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[68.80.13.9]) by smtp.gmail.com with ESMTPSA id f19sm660304qkh.2.2019.11.28.07.04.36 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 28 Nov 2019 07:04:36 -0800 (PST) Received: from ben by beast with local (Exim 4.92.1) (envelope-from ) id 1iaLLP-0008AH-MM for kernel-team@lists.ubuntu.com; Thu, 28 Nov 2019 10:04:35 -0500 From: Benjamin M Romer To: kernel-team@lists.ubuntu.com Subject: [xenial][PATCH 2/3] powerpc/book3s64: Fix link stack flush on context switch Date: Thu, 28 Nov 2019 10:04:34 -0500 Message-Id: <20191128150435.31340-3-benjamin.romer@canonical.com> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20191128150435.31340-1-benjamin.romer@canonical.com> References: <20191128150435.31340-1-benjamin.romer@canonical.com> MIME-Version: 1.0 X-BeenThere: kernel-team@lists.ubuntu.com X-Mailman-Version: 2.1.20 Precedence: list List-Id: Kernel team discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kernel-team-bounces@lists.ubuntu.com Sender: "kernel-team" From: Michael Ellerman BugLink: https://bugs.launchpad.net/bugs/1853142 commit 39e72bf96f5847ba87cc5bd7a3ce0fed813dc9ad upstream. In commit ee13cb249fab ("powerpc/64s: Add support for software count cache flush"), I added support for software to flush the count cache (indirect branch cache) on context switch if firmware told us that was the required mitigation for Spectre v2. As part of that code we also added a software flush of the link stack (return address stack), which protects against Spectre-RSB between user processes. That is all correct for CPUs that activate that mitigation, which is currently Power9 Nimbus DD2.3. What I got wrong is that on older CPUs, where firmware has disabled the count cache, we also need to flush the link stack on context switch. To fix it we create a new feature bit which is not set by firmware, which tells us we need to flush the link stack. We set that when firmware tells us that either of the existing Spectre v2 mitigations are enabled. Then we adjust the patching code so that if we see that feature bit we enable the link stack flush. If we're also told to flush the count cache in software then we fall through and do that also. On the older CPUs we don't need to do do the software count cache flush, firmware has disabled it, so in that case we patch in an early return after the link stack flush. The naming of some of the functions is awkward after this patch, because they're called "count cache" but they also do link stack. But we'll fix that up in a later commit to ease backporting. This is the fix for CVE-2019-18660. Reported-by: Anthony Steinhauser Fixes: ee13cb249fab ("powerpc/64s: Add support for software count cache flush") Cc: stable@vger.kernel.org # v4.4+ Signed-off-by: Michael Ellerman [dja: straightforward backport to v4.14 applies directly to Xenial's v4.4] Signed-off-by: Daniel Axtens CVE-2019-18660 Signed-off-by: Benjamin M Romer --- arch/powerpc/include/asm/asm-prototypes.h | 1 + arch/powerpc/include/asm/security_features.h | 3 ++ arch/powerpc/kernel/entry_64.S | 6 +++ arch/powerpc/kernel/security.c | 48 ++++++++++++++++++-- 4 files changed, 54 insertions(+), 4 deletions(-) diff --git a/arch/powerpc/include/asm/asm-prototypes.h b/arch/powerpc/include/asm/asm-prototypes.h index 8944c55591cf..7c74384c0146 100644 --- a/arch/powerpc/include/asm/asm-prototypes.h +++ b/arch/powerpc/include/asm/asm-prototypes.h @@ -15,6 +15,7 @@ /* Patch sites */ extern s32 patch__call_flush_count_cache; extern s32 patch__flush_count_cache_return; +extern s32 patch__flush_link_stack_return; extern long flush_count_cache; diff --git a/arch/powerpc/include/asm/security_features.h b/arch/powerpc/include/asm/security_features.h index 759597bf0fd8..ccf44c135389 100644 --- a/arch/powerpc/include/asm/security_features.h +++ b/arch/powerpc/include/asm/security_features.h @@ -81,6 +81,9 @@ static inline bool security_ftr_enabled(unsigned long feature) // Software required to flush count cache on context switch #define SEC_FTR_FLUSH_COUNT_CACHE 0x0000000000000400ull +// Software required to flush link stack on context switch +#define SEC_FTR_FLUSH_LINK_STACK 0x0000000000001000ull + // Features enabled by default #define SEC_FTR_DEFAULT \ diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S index 6d36a4fb4acf..e523d16c8b6e 100644 --- a/arch/powerpc/kernel/entry_64.S +++ b/arch/powerpc/kernel/entry_64.S @@ -477,6 +477,7 @@ flush_count_cache: /* Save LR into r9 */ mflr r9 + // Flush the link stack .rept 64 bl .