[PATCH] x86/hyperv/vtl: Replace real_mode_header only under Hyper-V

[Mathias Krause <minipli@grsecurity.net>]

Booting a CONFIG_HYPERV_VTL_MODE=y enabled kernel on bare metal or a
non-Hyper-V hypervisor leads to serve memory corruption as
hv_vtl_early_init() will run even though hv_vtl_init_platform() did not.
This skips no-oping the 'realmode_reserve' and 'realmode_init' platform
hooks, making init_real_mode() -> setup_real_mode() try to copy
'real_mode_blob' over 'real_mode_header' which we set to the stub
'hv_vtl_real_mode_header'. However, as 'real_mode_blob' isn't just a
'struct real_mode_header' -- it's the complete code! -- copying it over
'hv_vtl_real_mode_header' will corrupt quite some memory following it.

The real cause for this erroneous behaviour is that hv_vtl_early_init()
blindly assumes the kernel is running on Hyper-V, which it may not.

Fix this by making sure the code only replaces the real mode header with
the stub one iff the kernel is running under Hyper-V.

Fixes: 3be1bc2fe9d2 ("x86/hyperv: VTL support for Hyper-V")
Cc: Saurabh Sengar <ssengar@linux.microsoft.com>
Cc: stable@kernel.org
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
---
 arch/x86/hyperv/hv_vtl.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
index 57df7821d66c..54c06f4b8b4c 100644
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -12,6 +12,7 @@
 #include <asm/desc.h>
 #include <asm/i8259.h>
 #include <asm/mshyperv.h>
+#include <asm/hypervisor.h>
 #include <asm/realmode.h>
 
 extern struct boot_params boot_params;
@@ -214,6 +215,9 @@ static int hv_vtl_wakeup_secondary_cpu(int apicid, unsigned long start_eip)
 
 static int __init hv_vtl_early_init(void)
 {
+	if (!hypervisor_is_type(X86_HYPER_MS_HYPERV))
+		return 0;
+
 	/*
 	 * `boot_cpu_has` returns the runtime feature support,
 	 * and here is the earliest it can be used.
-- 
2.30.2