Re: [PATCH 02/18] x86/asm/64: Split the iret-to-user and iret-to-kernel paths

Borislav Petkov
2017-10-27T20:04:55Z

On Thu, Oct 26, 2017 at 01:26:34AM -0700, Andy Lutomirski wrote:
> These code paths will diverge soon.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> ---
> arch/x86/entry/entry_64.S | 32 +++++++++++++++++++++++---------
> arch/x86/entry/entry_64_compat.S | 2 +-
> arch/x86/kernel/head_64.S | 2 +-
> 3 files changed, 25 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index afe1f403fa0e..493e5e234d36 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -321,7 +321,7 @@ syscall_return_via_sysret:
>
> opportunistic_sysret_failed:
> SWAPGS
> - jmp restore_regs_and_iret
> + jmp restore_regs_and_return_to_usermode

rstor_regs_ret_user

still sounds pretty ok to me and it is shorter and it is using the
*RSTOR spelling the respective FPU insns use, so should be recognizable. :-)

And the other one could be

rstor_regs_ret_kernel

of course.

> END(entry_SYSCALL_64)
>
> ENTRY(stub_ptregs_64)
> @@ -423,7 +423,7 @@ ENTRY(ret_from_fork)
> call syscall_return_slowpath /* returns with IRQs disabled */
> TRACE_IRQS_ON /* user mode is traced as IRQS on */
> SWAPGS
> - jmp restore_regs_and_iret
> + jmp restore_regs_and_return_to_usermode
>
> 1:
> /* kernel thread */
> @@ -612,7 +612,19 @@ GLOBAL(retint_user)
> call prepare_exit_to_usermode
> TRACE_IRQS_IRETQ
> SWAPGS
> - jmp restore_regs_and_iret
> +
> +GLOBAL(restore_regs_and_return_to_usermode)
> +#ifdef CONFIG_DEBUG_ENTRY
> + testl $3, CS(%rsp)

testb

ditto for the other spot.

> + jnz 1f
> + ud2
> +1:
> +#endif

Why aren't we exploding here unconditionally? I mean, we don't want to
return with RPL != 3 CS for whatever reason...

> + RESTORE_EXTRA_REGS
> + RESTORE_C_REGS
> + REMOVE_PT_GPREGS_FROM_STACK 8
> + INTERRUPT_RETURN
> +
>
> /* Returning to kernel space */
> retint_kernel:
> @@ -632,11 +644,13 @@ retint_kernel:
> */
> TRACE_IRQS_IRETQ
>
> -/*
> - * At this label, code paths which return to kernel and to user,
> - * which come from interrupts/exception and from syscalls, merge.
> - */
> -GLOBAL(restore_regs_and_iret)
> +GLOBAL(restore_regs_and_return_to_kernel)
> +#ifdef CONFIG_DEBUG_ENTRY
> + testl $3, CS(%rsp)
> + jz 1f
> + ud2
> +1:
> +#endif

I guess here too, as a sanity-check. TEST and Jcc should be very cheap
compared to everything else jumping-through-hoops we'll be doing :-\

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--