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From:   Balbir Singh <sblbir@amazon.com>
To:     <tglx@linutronix.de>, <linux-kernel@vger.kernel.org>
CC:     <jpoimboe@redhat.com>, <tony.luck@intel.com>,
        <keescook@chromium.org>, <benh@kernel.crashing.org>,
        <x86@kernel.org>, <dave.hansen@intel.com>,
        "Balbir Singh" <sblbir@amazon.com>
Subject: [PATCH v4 3/6] arch/x86/mm: Refactor cond_ibpb() to support other use cases
Date:   Fri, 24 Apr 2020 00:01:22 +1000
Message-ID: <20200423140125.7332-4-sblbir@amazon.com>
In-Reply-To: <20200423140125.7332-1-sblbir@amazon.com>
References: <20200423140125.7332-1-sblbir@amazon.com>
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cond_ibpb() has the necessary bits required to track the
previous mm in switch_mm_irqs_off(). This can be reused for
other use cases like L1D flushing (on context switch out).

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
---
 arch/x86/include/asm/tlbflush.h |  2 +-
 arch/x86/mm/tlb.c               | 43 +++++++++++++++++----------------
 2 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 6f66d841262d..69e6ea20679c 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -172,7 +172,7 @@ struct tlb_state {
 	/* Last user mm for optimizing IBPB */
 	union {
 		struct mm_struct	*last_user_mm;
-		unsigned long		last_user_mm_ibpb;
+		unsigned long		last_user_mm_spec;
 	};
 
 	u16 loaded_mm_asid;
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 66f96f21a7b6..da5c94286c7d 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -33,10 +33,11 @@
  */
 
 /*
- * Use bit 0 to mangle the TIF_SPEC_IB state into the mm pointer which is
- * stored in cpu_tlb_state.last_user_mm_ibpb.
+ * Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
+ * stored in cpu_tlb_state.last_user_mm_spec.
  */
 #define LAST_USER_MM_IBPB	0x1UL
+#define LAST_USER_MM_SPEC_MASK	(LAST_USER_MM_IBPB)
 
 /*
  * We get here when we do something requiring a TLB invalidation
@@ -189,19 +190,24 @@ static void sync_current_stack_to_mm(struct mm_struct *mm)
 	}
 }
 
-static inline unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
+static inline unsigned long mm_mangle_tif_spec_bits(struct task_struct *next)
 {
 	unsigned long next_tif = task_thread_info(next)->flags;
-	unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;
+	unsigned long spec_bits = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_SPEC_MASK;
 
-	return (unsigned long)next->mm | ibpb;
+	return (unsigned long)next->mm | spec_bits;
 }
 
-static void cond_ibpb(struct task_struct *next)
+static void cond_mitigation(struct task_struct *next)
 {
+	unsigned long prev_mm, next_mm;
+
 	if (!next || !next->mm)
 		return;
 
+	next_mm = mm_mangle_tif_spec_bits(next);
+	prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_spec);
+
 	/*
 	 * Both, the conditional and the always IBPB mode use the mm
 	 * pointer to avoid the IBPB when switching between tasks of the
@@ -212,8 +218,6 @@ static void cond_ibpb(struct task_struct *next)
 	 * exposed data is not really interesting.
 	 */
 	if (static_branch_likely(&switch_mm_cond_ibpb)) {
-		unsigned long prev_mm, next_mm;
-
 		/*
 		 * This is a bit more complex than the always mode because
 		 * it has to handle two cases:
@@ -243,20 +247,14 @@ static void cond_ibpb(struct task_struct *next)
 		 * Optimize this with reasonably small overhead for the
 		 * above cases. Mangle the TIF_SPEC_IB bit into the mm
 		 * pointer of the incoming task which is stored in
-		 * cpu_tlbstate.last_user_mm_ibpb for comparison.
-		 */
-		next_mm = mm_mangle_tif_spec_ib(next);
-		prev_mm = this_cpu_read(cpu_tlbstate.last_user_mm_ibpb);
-
-		/*
+		 * cpu_tlbstate.last_user_mm_spec for comparison.
+		 *
 		 * Issue IBPB only if the mm's are different and one or
 		 * both have the IBPB bit set.
 		 */
 		if (next_mm != prev_mm &&
 		    (next_mm | prev_mm) & LAST_USER_MM_IBPB)
 			indirect_branch_prediction_barrier();
-
-		this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, next_mm);
 	}
 
 	if (static_branch_unlikely(&switch_mm_always_ibpb)) {
@@ -265,11 +263,12 @@ static void cond_ibpb(struct task_struct *next)
 		 * different context than the user space task which ran
 		 * last on this CPU.
 		 */
-		if (this_cpu_read(cpu_tlbstate.last_user_mm) != next->mm) {
+		if ((prev_mm & ~LAST_USER_MM_SPEC_MASK) !=
+					(unsigned long)next->mm)
 			indirect_branch_prediction_barrier();
-			this_cpu_write(cpu_tlbstate.last_user_mm, next->mm);
-		}
 	}
+
+	this_cpu_write(cpu_tlbstate.last_user_mm_spec, next_mm);
 }
 
 void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
@@ -374,8 +373,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		 * Avoid user/user BTB poisoning by flushing the branch
 		 * predictor when switching between processes. This stops
 		 * one process from doing Spectre-v2 attacks on another.
+		 * The hook can also be used for mitigations that rely
+		 * on switch_mm for hooks.
 		 */
-		cond_ibpb(tsk);
+		cond_mitigation(tsk);
 
 		if (IS_ENABLED(CONFIG_VMAP_STACK)) {
 			/*
@@ -501,7 +502,7 @@ void initialize_tlbstate_and_flush(void)
 	write_cr3(build_cr3(mm->pgd, 0));
 
 	/* Reinitialize tlbstate. */
-	this_cpu_write(cpu_tlbstate.last_user_mm_ibpb, LAST_USER_MM_IBPB);
+	this_cpu_write(cpu_tlbstate.last_user_mm_spec, LAST_USER_MM_IBPB);
 	this_cpu_write(cpu_tlbstate.loaded_mm_asid, 0);
 	this_cpu_write(cpu_tlbstate.next_asid, 1);
 	this_cpu_write(cpu_tlbstate.ctxs[0].ctx_id, mm->context.ctx_id);
-- 
2.17.1