linux-kernel - Re: [PATCH 3/5] x86/asm/entry/32: Jump from SYSENTER32 to SYSCALL32 code path

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Date:	Tue, 28 Jul 2015 13:13:04 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Krzysztof A. Sobiecki" <sobkas@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] x86/asm/entry/32: Jump from SYSENTER32 to SYSCALL32
 code path

On 07/28/2015 12:37 AM, Andy Lutomirski wrote:
>> Subject: [PATCH 3/5] x86/asm/entry/32: Jump from SYSENTER32 to SYSCALL32 code path
> 
> Shouldn't that be /64, not /32, or maybe /64/compat?
> 
> On Mon, Jul 27, 2015 at 1:33 PM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> In 32-bit SYSENTER code, load arg6 into R9 instead of EBP.
>> Jump to SYSCALL code path after we finish setting up pt_regs
>> and clearing FLAGS_NT.
>>
>> This leaves most of SYSENTER32 code path inaccessible.
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
>> CC: Ingo Molnar <mingo@...nel.org>
>> CC: Linus Torvalds <torvalds@...ux-foundation.org>
>> CC: Krzysztof A. Sobiecki <sobkas@...il.com>
>> CC: Steven Rostedt <rostedt@...dmis.org>
>> CC: Borislav Petkov <bp@...en8.de>
>> CC: "H. Peter Anvin" <hpa@...or.com>
>> CC: Andy Lutomirski <luto@...capital.net>
>> CC: Oleg Nesterov <oleg@...hat.com>
>> CC: Frederic Weisbecker <fweisbec@...il.com>
>> CC: Alexei Starovoitov <ast@...mgrid.com>
>> CC: Will Drewry <wad@...omium.org>
>> CC: Kees Cook <keescook@...omium.org>
>> CC: x86@...nel.org
>> CC: linux-kernel@...r.kernel.org
>> ---
>>  arch/x86/entry/entry_64_compat.S | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
>> index df102e8..d74745a 100644
>> --- a/arch/x86/entry/entry_64_compat.S
>> +++ b/arch/x86/entry/entry_64_compat.S
>> @@ -93,7 +93,7 @@ ENTRY(entry_SYSENTER_compat)
>>          * 32-bit zero extended
>>          */
>>         ASM_STAC
>> -1:     movl    (%rbp), %ebp
>> +1:     movl    (%rbp), %r9d
> 
> You're sticking arg6 into r9d here, I think, and then:
> 
> 
>>
>>         orl     $TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
>>         testl   $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
>> @@ -343,6 +344,7 @@ ENTRY(entry_SYSCALL_compat)
>>         _ASM_EXTABLE(1b, ia32_badarg)
>>         ASM_CLAC
>>
>> +sysenter_jumps_here:
>>         orl     $TS_COMPAT, ASM_THREAD_INFO(TI_status, %rsp, SIZEOF_PTREGS)
>>         testl   $_TIF_WORK_SYSCALL_ENTRY, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
>>         jnz     cstar_tracesys
> 
> you land here, which eventually does:
> 
>     movl    %ebp, %r9d        /* arg6 */
> 
> What am I missing?

Please "git pull" from Ingo's tree. There was a revert,
arg6 is no longer held in EBP in SYSCALL code:

cstar_do_call:
        /* 32-bit syscall -> 64-bit C ABI argument conversion */
        movl    %edi, %r8d              /* arg5 */
        /* r9 already loaded */         /* arg6 */
        xchg    %ecx, %esi              /* rsi:arg2, rcx:arg4 */
        movl    %ebx, %edi              /* arg1 */
        movl    %edx, %edx              /* arg3 (zero extension) */

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