The patch below does not apply to the 5.10-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y git checkout FETCH_HEAD git cherry-pick -x 07fa619f2a40c221ea27747a3323cabc59ab25eb # <resolve conflicts, build, test, etc.> git commit -s git send-email --to 'stable@vger.kernel.org' --in-reply-to '2024121048-dreamlike-devourer-7135@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 07fa619f2a40c221ea27747a3323cabc59ab25eb Mon Sep 17 00:00:00 2001 From: David Woodhouse dwmw@amazon.co.uk Date: Thu, 5 Dec 2024 15:05:07 +0000 Subject: [PATCH] x86/kexec: Restore GDT on return from ::preserve_context kexec

The restore_processor_state() function explicitly states that "the asm code that gets us here will have restored a usable GDT". That wasn't true in the case of returning from a ::preserve_context kexec. Make it so.

Without this, the kernel was depending on the called function to reload a GDT which is appropriate for the kernel before returning.

Test program:

#include <unistd.h> #include <errno.h> #include <stdio.h> #include <stdlib.h> #include <linux/kexec.h> #include <linux/reboot.h> #include <sys/reboot.h> #include <sys/syscall.h>

int main (void) { struct kexec_segment segment = {}; unsigned char purgatory[] = { 0x66, 0xba, 0xf8, 0x03, // mov $0x3f8, %dx 0xb0, 0x42, // mov $0x42, %al 0xee, // outb %al, (%dx) 0xc3, // ret }; int ret;

segment.buf = &purgatory; segment.bufsz = sizeof(purgatory); segment.mem = (void *)0x400000; segment.memsz = 0x1000; ret = syscall(__NR_kexec_load, 0x400000, 1, &segment, KEXEC_PRESERVE_CONTEXT); if (ret) { perror("kexec_load"); exit(1); }

ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2, LINUX_REBOOT_CMD_KEXEC); if (ret) { perror("kexec reboot"); exit(1); } printf("Success\n"); return 0; }

Signed-off-by: David Woodhouse dwmw@amazon.co.uk Signed-off-by: Ingo Molnar mingo@kernel.org Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20241205153343.3275139-2-dwmw2@infradead.org

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S index e9e88c342f75..1236f25fc8d1 100644 --- a/arch/x86/kernel/relocate_kernel_64.S +++ b/arch/x86/kernel/relocate_kernel_64.S @@ -242,6 +242,13 @@ SYM_CODE_START_LOCAL_NOALIGN(virtual_mapped) movq CR0(%r8), %r8 movq %rax, %cr3 movq %r8, %cr0 + +#ifdef CONFIG_KEXEC_JUMP + /* Saved in save_processor_state. */ + movq $saved_context, %rax + lgdt saved_context_gdt_desc(%rax) +#endif + movq %rbp, %rax

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