[PATCH v8 5/7] x86/kexec: Invalidate GDT/IDT from relocate_kernel() instead of earlier

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Commit 2114796ca041 ("x86/kexec: Mark machine_kexec() with __nocfi") marked
the machine_kexec() function with __nocfi because I failed to make objtool
happy when actually *providing* CFI information for the relocate_kernel()
function that it calls, and __nocfi was the easy workaround.

When that eventually gets fixed, the check on calling the relocate_kernel()
function pointer may trap. Obviously that should never happen unless the
function prototype gets messed up, but that's exactly what the CFI check is
for.

So it would be kind of helpful if the IDT was still in place and the trap
could be *handled*. It can remain valid until the moment that %cr3 gets
replaced, so shift the invalidation down into relocate_kernel() itself.

This is a useful cleanup regardless of making CFI work, so do it now.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kernel/machine_kexec_64.c   | 10 ++--------
 arch/x86/kernel/relocate_kernel_64.S |  9 +++++++--
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
index a2e00a814708..7abc7aa0261b 100644
--- a/arch/x86/kernel/machine_kexec_64.c
+++ b/arch/x86/kernel/machine_kexec_64.c
@@ -434,16 +434,10 @@ void __nocfi machine_kexec(struct kimage *image)
 	 * with from a table in memory.  At no other time is the
 	 * descriptor table in memory accessed.
 	 *
-	 * I take advantage of this here by force loading the
-	 * segments, before I zap the gdt with an invalid value.
+	 * Take advantage of this here by force loading the segments,
+	 * before the GDT is zapped with an invalid value.
 	 */
 	load_segments();
-	/*
-	 * The gdt & idt are now invalid.
-	 * If you want to load them you must set up your own idt & gdt.
-	 */
-	native_idt_invalidate();
-	native_gdt_invalidate();
 
 	/* now call it */
 	image->start = relocate_kernel_ptr((unsigned long)image->head,
diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index cf736bbaec98..4f8b7d318025 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -79,8 +79,13 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	pushq %r15
 	pushf
 
-	/* zero out flags, and disable interrupts */
-	pushq $0
+	/* Invalidate GDT/IDT, zero out flags */
+	pushq	$0
+	pushq	$0
+
+	lidt	(%rsp)
+	lgdt	(%rsp)
+	addq	$8, %rsp
 	popfq
 
 	/* Switch to the identity mapped page tables */
-- 
2.48.1