Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call

[Andy Lutomirski]

> On Jan 10, 2018, at 4:16 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> 
>> On Tue, Jan 09, 2018 at 09:46:16PM -0500, Brian Gerst wrote:
>>> On Tue, Jan 9, 2018 at 8:03 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
>>> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>> 
>>> Remove the partial stack frame in the 64bit syscall fast path.
>>> In the next patch we want to clear the extra registers, which requires
>>> to always save all registers. So remove the partial stack frame
>>> in the syscall fast path and always save everything.
>>> 
>>> This actually simplifies the code because the ptregs stubs
>>> are not needed anymore.
>>> 
>>> arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------------------
>>> arch/x86/entry/syscall_64.c |  2 +-
>>> 
>>> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>> ---
>>> arch/x86/entry/entry_64.S   | 57 ++++-----------------------------------------
>>> arch/x86/entry/syscall_64.c |  2 +-
>>> 2 files changed, 5 insertions(+), 54 deletions(-)
>>> 
>>> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
>>> index 58dbf7a12a05..bbdfbdd817d6 100644
>>> --- a/arch/x86/entry/entry_64.S
>>> +++ b/arch/x86/entry/entry_64.S
>>> @@ -234,7 +234,9 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
>>>        pushq   %r9                             /* pt_regs->r9 */
>>>        pushq   %r10                            /* pt_regs->r10 */
>>>        pushq   %r11                            /* pt_regs->r11 */
>>> -       sub     $(6*8), %rsp                    /* pt_regs->bp, bx, r12-15 not saved */
>>> +       sub     $(6*8), %rsp
>>> +       SAVE_EXTRA_REGS
>>> +
>> 
>> Continue using pushes here
>> 
>>>        UNWIND_HINT_REGS extra=0
>>> 
>>>        /*
>>> @@ -262,11 +264,6 @@ entry_SYSCALL_64_fastpath:
>>>        ja      1f                              /* return -ENOSYS (already in pt_regs->ax) */
>>>        movq    %r10, %rcx
>>> 
>>> -       /*
>>> -        * This call instruction is handled specially in stub_ptregs_64.
>>> -        * It might end up jumping to the slow path.  If it jumps, RAX
>>> -        * and all argument registers are clobbered.
>>> -        */
>>> #ifdef CONFIG_RETPOLINE
>>>        movq    sys_call_table(, %rax, 8), %rax
>>>        call    __x86_indirect_thunk_rax
>>> @@ -293,9 +290,7 @@ entry_SYSCALL_64_fastpath:
>>>        TRACE_IRQS_ON           /* user mode is traced as IRQs on */
>>>        movq    RIP(%rsp), %rcx
>>>        movq    EFLAGS(%rsp), %r11
>>> -       addq    $6*8, %rsp      /* skip extra regs -- they were preserved */
>>> -       UNWIND_HINT_EMPTY
>>> -       jmp     .Lpop_c_regs_except_rcx_r11_and_sysret
>>> +       jmp     syscall_return_via_sysret
>>> 
>>> 1:
>>>        /*
>>> @@ -305,14 +300,12 @@ entry_SYSCALL_64_fastpath:
>>>         */
>>>        TRACE_IRQS_ON
>>>        ENABLE_INTERRUPTS(CLBR_ANY)
>>> -       SAVE_EXTRA_REGS
>>>        movq    %rsp, %rdi
>>>        call    syscall_return_slowpath /* returns with IRQs disabled */
>>>        jmp     return_from_SYSCALL_64
>>> 
>>> entry_SYSCALL64_slow_path:
>>>        /* IRQs are off. */
>>> -       SAVE_EXTRA_REGS
>>>        movq    %rsp, %rdi
>>>        call    do_syscall_64           /* returns with IRQs disabled */
>>> 
>>> @@ -389,7 +382,6 @@ syscall_return_via_sysret:
>>>        /* rcx and r11 are already restored (see code above) */
>>>        UNWIND_HINT_EMPTY
>>>        POP_EXTRA_REGS
>>> -.Lpop_c_regs_except_rcx_r11_and_sysret:
>>>        popq    %rsi    /* skip r11 */
>>>        popq    %r10
>>>        popq    %r9
>>> @@ -420,47 +412,6 @@ syscall_return_via_sysret:
>>>        USERGS_SYSRET64
>>> END(entry_SYSCALL_64)
>>> 
>>> -ENTRY(stub_ptregs_64)
>>> -       /*
>>> -        * Syscalls marked as needing ptregs land here.
>>> -        * If we are on the fast path, we need to save the extra regs,
>>> -        * which we achieve by trying again on the slow path.  If we are on
>>> -        * the slow path, the extra regs are already saved.
>>> -        *
>>> -        * RAX stores a pointer to the C function implementing the syscall.
>>> -        * IRQs are on.
>>> -        */
>>> -       cmpq    $.Lentry_SYSCALL_64_after_fastpath_call, (%rsp)
>>> -       jne     1f
>>> -
>>> -       /*
>>> -        * Called from fast path -- disable IRQs again, pop return address
>>> -        * and jump to slow path
>>> -        */
>>> -       DISABLE_INTERRUPTS(CLBR_ANY)
>>> -       TRACE_IRQS_OFF
>>> -       popq    %rax
>>> -       UNWIND_HINT_REGS extra=0
>>> -       jmp     entry_SYSCALL64_slow_path
>>> -
>>> -1:
>>> -       JMP_NOSPEC %rax                         /* Called from C */
>>> -END(stub_ptregs_64)
>>> -
>>> -.macro ptregs_stub func
>>> -ENTRY(ptregs_\func)
>>> -       UNWIND_HINT_FUNC
>>> -       leaq    \func(%rip), %rax
>>> -       jmp     stub_ptregs_64
>>> -END(ptregs_\func)
>>> -.endm
>>> -
>>> -/* Instantiate ptregs_stub for each ptregs-using syscall */
>>> -#define __SYSCALL_64_QUAL_(sym)
>>> -#define __SYSCALL_64_QUAL_ptregs(sym) ptregs_stub sym
>>> -#define __SYSCALL_64(nr, sym, qual) __SYSCALL_64_QUAL_##qual(sym)
>>> -#include <asm/syscalls_64.h>
>>> -
>> 
>> You can't just blindly remove this.  We need to make sure that
>> syscalls that modify registers take the slow path exit, because they
>> may change the registers to be incompatible with SYSRET.
> 
> That's a good point. I checked the ptregs calls:
> 
> iopl: should be fine, we will be restoring the correct IOPL through
>    SYSRET
> 
> clone/fork: fine too, the original return is fine and ret_from_fork
>            takes care of the child
> 
> execve et.al.: we will be leaking r11(rflags), rcx(orig return) into
>        the new process. but that seems acceptable.
> 
> rt_sigreturn:  that's the only one who has problems. I added a new
>            TIF_FULL_RESTORE to force it into the slow path.        
>     

So your series removes the old declarative annotation and then will add a new TI flag to make it work again?

This whole thing seems to be at the wrong end of the cost benefit curve.