x86/boot/compressed/64: Do not corrupt EDX on EFER.LME=1 setting

RDMSR in the trampoline code overwrites EDX but that register is used
to indicate whether 5-level paging has to be enabled and if clobbered,
leads to failure to boot on a 5-level paging machine.

Preserve EDX on the stack while we are dealing with EFER.

Fixes: b677dfae5aa1 ("x86/boot/compressed/64: Set EFER.LME=1 in 32-bit trampoline before returning to long mode")
Reported-by: Kyle D Pelton <kyle.d.pelton@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: dave.hansen@linux.intel.com
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wei Huang <wei@redhat.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190206115253.1907-1-kirill.shutemov@linux.intel.com

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f105ae8651c9425429982fccabd47a8114edd5ff..f62e347862ccc61ba417d80dabee304ef28b6ec7 100644 (file)
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -602,10 +602,12 @@ ENTRY(trampoline_32bit_src)
 3:
        /* Set EFER.LME=1 as a precaution in case hypervsior pulls the rug */
        pushl   %ecx
+       pushl   %edx
        movl    $MSR_EFER, %ecx
        rdmsr
        btsl    $_EFER_LME, %eax
        wrmsr
+       popl    %edx
        popl    %ecx
 
        /* Enable PAE and LA57 (if required) paging modes */