[RFC PATCH 23/47] mm: asi: Add support for mapping all userspace memory into ASI

Junaid Shahid
2022-02-23T05:26:12Z

This adds a new ASI class flag, ASI_MAP_ALL_USERSPACE, which if set,
would automatically map all userspace addresses into that ASI address
space. This is achieved by lazily cloning the userspace PGD entries
during page faults encountered while in that restricted address space.

When the userspace PGD entry is cleared (e.g. in munmap()), we go
through all restricted address spaces with the ASI_MAP_ALL_USERSPACE
flag and clear the corresponding entry in those address spaces as well.

Signed-off-by: Junaid Shahid <junaids@google.com>

---
arch/x86/include/asm/asi.h | 2 +
arch/x86/mm/asi.c | 81 ++++++++++++++++++++++++++++++++++++++
include/asm-generic/asi.h | 7 ++++
mm/memory.c | 2 +
4 files changed, 92 insertions(+)

diff --git a/arch/x86/include/asm/asi.h b/arch/x86/include/asm/asi.h
index 2dc465f78bcc..062ccac07fd9 100644
--- a/arch/x86/include/asm/asi.h
+++ b/arch/x86/include/asm/asi.h
@@ -68,6 +68,8 @@ void asi_unmap(struct asi *asi, void *addr, size_t len, bool flush_tlb);
void asi_flush_tlb_range(struct asi *asi, void *addr, size_t len);
void asi_sync_mapping(struct asi *asi, void *addr, size_t len);
void asi_do_lazy_map(struct asi *asi, size_t addr);
+void asi_clear_user_pgd(struct mm_struct *mm, size_t addr);
+void asi_clear_user_p4d(struct mm_struct *mm, size_t addr);

static inline void asi_init_thread_state(struct thread_struct *thread)
{
diff --git a/arch/x86/mm/asi.c b/arch/x86/mm/asi.c
index ac35323193a3..a3d96be76fa9 100644
--- a/arch/x86/mm/asi.c
+++ b/arch/x86/mm/asi.c
@@ -702,6 +702,41 @@ static bool is_addr_in_local_nonsensitive_range(size_t addr)
addr < VMALLOC_GLOBAL_NONSENSITIVE_START;
}

+static void asi_clone_user_pgd(struct asi *asi, size_t addr)
+{
+ pgd_t *src = pgd_offset_pgd(asi->mm->pgd, addr);
+ pgd_t *dst = pgd_offset_pgd(asi->pgd, addr);
+ pgdval_t old_src, curr_src;
+
+ if (pgd_val(*dst))
+ return;
+
+ VM_BUG_ON(!irqs_disabled());
+
+ /*
+ * This synchronizes against the PGD entry getting cleared by
+ * free_pgd_range(). That path has the following steps:
+ * 1. pgd_clear
+ * 2. asi_clear_user_pgd
+ * 3. Remote TLB Flush
+ * 4. Free page tables
+ *
+ * (3) will be blocked for the duration of this function because the
+ * IPI will remain pending until interrupts are re-enabled.
+ *
+ * The following loop ensures that if we read the PGD value before
+ * (1) and write it after (2), we will re-read the value and write
+ * the new updated value.
+ */
+ curr_src = pgd_val(*src);
+ do {
+ set_pgd(dst, __pgd(curr_src));
+ smp_mb();
+ old_src = curr_src;
+ curr_src = pgd_val(*src);
+ } while (old_src != curr_src);
+}
+
void asi_do_lazy_map(struct asi *asi, size_t addr)
{
if (!static_cpu_has(X86_FEATURE_ASI) || !asi)
@@ -710,6 +745,9 @@ void asi_do_lazy_map(struct asi *asi, size_t addr)
if ((asi->class->flags & ASI_MAP_STANDARD_NONSENSITIVE) &&
is_addr_in_local_nonsensitive_range(addr))
asi_clone_pgd(asi->pgd, asi->mm->asi[0].pgd, addr);
+ else if ((asi->class->flags & ASI_MAP_ALL_USERSPACE) &&
+ addr < TASK_SIZE_MAX)
+ asi_clone_user_pgd(asi, addr);
}

