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -94,13 +94,19 @@ SYM_CODE_START_NOALIGN(startup_64)
/* Sanitize CPU configuration */
call verify_cpu

+ /*
+ * Derive the kernel's physical-to-virtual offset from the physical and
+ * virtual addresses of common_startup_64().
+ */
+ leaq common_startup_64(%rip), %rdi
+ subq .Lcommon_startup_64(%rip), %rdi
+
/*
* Perform pagetable fixups. Additionally, if SME is active, encrypt
* the kernel and retrieve the modifier (SME encryption mask if SME
* is active) to be added to the initial pgdir entry that will be
* programmed into CR3.
*/
- leaq _text(%rip), %rdi
movq %r15, %rsi
call __startup_64

@@ -128,11 +134,11 @@ SYM_CODE_START_NOALIGN(startup_64)

/* Branch to the common startup code at its kernel virtual address */
ANNOTATE_RETPOLINE_SAFE
- jmp *0f(%rip)
+ jmp *.Lcommon_startup_64(%rip)
SYM_CODE_END(startup_64)

__INITRODATA
-0: .quad common_startup_64
+SYM_DATA_LOCAL(.Lcommon_startup_64, .quad common_startup_64)

.text
SYM_CODE_START(secondary_startup_64)
--
2.47.0.338.g60cca15819-goog