[PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL_CS after GDT is loaded

Joerg Roedel
2020-04-28T15:16:22Z

From: Joerg Roedel <jroedel@suse.de>

When the pre-decompression code loads its first GDT in startup_64, it is
still running on the CS value of the previous GDT. In the case of SEV-ES
this is the EFI GDT.

To make exception handling work (especially IRET) the CPU needs to
switch to a CS value in the current GDT, so jump to __KERNEL_CS after
the first GDT is loaded.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/boot/compressed/head_64.S | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 4f7e6b84be07..6b11060c3a0f 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -393,6 +393,14 @@ SYM_CODE_START(startup_64)
addq %rax, 2(%rax)
lgdt (%rax)

+ /* Reload CS so IRET returns to a CS actually in the GDT */
+ pushq $__KERNEL_CS
+ leaq .Lon_kernel_cs(%rip), %rax
+ pushq %rax
+ lretq
+
+.Lon_kernel_cs:
+
/*
* paging_prepare() sets up the trampoline and checks if we need to
* enable 5-level paging.
--
2.17.1