Re: [PATCH] x86/entry/64: Fix context tracking state warning when load_gs_index fails

[Wanpeng Li]

2016-09-30 5:01 GMT+08:00 Andy Lutomirski <luto@xxxxxxxxxxxxxx>:
> On Mon, Sep 26, 2016 at 4:49 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>
>>  WARNING: CPU: 0 PID: 3331 at arch/x86/entry/common.c:45 enter_from_user_mode+0x32/0x50
>>  CPU: 0 PID: 3331 Comm: ldt_gdt_64 Not tainted 4.8.0-rc7+ #13
>>  Call Trace:
>>   dump_stack+0x99/0xd0
>>   __warn+0xd1/0xf0
>>   warn_slowpath_null+0x1d/0x20
>>   enter_from_user_mode+0x32/0x50
>>   error_entry+0x6d/0xc0
>>   ? general_protection+0x12/0x30
>>   ? native_load_gs_index+0xd/0x20
>>   ? do_set_thread_area+0x19c/0x1f0
>>   SyS_set_thread_area+0x24/0x30
>>   do_int80_syscall_32+0x7c/0x220
>>   entry_INT80_compat+0x38/0x50
>>
>> This can be reproduced by running the GS testcase of ldt_gdt test unit in
>> selftests.
>>
>> do_int80_syscall_32() will call enter_form_user_mode() to convert context
>> tracking state from user state to kernel state. The load_gs_index can fail
>> with user gsbase, gsbase will be fixed up and proceed if this happen.
>> However, enter_from_user_mode() will be called again in the fixed up path
>> though it is context tracking kernel state currently.
>>
>> This patch fix it by just fixing up gsbase and telling lockdep that IRQs
>> are off once load_gs_index failed with user gsbase.
>>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>> ---
>>  arch/x86/entry/entry_64.S | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index d172c61..dc1ec23 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1045,7 +1045,9 @@ ENTRY(error_entry)
>>          * gsbase and proceed.  We'll fix up the exception and land in
>>          * .Lgs_change's error handler with kernel gsbase.
>>          */
>> -       jmp     .Lerror_entry_from_usermode_swapgs
>> +       SWAPGS
>> +       TRACE_IRQS_OFF
>
> Let's make this more readable: can you change this to:
>
> SWAPGS
> jmp .Lerror_entry_done
>
> and remove the .Lerror_entry_from_usermode_swapgs label as well?

I will do this in v2. Btw, could you point out why the GS testcase in
tools/testing/selftests/x86/ldt_gdt.c will #GP? SDM said that swapgs
will #GP if CPL != 0, however, native_load_gs_index() is CPL == 0.

Regards,
Wanpeng Li