[PATCH 2/3] x86/mm: Fix LAM inconsistency during context switch
From: Yosry Ahmed
Date: Tue Mar 12 2024 - 00:00:12 EST
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@xxxxxxxxxx>
---
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..748d2b3bdb985 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 = mm_lam_cr3_mask(mm);
+ u64 untag_mask = mm_untag_mask(mm);
if (this_cpu_read(cpu_tlbstate.loaded_mm) == mm) {
- write_cr3(__read_cr3() | mm->context.lam_cr3_mask);
- set_tlbstate_lam_mode(mm);
+ write_cr3(__read_cr3() | lam);
+ cpu_tlbstate_update_lam(lam, untag_mask);
}
}
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