+4 .endr @@ -486,6 +487,11 @@ flush_count_cache: .balign 32 /* Restore LR */ 1: mtlr r9 + + // If we're just flushing the link stack, return here +3: nop + patch_site 3b patch__flush_link_stack_return + li r9,0x7fff mtctr r9 diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c index 37d96227e7cf..ce6b612c605a 100644 --- a/arch/powerpc/kernel/security.c +++ b/arch/powerpc/kernel/security.c @@ -25,6 +25,7 @@ enum count_cache_flush_type { COUNT_CACHE_FLUSH_HW = 0x4, }; static enum count_cache_flush_type count_cache_flush_type = COUNT_CACHE_FLUSH_NONE; +static bool link_stack_flush_enabled; bool barrier_nospec_enabled; static bool no_nospec; @@ -205,11 +206,19 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr, c if (ccd) seq_buf_printf(&s, "Indirect branch cache disabled"); + + if (link_stack_flush_enabled) + seq_buf_printf(&s, ", Software link stack flush"); + } else if (count_cache_flush_type != COUNT_CACHE_FLUSH_NONE) { seq_buf_printf(&s, "Mitigation: Software count cache flush"); if (count_cache_flush_type == COUNT_CACHE_FLUSH_HW) seq_buf_printf(&s, " (hardware accelerated)"); + + if (link_stack_flush_enabled) + seq_buf_printf(&s, ", Software link stack flush"); + } else if (btb_flush_enabled) { seq_buf_printf(&s, "Mitigation: Branch predictor state flush"); } else { @@ -368,18 +377,40 @@ static __init int stf_barrier_debugfs_init(void) device_initcall(stf_barrier_debugfs_init); #endif /* CONFIG_DEBUG_FS */ +static void no_count_cache_flush(void) +{ + count_cache_flush_type = COUNT_CACHE_FLUSH_NONE; + pr_info("count-cache-flush: software flush disabled.\n"); +} + static void toggle_count_cache_flush(bool enable) { - if (!enable || !security_ftr_enabled(SEC_FTR_FLUSH_COUNT_CACHE)) { + if (!security_ftr_enabled(SEC_FTR_FLUSH_COUNT_CACHE) && + !security_ftr_enabled(SEC_FTR_FLUSH_LINK_STACK)) + enable = false; + + if (!enable) { patch_instruction_site(&patch__call_flush_count_cache, PPC_INST_NOP); - count_cache_flush_type = COUNT_CACHE_FLUSH_NONE; - pr_info("count-cache-flush: software flush disabled.\n"); + pr_info("link-stack-flush: software flush disabled.\n"); + link_stack_flush_enabled = false; + no_count_cache_flush(); return; } + // This enables the branch from _switch to flush_count_cache patch_branch_site(&patch__call_flush_count_cache, (u64)&flush_count_cache, BRANCH_SET_LINK); + pr_info("link-stack-flush: software flush enabled.\n"); + link_stack_flush_enabled = true; + + // If we just need to flush the link stack, patch an early return + if (!security_ftr_enabled(SEC_FTR_FLUSH_COUNT_CACHE)) { + patch_instruction_site(&patch__flush_link_stack_return, PPC_INST_BLR); + no_count_cache_flush(); + return; + } + if (!security_ftr_enabled(SEC_FTR_BCCTR_FLUSH_ASSIST)) { count_cache_flush_type = COUNT_CACHE_FLUSH_SW; pr_info("count-cache-flush: full software flush sequence enabled.\n"); @@ -398,11 +429,20 @@ void setup_count_cache_flush(void) if (no_spectrev2 || cpu_mitigations_off()) { if (security_ftr_enabled(SEC_FTR_BCCTRL_SERIALISED) || security_ftr_enabled(SEC_FTR_COUNT_CACHE_DISABLED)) - pr_warn("Spectre v2 mitigations not under software control, can't disable\n"); + pr_warn("Spectre v2 mitigations not fully under software control, can't disable\n"); enable = false; } + /* + * There's no firmware feature flag/hypervisor bit to tell us we need to + * flush the link stack on context switch. So we set it here if we see + * either of the Spectre v2 mitigations that aim to protect userspace. + */ + if (security_ftr_enabled(SEC_FTR_COUNT_CACHE_DISABLED) || + security_ftr_enabled(SEC_FTR_FLUSH_COUNT_CACHE)) + security_ftr_set(SEC_FTR_FLUSH_LINK_STACK); + toggle_count_cache_flush(enable); }