4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jiri Kosina jkosina@suse.cz
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@suse.cz Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Josh Poimboeuf jpoimboe@redhat.com Acked-by: Tim Chen tim.c.chen@linux.intel.com Cc: Konrad Rzeszutek Wilk konrad.wilk@oracle.com Cc: Borislav Petkov bp@suse.de Cc: David Woodhouse dwmw@amazon.co.uk Cc: Peter Zijlstra peterz@infradead.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/nycvar.YFH.7.76.1807261308190.997@cbobk.fhfr.pm Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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 @@ -309,23 +309,6 @@ 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(); @@ -386,22 +369,15 @@ 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_KAISER) && - !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)) {