Re: [PATCH] x86/asm/entry/64: Fix MSR_IA32_SYSENTER_CS MSR value

[Denys Vlasenko <dvlasenk@redhat.com>]

On 04/03/2015 01:20 PM, Borislav Petkov wrote:
> On Fri, Apr 03, 2015 at 11:42:10AM +0200, Borislav Petkov wrote:
>> From: Borislav Petkov <bp@suse.de>
>>
>> Commit
>>
>>   d56fe4bf5f3c ("x86/asm/entry/64: Always set up SYSENTER MSRs")
>>
>> missed to add "ULL" to the 0 and wrmsrl_safe() complains:
>>
>>   arch/x86/kernel/cpu/common.c: In function ‘syscall_init’:
>>   arch/x86/kernel/cpu/common.c:1226:2: warning: right shift count >= width of type
>>     wrmsrl_safe(MSR_IA32_SYSENTER_CS, 0);
>>
>> Fix it.
>>
>> Signed-off-by: Borislav Petkov <bp@suse.de>
>> Cc: Denys Vlasenko <dvlasenk@redhat.com>
>> Cc: Andy Lutomirski <luto@amacapital.net>
>> Cc: Alexei Starovoitov <ast@plumgrid.com>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Oleg Nesterov <oleg@redhat.com>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Cc: Will Drewry <wad@chromium.org>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> ---
>>  arch/x86/kernel/cpu/common.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index bb6633a87a19..2c249b57e4b5 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -1223,7 +1223,7 @@ void syscall_init(void)
>>  	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)ia32_sysenter_target);
>>  #else
>>  	wrmsrl(MSR_CSTAR, ignore_sysret);
>> -	wrmsrl_safe(MSR_IA32_SYSENTER_CS, 0);
>> +	wrmsrl_safe(MSR_IA32_SYSENTER_CS, 0ULL);
> 
> Btw, we probably should define an invalid segment
> 
> #define GDT_ENTRY_INVALID_SEGMENT		0ULL
> 
> and use that value instead of the naked 0.

Why? Segment selectors aren't "long long", they are in fact "short"
(16-bit ints).

The "ULL" is used here in wrmsrl not because it's a segment,
but because wrmsrl expects a u64 parameter.

We can probably add a cast in its definition...