Re: [PATCH v3 12/75] x86/boot/compressed/64: Switch to __KERNEL_CS after GDT is loaded
From: Borislav Petkov <bp@alien8.de>
Date: 2020-05-04 10:41:37
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kvm, lkml
On Tue, Apr 28, 2020 at 05:16:22PM +0200, Joerg Roedel wrote:
quoted hunk ↗ jump to hunk
From: Joerg Roedel <redacted> 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 <redacted> --- 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.--
So I'm thinking I should take this one even now on the grounds that
it sanitizes CS to something known-good than what was there before and
who knows what set it and loaded the kernel...?
And that is a good thing in itself.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette