[PATCH v3 03/19] x86/startup_64: Drop long return to initial_code pointer

Ard Biesheuvel
2024-01-29T18:05:36Z

From: Ard Biesheuvel <ardb@kernel.org>

Since commit 866b556efa12 ("x86/head/64: Install startup GDT"), the
primary startup sequence sets the code segment register (CS) to __KERNEL_CS
before calling into the startup code shared between primary and
secondary boot.

This means a simple indirect call is sufficient here.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/kernel/head_64.S | 35 ++------------------
1 file changed, 3 insertions(+), 32 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index d4918d03efb4..4017a49d7b76 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -428,39 +428,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
movq %r15, %rdi

.Ljump_to_C_code:
- /*
- * Jump to run C code and to be on a real kernel address.
- * Since we are running on identity-mapped space we have to jump
- * to the full 64bit address, this is only possible as indirect
- * jump. In addition we need to ensure %cs is set so we make this
- * a far return.
- *
- * Note: do not change to far jump indirect with 64bit offset.
- *
- * AMD does not support far jump indirect with 64bit offset.
- * AMD64 Architecture Programmer's Manual, Volume 3: states only
- * JMP FAR mem16:16 FF /5 Far jump indirect,
- * with the target specified by a far pointer in memory.
- * JMP FAR mem16:32 FF /5 Far jump indirect,
- * with the target specified by a far pointer in memory.
- *
- * Intel64 does support 64bit offset.
- * Software Developer Manual Vol 2: states:
- * FF /5 JMP m16:16 Jump far, absolute indirect,
- * address given in m16:16
- * FF /5 JMP m16:32 Jump far, absolute indirect,
- * address given in m16:32.
- * REX.W + FF /5 JMP m16:64 Jump far, absolute indirect,
- * address given in m16:64.
- */
- pushq $.Lafter_lret # put return address on stack for unwinder
xorl %ebp, %ebp # clear frame pointer
- movq initial_code(%rip), %rax
- pushq $__KERNEL_CS # set correct cs
- pushq %rax # target address in negative space
- lretq
-.Lafter_lret:
- ANNOTATE_NOENDBR
+ ANNOTATE_RETPOLINE_SAFE
+ callq *initial_code(%rip)
+ int3
SYM_CODE_END(secondary_startup_64)

#include "verify_cpu.S"
--
2.43.0.429.g432eaa2c6b-goog