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

[Brian Gerst]

On Wed, Jan 10, 2018 at 7:16 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.

We still need to check if we are loading a 32-bit binary.  That must
return with IRET.

> rt_sigreturn:  that's the only one who has problems. I added a new
>             TIF_FULL_RESTORE to force it into the slow path.

--
Brian Gerst