linux-kernel - Re: [PATCH] x86/bugs: Allow STIBP with IBRS

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Date:   Tue, 21 Feb 2023 17:26:31 -0800
From:   KP Singh <kpsingh@...nel.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        pjt@...gle.com, evn@...gle.com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, peterz@...radead.org, kim.phillips@....com,
        alexandre.chartre@...cle.com, daniel.sneddon@...ux.intel.com,
        José Oliveira <joseloliveira11@...il.com>,
        Rodrigo Branco <rodrigo@...nelhacking.com>,
        Alexandra Sandulescu <aesa@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] x86/bugs: Allow STIBP with IBRS

On Tue, Feb 21, 2023 at 5:20 PM Pawan Gupta
<pawan.kumar.gupta@...ux.intel.com> wrote:
>
> On Mon, Feb 20, 2023 at 11:09:08AM -0800, Josh Poimboeuf wrote:
> > On Mon, Feb 20, 2023 at 07:34:59PM +0100, Borislav Petkov wrote:
> > > Drop stable@ again.
> > >
> > > On Mon, Feb 20, 2023 at 10:27:17AM -0800, Josh Poimboeuf wrote:
> > > > IBRS is only enabled in kernel space.  Since it's not enabled in user
> > > > space, user space isn't protected from indirect branch prediction
> > > > attacks from a sibling CPU thread.
> > > >
> > > > Allow STIBP to be enabled to protect against such attacks.
> > > >
> > > > Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> > >
> > > Yah, look at that one:
> > >
> > > commit 7c693f54c873691a4b7da05c7e0f74e67745d144
> > > Author: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > > Date:   Tue Jun 14 23:15:55 2022 +0200
> > >
> > >     x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS
> > >
> > >     Extend spectre_v2= boot option with Kernel IBRS.
> > >
> > >     [jpoimboe: no STIBP with IBRS]
> > >     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > >
> > > I'm assuming this was supposed to mean no STIBP in *kernel mode* when
> > > IBRS is selected?
> >
> > No it was supposed to be "no STIBP with *eIBRS*".
>
> Maybe not, "no STIBP with eIBRS" was the state before the said patch.
>
> In an offlist discussion during Retbleed embargo(copied below), it
> appears to mean "no STIBP *in kernel* with IBRS". But anyways, we missed
> to consider userspace.
>
> (BTW replying late because yesterday was a holiday in my geo).
>
> ---
> > > Subject: [PATCH v5 26/30] x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS
> > >
> > > From: Peter Zijlstra <peterz@...radead.org>
> > >
> > > From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > >
> > > The "spectre_v2=" boot option is extended to enable Kernel IBRS.
> > >
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >  Documentation/admin-guide/kernel-parameters.txt |    1
> > >  arch/x86/include/asm/nospec-branch.h            |    1
> > >  arch/x86/kernel/cpu/bugs.c                      |   29 ++++++++++++++++++++++--
> > >  3 files changed, 29 insertions(+), 2 deletions(-)
> > >
> > > @@ -1163,6 +1182,10 @@ static void __init spectre_v2_select_mit
> > >     case SPECTRE_V2_EIBRS:
> > >             break;
> > >
> > > +   case SPECTRE_V2_IBRS:
> > > +           setup_force_cpu_cap(X86_FEATURE_KERNEL_IBRS);
> > > +           break;
> >
> > Don't we also need to set SPEC_CTRL_IBRS in x86_spec_ctrl_base?
>
> Also, STIBP isn't needed with IBRS.  Suggested changes:
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 344ab7c9a4e2..498cb36587a3 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -897,11 +897,13 @@ spectre_v2_parse_user_cmdline(enum spectre_v2_mitigation_cmd v2_cmd)
>         return SPECTRE_V2_USER_CMD_AUTO;
>  }
>
> -static inline bool spectre_v2_in_eibrs_mode(enum spectre_v2_mitigation mode)
> +static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
>  {
> -       return (mode == SPECTRE_V2_EIBRS ||
> -               mode == SPECTRE_V2_EIBRS_RETPOLINE ||
> -               mode == SPECTRE_V2_EIBRS_LFENCE);
> +
> +       return spectre_v2_mode == SPECTRE_V2_IBRS
> +              spectre_v2_mode == SPECTRE_V2_EIBRS ||
> +              spectre_v2_mode == SPECTRE_V2_EIBRS_RETPOLINE ||
> +              spectre_v2_mode == SPECTRE_V2_EIBRS_LFENCE;
>  }
>
>  static void __init
> @@ -966,12 +968,12 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
>         }
>
>         /*
> -        * If no STIBP, enhanced IBRS is enabled or SMT impossible, STIBP is not
> -        * required.
> +        * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible,
> +        * STIBP is not required.
>          */
>         if (!boot_cpu_has(X86_FEATURE_STIBP) ||
>             !smt_possible ||
> -           spectre_v2_in_eibrs_mode(spectre_v2_enabled))
> +           spectre_v2_in_ibrs_mode(spectre_v2_enabled))
>                 return;
>
>         /*
> @@ -1171,7 +1173,7 @@ static void __init spectre_v2_select_mitigation(void)
>         if (mode == SPECTRE_V2_EIBRS && unprivileged_ebpf_enabled())
>                 pr_err(SPECTRE_V2_EIBRS_EBPF_MSG);
>
> -       if (spectre_v2_in_eibrs_mode(mode)) {
> +       if (spectre_v2_in_ibrs_mode(mode)) {

Pawan can you review the v2 that I sent out here:

https://lore.kernel.org/lkml/20230221184908.2349578-1-kpsingh@kernel.org/T/#t

>                 /* Force it so VMEXIT will restore correctly */
>                 x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
>                 wr_spec_ctrl(x86_spec_ctrl_base, true);
> @@ -1212,19 +1214,17 @@ static void __init spectre_v2_select_mitigation(void)
>         pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
>
>         /*
> -        * Retpoline means the kernel is safe because it has no indirect
> -        * branches. Enhanced IBRS protects firmware too, so, enable restricted
> -        * speculation around firmware calls only when Enhanced IBRS isn't
> -        * supported or kernel IBRS isn't enabled.
> +        * Retpoline protects the kernel, but doesn't protect firmware.  IBRS
> +        * and Enhanced IBRS protect firmware too, so enable IBRS around
> +        * firmware calls only when IBRS / Enhanced IBRS aren't otherwise
> +        * enabled.
>          *
>          * Use "mode" to check Enhanced IBRS instead of boot_cpu_has(), because
>          * the user might select retpoline on the kernel command line and if
>          * the CPU supports Enhanced IBRS, kernel might un-intentionally not
>          * enable IBRS around firmware calls.
>          */
> -       if (boot_cpu_has(X86_FEATURE_IBRS) &&
> -           !boot_cpu_has(X86_FEATURE_KERNEL_IBRS) &&
> -           !spectre_v2_in_eibrs_mode(mode)) {
> +       if (boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode)) {
>                 setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
>                 pr_info("Enabling Restricted Speculation for firmware calls\n");
>         }
> @@ -1937,7 +1937,7 @@ static ssize_t tsx_async_abort_show_state(char *buf)
>
>  static char *stibp_state(void)
>  {
> -       if (spectre_v2_in_eibrs_mode(spectre_v2_enabled))
> +       if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
>                 return "";
>
>         switch (spectre_v2_user_stibp) {