Messages in this thread Patch in this message | | | Date | Tue, 21 Feb 2023 17:20:09 -0800 | From | Pawan Gupta <> | Subject | Re: [PATCH] x86/bugs: Allow STIBP with IBRS |
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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)) { /* 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) {
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