[PATCH v2 2/3] x86/mm: Fix LAM inconsistency during context switch

[Yosry Ahmed <yosryahmed@google.com>]

LAM can only be enabled when a process is single-threaded.  But _kernel_
threads can temporarily use a single-threaded process's mm.  That means
that a context-switching kernel thread can race and observe the mm's LAM
metadata (mm->context.lam_cr3_mask) change.

The context switch code does two logical things with that metadata:
populate CR3 and populate 'cpu_tlbstate.lam'.  If it hits this race,
'cpu_tlbstate.lam' and CR3 can end up out of sync.

This de-synchronization is currently harmless.  But it is confusing and
might lead to warnings or real bugs.

Update set_tlbstate_lam_mode() to take in the LAM mask and untag mask
instead of an mm_struct pointer, and while we are at it, rename it to
cpu_tlbstate_update_lam(). This should also make it clearer that we are
updating cpu_tlbstate. In switch_mm_irqs_off(), read the LAM mask once
and use it for both the cpu_tlbstate update and the CR3 update.

Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
---
 arch/x86/include/asm/mmu_context.h | 8 +++++++-
 arch/x86/include/asm/tlbflush.h    | 9 ++++-----
 arch/x86/kernel/process_64.c       | 6 ++++--
 arch/x86/mm/tlb.c                  | 8 +++++---
 4 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 8dac45a2c7fcf..19091ebb86338 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -88,7 +88,13 @@ static inline void switch_ldt(struct mm_struct *prev, struct mm_struct *next)
 #ifdef CONFIG_ADDRESS_MASKING
 static inline unsigned long mm_lam_cr3_mask(struct mm_struct *mm)
 {
-	return mm->context.lam_cr3_mask;
+	/*
+	 * When switch_mm_irqs_off() is called for a kthread, it may race with
+	 * LAM enablement. switch_mm_irqs_off() uses the LAM mask to do two
+	 * things: populate CR3 and populate 'cpu_tlbstate.lam'. Make sure it
+	 * reads a single value for both.
+	 */
+	return READ_ONCE(mm->context.lam_cr3_mask);
 }
 
 static inline void dup_lam(struct mm_struct *oldmm, struct mm_struct *mm)
diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 25726893c6f4d..69e79fff41b80 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -399,11 +399,10 @@ static inline  u64 tlbstate_lam_cr3_mask(void)
 	return lam << X86_CR3_LAM_U57_BIT;
 }
 
-static inline void set_tlbstate_lam_mode(struct mm_struct *mm)
+static inline void cpu_tlbstate_update_lam(unsigned long lam, u64 untag_mask)
 {
-	this_cpu_write(cpu_tlbstate.lam,
-		       mm->context.lam_cr3_mask >> X86_CR3_LAM_U57_BIT);
-	this_cpu_write(tlbstate_untag_mask, mm->context.untag_mask);
+	this_cpu_write(cpu_tlbstate.lam, lam >> X86_CR3_LAM_U57_BIT);
+	this_cpu_write(tlbstate_untag_mask, untag_mask);
 }
 
 #else
@@ -413,7 +412,7 @@ static inline u64 tlbstate_lam_cr3_mask(void)
 	return 0;
 }
 
-static inline void set_tlbstate_lam_mode(struct mm_struct *mm)
+static inline void cpu_tlbstate_update_lam(unsigned long lam, u64 untag_mask)
 {
 }
 #endif
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 76e91fc68c5f3..3dd8c1f51e45c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -753,10 +753,12 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
 static void enable_lam_func(void *__mm)
 {
 	struct mm_struct *mm = __mm;
+	unsigned long lam;
 
 	if (this_cpu_read(cpu_tlbstate.loaded_mm) == mm) {
-		write_cr3(__read_cr3() | mm->context.lam_cr3_mask);
-		set_tlbstate_lam_mode(mm);
+		lam = mm_lam_cr3_mask(mm);
+		write_cr3(__read_cr3() | lam);
+		cpu_tlbstate_update_lam(lam, mm_untag_mask(mm));
 	}
 }
 
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index e8feb2e154db2..b2f74c451b3d7 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -11,6 +11,7 @@
 #include <linux/sched/smt.h>
 #include <linux/task_work.h>
 #include <linux/mmu_notifier.h>
+#include <linux/mmu_context.h>
 
 #include <asm/tlbflush.h>
 #include <asm/mmu_context.h>
@@ -635,7 +636,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 	}
 
 	new_lam = mm_lam_cr3_mask(next);
-	set_tlbstate_lam_mode(next);
 	if (need_flush) {
 		this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
 		this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
@@ -654,6 +654,7 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 
 	this_cpu_write(cpu_tlbstate.loaded_mm, next);
 	this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
+	cpu_tlbstate_update_lam(new_lam, mm_untag_mask(next));
 
 	if (next != real_prev) {
 		cr4_update_pce_mm(next);
@@ -700,6 +701,7 @@ void initialize_tlbstate_and_flush(void)
 	int i;
 	struct mm_struct *mm = this_cpu_read(cpu_tlbstate.loaded_mm);
 	u64 tlb_gen = atomic64_read(&init_mm.context.tlb_gen);
+	unsigned long lam = mm_lam_cr3_mask(mm);
 	unsigned long cr3 = __read_cr3();
 
 	/* Assert that CR3 already references the right mm. */
@@ -707,7 +709,7 @@ void initialize_tlbstate_and_flush(void)
 
 	/* LAM expected to be disabled */
 	WARN_ON(cr3 & (X86_CR3_LAM_U48 | X86_CR3_LAM_U57));
-	WARN_ON(mm_lam_cr3_mask(mm));
+	WARN_ON(lam);
 
 	/*
 	 * Assert that CR4.PCIDE is set if needed.  (CR4.PCIDE initialization
@@ -726,7 +728,7 @@ void initialize_tlbstate_and_flush(void)
 	this_cpu_write(cpu_tlbstate.next_asid, 1);
 	this_cpu_write(cpu_tlbstate.ctxs[0].ctx_id, mm->context.ctx_id);
 	this_cpu_write(cpu_tlbstate.ctxs[0].tlb_gen, tlb_gen);
-	set_tlbstate_lam_mode(mm);
+	cpu_tlbstate_update_lam(lam, mm_untag_mask(mm));
 
 	for (i = 1; i < TLB_NR_DYN_ASIDS; i++)
 		this_cpu_write(cpu_tlbstate.ctxs[i].ctx_id, 0);
-- 
2.44.0.278.ge034bb2e1d-goog