4.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jiri Kosina <jkosina@xxxxxxx> commit fdf82a7856b32d905c39afc85e34364491e46346 upstream. The article "Spectre Returns! Speculation Attacks using the Return Stack Buffer" [1] describes two new (sub-)variants of spectrev2-like attacks, making use solely of the RSB contents even on CPUs that don't fallback to BTB on RSB underflow (Skylake+). Mitigate userspace-userspace attacks by always unconditionally filling RSB on context switch when the generic spectrev2 mitigation has been enabled. [1] https://arxiv.org/pdf/1807.07940.pdf Signed-off-by: Jiri Kosina <jkosina@xxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> Acked-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxx> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Link: https://lkml.kernel.org/r/nycvar.YFH.7.76.1807261308190.997@xxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kernel/cpu/bugs.c | 38 +++++++------------------------------- 1 file changed, 7 insertions(+), 31 deletions(-) --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -313,23 +313,6 @@ static enum spectre_v2_mitigation_cmd __ return cmd; } -/* Check for Skylake-like CPUs (for RSB handling) */ -static bool __init is_skylake_era(void) -{ - if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL && - boot_cpu_data.x86 == 6) { - switch (boot_cpu_data.x86_model) { - case INTEL_FAM6_SKYLAKE_MOBILE: - case INTEL_FAM6_SKYLAKE_DESKTOP: - case INTEL_FAM6_SKYLAKE_X: - case INTEL_FAM6_KABYLAKE_MOBILE: - case INTEL_FAM6_KABYLAKE_DESKTOP: - return true; - } - } - return false; -} - static void __init spectre_v2_select_mitigation(void) { enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline(); @@ -390,22 +373,15 @@ retpoline_auto: pr_info("%s\n", spectre_v2_strings[mode]); /* - * If neither SMEP nor PTI are available, there is a risk of - * hitting userspace addresses in the RSB after a context switch - * from a shallow call stack to a deeper one. To prevent this fill - * the entire RSB, even when using IBRS. + * If spectre v2 protection has been enabled, unconditionally fill + * RSB during a context switch; this protects against two independent + * issues: * - * Skylake era CPUs have a separate issue with *underflow* of the - * RSB, when they will predict 'ret' targets from the generic BTB. - * The proper mitigation for this is IBRS. If IBRS is not supported - * or deactivated in favour of retpolines the RSB fill on context - * switch is required. + * - RSB underflow (and switch to BTB) on Skylake+ + * - SpectreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs */ - if ((!boot_cpu_has(X86_FEATURE_PTI) && - !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) { - setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW); - pr_info("Spectre v2 mitigation: Filling RSB on context switch\n"); - } + setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW); + pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n"); /* Initialize Indirect Branch Prediction Barrier if supported */ if (boot_cpu_has(X86_FEATURE_IBPB)) {
