linux-kernel - Re: [PATCH RESEND] x86/asm/entry/32: simplify pushes of zeroed pt

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Date:	Tue, 3 May 2016 10:55:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs

On Tue, May 3, 2016 at 10:45 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Mon, May 2, 2016 at 10:56 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> Use of a temporary R8 register here seems to be unnecessary.
>>
>> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
>> "push $0" is two-byte too. It seems just using the latter would be
>> no worse.
>>
>> Thus, code had an unnecessary "xorq %r8,%r8" insn.
>> It probably costs nothing in execution time here since we are probably
>> limited by store bandwidth at this point, but still.
>>
>> Run-tested under QEMU: 32-bit calls still work:
>>
>> / # ./test_syscall_vdso32
>> [RUN]   Executing 6-argument 32-bit syscall via VDSO
>> [OK]    Arguments are preserved across syscall
>> [NOTE]  R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
>> [OK]    R8..R15 did not leak kernel data
>> [RUN]   Executing 6-argument 32-bit syscall via INT 80
>> [OK]    Arguments are preserved across syscall
>> [OK]    R8..R15 did not leak kernel data
>> [RUN]   Running tests under ptrace
>> [RUN]   Executing 6-argument 32-bit syscall via VDSO
>> [OK]    Arguments are preserved across syscall
>> [NOTE]  R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
>> [OK]    R8..R15 did not leak kernel data
>> [RUN]   Executing 6-argument 32-bit syscall via INT 80
>> [OK]    Arguments are preserved across syscall
>> [OK]    R8..R15 did not leak kernel data
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
>> CC: Ingo Molnar <mingo@...nel.org>
>> 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: Frederic Weisbecker <fweisbec@...il.com>
>> CC: Will Drewry <wad@...omium.org>
>> CC: Kees Cook <keescook@...omium.org>
>> CC: x86@...nel.org
>> CC: linux-kernel@...r.kernel.org
>> ---
>>
>> Resending. Still applies to current Ingo's tip tree
>>
>>  arch/x86/entry/entry_64_compat.S | 45 +++++++++++++++++++---------------------
>>  1 file changed, 21 insertions(+), 24 deletions(-)
>>
>> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
>> index 847f2f0..e1721da 100644
>> --- a/arch/x86/entry/entry_64_compat.S
>> +++ b/arch/x86/entry/entry_64_compat.S
>> @@ -72,24 +72,23 @@ ENTRY(entry_SYSENTER_compat)
>>         pushfq                          /* pt_regs->flags (except IF = 0) */
>>         orl     $X86_EFLAGS_IF, (%rsp)  /* Fix saved flags */
>>         pushq   $__USER32_CS            /* pt_regs->cs */
>> -       xorq    %r8,%r8
>> -       pushq   %r8                     /* pt_regs->ip = 0 (placeholder) */
>> +       pushq   $0                      /* pt_regs->ip = 0 (placeholder) */
>>         pushq   %rax                    /* pt_regs->orig_ax */
>>         pushq   %rdi                    /* pt_regs->di */
>>         pushq   %rsi                    /* pt_regs->si */
>>         pushq   %rdx                    /* pt_regs->dx */
>>         pushq   %rcx                    /* pt_regs->cx */
>>         pushq   $-ENOSYS                /* pt_regs->ax */
>> -       pushq   %r8                     /* pt_regs->r8  = 0 */
>> -       pushq   %r8                     /* pt_regs->r9  = 0 */
>> -       pushq   %r8                     /* pt_regs->r10 = 0 */
>> -       pushq   %r8                     /* pt_regs->r11 = 0 */
>> +       pushq   $0                      /* pt_regs->r8  = 0 */
>> +       pushq   $0                      /* pt_regs->r9  = 0 */
>> +       pushq   $0                      /* pt_regs->r10 = 0 */
>> +       pushq   $0                      /* pt_regs->r11 = 0 */
>>         pushq   %rbx                    /* pt_regs->rbx */
>>         pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
>> -       pushq   %r8                     /* pt_regs->r12 = 0 */
>> -       pushq   %r8                     /* pt_regs->r13 = 0 */
>> -       pushq   %r8                     /* pt_regs->r14 = 0 */
>> -       pushq   %r8                     /* pt_regs->r15 = 0 */
>> +       pushq   $0                      /* pt_regs->r12 = 0 */
>> +       pushq   $0                      /* pt_regs->r13 = 0 */
>> +       pushq   $0                      /* pt_regs->r14 = 0 */
>> +       pushq   $0                      /* pt_regs->r15 = 0 */
>
> I think it actually should push r12-r15, since they are callee-saved
> and we don't explicitly zero them out on SYSRET like r8-r10.  If it
> exited via IRET it would reload them as zero, so there is an
> inconsistency there.

Hmm.  We could do this or we could zero them on the way out from the
fast path.  I have no real preference.  Preserving the values is
probably a bit nicer.

--Andy