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

[Mathias Krause <minipli@grsecurity.net>]

On 08.09.23 17:02, Saurabh Singh Sengar wrote:
> On Fri, Sep 08, 2023 at 12:26:10PM +0200, Mathias Krause wrote:
>> Booting a CONFIG_HYPERV_VTL_MODE=y enabled kernel on bare metal or a
>> non-Hyper-V hypervisor leads to serve memory corruption as
> 
> FWIW, CONFIG_HYPERV_VTL_MODE is not expected to be enabled for non VTL
> platforms.

Fair enough, but there's really no excuse to randomly crashing the
kernel if one forgot to RTFM like I did. The code should (and easily
can) handle such situations, especially if it's just a matter of a two
line change.

> Referring Kconfig documentation:
> "A kernel built with this option must run at VTL2, and will not run as
> a normal guest."

So, maybe, the 'return 0' below should be a 'panic("Need to run on
Hyper-V!")' instead?

But then, looking at the code, most of the VTL specifics only run when
the Hyper-V hypervisor was actually detected during early boot. It's
just hv_vtl_early_init() that runs unconditionally and spoils the game.

Is there really a *hard* requirement / reason for having VTL support
disabled when not running under Hyper-V? At least I can't find any from
the code side. It'll all be fine with the below patch, also enabling
running the same kernel on multiple platforms -- bare metal, KVM, Hyper-V,..

Thanks,
Mathias

> 
> - Saurabh
> 
>> 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
>>