Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

[Linus Torvalds <torvalds@linux-foundation.org>]

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All of this is pure garbage.

Is Intel really planning on making this shit architectural? Has anybody
talked to them and told them they are f*cking insane?

Please, any Intel engineers here - talk to your managers.

        Linus

On Jan 20, 2018 11:23, "KarimAllah Ahmed" <karahmed@amazon.de> wrote:

> From: Tim Chen <tim.c.chen@linux.intel.com>
>
> Create macros to control Indirect Branch Speculation.
>
> Name them so they reflect what they are actually doing.
> The macros are used to restrict and unrestrict the indirect branch
> speculation.
> They do not *disable* (or *enable*) indirect branch speculation. A trip
> back to
> user-space after *restricting* speculation would still affect the BTB.
>
> Quoting from a commit by Tim Chen:
>
> """
>     If IBRS is set, near returns and near indirect jumps/calls will not
> allow
>     their predicted target address to be controlled by code that executed
> in a
>     less privileged prediction mode *BEFORE* the IBRS mode was last
> written with
>     a value of 1 or on another logical processor so long as all Return
> Stack
>     Buffer (RSB) entries from the previous less privileged prediction mode
> are
>     overwritten.
>
>     Thus a near indirect jump/call/return may be affected by code in a less
>     privileged prediction mode that executed *AFTER* IBRS mode was last
> written
>     with a value of 1.
> """
>
> [ tglx: Changed macro names and rewrote changelog ]
> [ karahmed: changed macro names *again* and rewrote changelog ]
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Greg KH <gregkh@linuxfoundation.org>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Arjan Van De Ven <arjan.van.de.ven@intel.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: Ashok Raj <ashok.raj@intel.com>
> Link: https://lkml.kernel.org/r/3aab341725ee6a9aafd3141387453b
> 45d788d61a.1515542293.git.tim.c.chen@linux.intel.com
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>  arch/x86/entry/calling.h | 73 ++++++++++++++++++++++++++++++
> ++++++++++++++++++
>  1 file changed, 73 insertions(+)
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 3f48f69..5aafb51 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -6,6 +6,8 @@
>  #include <asm/percpu.h>
>  #include <asm/asm-offsets.h>
>  #include <asm/processor-flags.h>
> +#include <asm/msr-index.h>
> +#include <asm/cpufeatures.h>
>
>  /*
>
> @@ -349,3 +351,74 @@ For 32-bit we have the following conventions - kernel
> is built with
>  .Lafter_call_\@:
>  #endif
>  .endm
> +
> +/*
> + * IBRS related macros
> + */
> +.macro PUSH_MSR_REGS
> +       pushq   %rax
> +       pushq   %rcx
> +       pushq   %rdx
> +.endm
> +
> +.macro POP_MSR_REGS
> +       popq    %rdx
> +       popq    %rcx
> +       popq    %rax
> +.endm
> +
> +.macro WRMSR_ASM msr_nr:req edx_val:req eax_val:req
> +       movl    \msr_nr, %ecx
> +       movl    \edx_val, %edx
> +       movl    \eax_val, %eax
> +       wrmsr
> +.endm
> +
> +.macro RESTRICT_IB_SPEC
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       PUSH_MSR_REGS
> +       WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $SPEC_CTRL_IBRS
> +       POP_MSR_REGS
> +.Lskip_\@:
> +.endm
> +
> +.macro UNRESTRICT_IB_SPEC
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       PUSH_MSR_REGS
> +       WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0
> +       POP_MSR_REGS
> +.Lskip_\@:
> +.endm
> +
> +.macro RESTRICT_IB_SPEC_CLOBBER
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $SPEC_CTRL_IBRS
> +.Lskip_\@:
> +.endm
> +
> +.macro UNRESTRICT_IB_SPEC_CLOBBER
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0
> +.Lskip_\@:
> +.endm
> +
> +.macro RESTRICT_IB_SPEC_SAVE_AND_CLOBBER save_reg:req
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       movl    $MSR_IA32_SPEC_CTRL, %ecx
> +       rdmsr
> +       movl    %eax, \save_reg
> +       movl    $0, %edx
> +       movl    $SPEC_CTRL_IBRS, %eax
> +       wrmsr
> +.Lskip_\@:
> +.endm
> +
> +.macro RESTORE_IB_SPEC_CLOBBER save_reg:req
> +       ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS
> +       /* Set IBRS to the value saved in the save_reg */
> +       movl    $MSR_IA32_SPEC_CTRL, %ecx
> +       movl    $0, %edx
> +       movl    \save_reg, %eax
> +       wrmsr
> +.Lskip_\@:
> +.endm
> --
> 2.7.4
>
>

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