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

From: Brian Gerst
Date: Wed Jan 10 2018 - 20:02:06 EST

On Wed, Jan 10, 2018 at 7:55 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>
>> 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.

We already check TIF flags after the syscall on the fast path. Adding
another bit to the mask costs nothing.

--
Brian Gerst