/*
@@ -766,3 +804,46 @@ void __init asi_vmalloc_init(void)
VM_BUG_ON(vmalloc_local_nonsensitive_end >= VMALLOC_END);
VM_BUG_ON(vmalloc_global_nonsensitive_start <= VMALLOC_START);
}
+
+static void __asi_clear_user_pgd(struct mm_struct *mm, size_t addr)
+{
+ uint i;
+
+ if (!static_cpu_has(X86_FEATURE_ASI) || !mm_asi_enabled(mm))
+ return;
+
+ /*
+ * This function is called right after pgd_clear/p4d_clear.
+ * We need to be sure that the preceding pXd_clear is visible before
+ * the ASI pgd clears below. Compare with asi_clone_user_pgd().
+ */
+ smp_mb__before_atomic();
+
+ /*
+ * We need to ensure that the ASI PGD tables do not get freed from
+ * under us. We can potentially use RCU to avoid that, but since
+ * this path is probably not going to be too performance sensitive,
+ * so we just acquire the lock to block asi_destroy().
+ */
+ mutex_lock(&mm->asi_init_lock);
+
+ for (i = 1; i < ASI_MAX_NUM; i++)
+ if (mm->asi[i].class &&
+ (mm->asi[i].class->flags & ASI_MAP_ALL_USERSPACE))
+ set_pgd(pgd_offset_pgd(mm->asi[i].pgd, addr),
+ native_make_pgd(0));
+
+ mutex_unlock(&mm->asi_init_lock);
+}
+
+void asi_clear_user_pgd(struct mm_struct *mm, size_t addr)
+{
+ if (pgtable_l5_enabled())
+ __asi_clear_user_pgd(mm, addr);
+}
+
+void asi_clear_user_p4d(struct mm_struct *mm, size_t addr)
+{
+ if (!pgtable_l5_enabled())
+ __asi_clear_user_pgd(mm, addr);
+}
diff --git a/include/asm-generic/asi.h b/include/asm-generic/asi.h
index 7c50d8b64fa4..8513d0d7865a 100644
--- a/include/asm-generic/asi.h
+++ b/include/asm-generic/asi.h
@@ -6,6 +6,7 @@

/* ASI class flags */
#define ASI_MAP_STANDARD_NONSENSITIVE 1
+#define ASI_MAP_ALL_USERSPACE 2

#ifndef CONFIG_ADDRESS_SPACE_ISOLATION

@@ -85,6 +86,12 @@ void asi_unmap(struct asi *asi, void *addr, size_t len, bool flush_tlb) { }
static inline
void asi_do_lazy_map(struct asi *asi, size_t addr) { }

+static inline
+void asi_clear_user_pgd(struct mm_struct *mm, size_t addr) { }
+
+static inline
+void asi_clear_user_p4d(struct mm_struct *mm, size_t addr) { }
+
static inline
void asi_flush_tlb_range(struct asi *asi, void *addr, size_t len) { }

diff --git a/mm/memory.c b/mm/memory.c
index 8f1de811a1dc..667ece86e051 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -296,6 +296,7 @@ static inline void free_pud_range(struct mmu_gather *tlb, p4d_t *p4d,

pud = pud_offset(p4d, start);
p4d_clear(p4d);
+ asi_clear_user_p4d(tlb->mm, start);
pud_free_tlb(tlb, pud, start);
mm_dec_nr_puds(tlb->mm);
}
@@ -330,6 +331,7 @@ static inline void free_p4d_range(struct mmu_gather *tlb, pgd_t *pgd,

p4d = p4d_offset(pgd, start);
pgd_clear(pgd);
+ asi_clear_user_pgd(tlb->mm, start);
p4d_free_tlb(tlb, p4d, start);
}

--
2.35.1.473.g83b2b277ed-goog