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

Commit-ID: 07fa619f2a40c221ea27747a3323cabc59ab25eb Gitweb: https://git.kernel.org/tip/07fa619f2a40c221ea27747a3323cabc59ab25eb Author: David Woodhouse dwmw@amazon.co.uk AuthorDate: Thu, 05 Dec 2024 15:05:07 Committer: Ingo Molnar mingo@kernel.org CommitterDate: Fri, 06 Dec 2024 10:35:49 +01:00

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 --- arch/x86/kernel/relocate_kernel_64.S | 7 +++++++ 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S index e9e88c3..1236f25 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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