linux-kernel - Re: [PATCH v4 10/17] x86/entry/64: Really create an error-entry-from-usermode code path

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Date:	Thu, 2 Jul 2015 08:33:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 10/17] x86/entry/64: Really create an
 error-entry-from-usermode code path

On Thu, Jul 2, 2015 at 3:25 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jun 29, 2015 at 12:33:42PM -0700, Andy Lutomirski wrote:
>> In 539f51136500 ("x86/asm/entry/64: Disentangle error_entry/exit
>> gsbase/ebx/usermode code"), I arranged the code slightly wrong --
>> IRET faults would skip the code path that was intended to execute on
>> all error entries from user mode.  Fix it up.
>>
>> This does not fix a bug, but we'll need it, and it slightly shrinks
>> the code.
>>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/entry/entry_64.S | 12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>> index 141a5d49dddc..cd9cbc62159c 100644
>> --- a/arch/x86/entry/entry_64.S
>> +++ b/arch/x86/entry/entry_64.S
>> @@ -1145,9 +1145,14 @@ ENTRY(error_entry)
>>       testb   $3, CS+8(%rsp)
>>       jz      error_kernelspace
>>
>> -     /* We entered from user mode */
>> +error_entry_from_usermode_swapgs:
>> +     /*
>> +      * We entered from user mode or we're pretending to have entered
>> +      * from user mode due to an IRET fault.
>> +      */
>>       SWAPGS
>>
>> +error_entry_from_usermode_after_swapgs:
>>  error_entry_done:
>
> Why the second label?

"Migrate error and interrupt exit work to C" puts code between those
labels.  I could squash the patches together to make it less
mysterious.

>
> Also, please make all those labels local by prefixing them with .L
> because they appear unnecesasrily in objdump output as global symbols.

Will do.

>
> $ objdump -d vmlinux | grep -E "^[0-9a-f]+ <(error_entry_\w*|error_kernelspace|bstep_iret|error_bad_iret)"
> ffffffff8167a4a8 <error_entry_from_usermode_swapgs>:
> ffffffff8167a4ab <error_entry_done>:
> ffffffff8167a4b1 <error_kernelspace>:
> ffffffff8167a4e0 <bstep_iret>:
> ffffffff8167a4e8 <error_bad_iret>:
>
> Oh, and shorter please :)
>

I'll try to think of a good shorter name.  No promises :)

--Andy
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