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

KP Singh
2023-02-22T01:26:54Z

On Tue, Feb 21, 2023 at 5:20 PM Pawan Gupta
<pawan.kumar.gupta@linux.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@linux.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@infradead.org>
> > >
> > > From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > >
> > > The "spectre_v2=" boot option is extended to enable Kernel IBRS.
> > >
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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) {