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Message-Id: <550472D4-8A2E-4219-B1ED-71EA5597E3A2@amacapital.net>
Date: Wed, 10 Jan 2018 16:55:27 -0800
From: Andy Lutomirski <luto@...capital.net>
To: Andi Kleen <andi@...stfloor.org>
Cc: Brian Gerst <brgerst@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
the arch/x86 maintainers <x86@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
David Woodhouse <dwmw@...zon.co.uk>,
Paul Turner <pjt@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Tom Lendacky <thomas.lendacky@....com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
Dave Hansen <dave.hansen@...el.com>,
Jiri Kosina <jikos@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v1 1/8] x86/entry/clearregs: Remove partial stack frame in fast system call
> On Jan 10, 2018, at 4:16 PM, Andi Kleen <andi@...stfloor.org> 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@...stfloor.org> wrote:
>>> From: Andi Kleen <ak@...ux.intel.com>
>>>
>>> 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@...ux.intel.com>
>>> ---
>>> 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.
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