[PATCH v5 13/27] x86/boot/64: Build head64.c as mcmodel large when PIE is enabled

[Thomas Garnier]

The __startup_64 function assumes all symbols have relocated addresses
instead of the current boot virtual address. PIE generated code favor
relative addresses making all virtual and physical address math incorrect.
If PIE is enabled, build head64.c as mcmodel large instead to ensure absolute
references on all memory access. Add a global __force_order variable required
when using a large model with read_cr* functions.

To build head64.c as mcmodel=large, disable the retpoline gcc flags.
This code is used at early boot and removed later, it doesn't need
retpoline mitigation.

Position Independent Executable (PIE) support will allow to extend the
KASLR randomization range 0xffffffff80000000.

Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
---
 arch/x86/kernel/Makefile | 6 ++++++
 arch/x86/kernel/head64.c | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 02d6f5cf4e70..0f6da4b216e0 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -22,6 +22,12 @@ CFLAGS_REMOVE_early_printk.o = -pg
 CFLAGS_REMOVE_head64.o = -pg
 endif
 
+ifdef CONFIG_X86_PIE
+# Remove PIE and retpoline flags that are incompatible with mcmodel=large
+CFLAGS_REMOVE_head64.o += -fPIE -mindirect-branch=thunk-extern -mindirect-branch-register
+CFLAGS_head64.o = -mcmodel=large
+endif
+
 KASAN_SANITIZE_head$(BITS).o				:= n
 KASAN_SANITIZE_dumpstack.o				:= n
 KASAN_SANITIZE_dumpstack_$(BITS).o			:= n
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 8047379e575a..49df0386098c 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -62,6 +62,9 @@ EXPORT_SYMBOL(vmemmap_base);
 
 #define __head	__section(.head.text)
 
+/* Required for read_cr3 when building as PIE */
+unsigned long __force_order;
+
 static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
 {
 	return ptr - (void *)_text + (void *)physaddr;
-- 
2.18.0.rc2.346.g013aa6912e-goog