[tip: x86/boot] x86/kexec: Ensure preserve_context flag is set on return to kernel

[tip-bot2 for David Woodhouse]

The following commit has been merged into the x86/boot branch of tip:

Commit-ID:     4d5f1da98f5e94ecc39d6e7a990db2efe0ae3810
Gitweb:        https://git.kernel.org/tip/4d5f1da98f5e94ecc39d6e7a990db2efe0ae3810
Author:        David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate:    Thu, 09 Jan 2025 14:04:14 
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Tue, 14 Jan 2025 12:52:47 +01:00

x86/kexec: Ensure preserve_context flag is set on return to kernel

The swap_pages() function will only actually *swap*, as its name implies, if
the preserve_context flag in the %r11 register is non-zero. On the way back
from a ::preserve_context kexec, ensure that the %r11 register is non-zero so
that the pages get swapped back.

Fixes: 9e5683e2d0b5 ("x86/kexec: Only swap pages for ::preserve_context mode")
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20250109140757.2841269-3-dwmw2@xxxxxxxxxxxxx
---
 arch/x86/kernel/relocate_kernel_64.S | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 9bd601d..6fce4b4 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -220,6 +220,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movq	kexec_pa_table_page(%rip), %rax
 	movq	%rax, %cr3
 	lea	PAGE_SIZE(%r8), %rsp
+	movl	$1, %r11d	/* Ensure preserve_context flag is set */
 	call	swap_pages
 	movq	kexec_va_control_page(%rip), %rax
 	addq	$(virtual_mapped - relocate_kernel), %rax