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Subject[patch 15/16] x86/ldt: Add VMA management code
From: Thomas Gleixner <tglx@linutronix.de>

Add the VMA management code to LDT which allows to install the LDT as a
special mapping, like VDSO and uprobes. The mapping is in the user address
space, but without the usr bit set and read only. Split out for ease of
review.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/ldt.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 102 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -31,6 +31,7 @@
struct ldt_mapping {
struct ldt_struct ldts[2];
unsigned int ldt_index;
+ unsigned int ldt_mapped;
};

/* After calling this, the LDT is immutable. */
@@ -208,6 +209,105 @@ bool __ldt_write_fault(unsigned long add
return true;
}

+static int ldt_fault(const struct vm_special_mapping *sm,
+ struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+ struct ldt_mapping *lmap = vma->vm_mm->context.ldt_mapping;
+ struct ldt_struct *ldt = lmap->ldts;
+ pgoff_t pgo = vmf->pgoff;
+ struct page *page;
+
+ if (pgo >= LDT_ENTRIES_PAGES) {
+ pgo -= LDT_ENTRIES_PAGES;
+ ldt++;
+ }
+ if (pgo >= LDT_ENTRIES_PAGES)
+ return VM_FAULT_SIGBUS;
+
+ page = ldt->pages[pgo];
+ if (!page)
+ return VM_FAULT_SIGBUS;
+ get_page(page);
+ vmf->page = page;
+ return 0;
+}
+
+static int ldt_mremap(const struct vm_special_mapping *sm,
+ struct vm_area_struct *new_vma)
+{
+ return -EINVAL;
+}
+
+static void ldt_close(const struct vm_special_mapping *sm,
+ struct vm_area_struct *vma)
+{
+ struct mm_struct *mm = vma->vm_mm;
+ struct ldt_struct *ldt;
+
+ /*
+ * Orders against ldt_install().
+ */
+ mutex_lock(&mm->context.lock);
+ ldt = mm->context.ldt;
+ ldt_install_mm(mm, NULL);
+ cleanup_ldt_struct(ldt);
+ mm->context.ldt_mapping->ldt_mapped = 0;
+ mutex_unlock(&mm->context.lock);
+}
+
+static const struct vm_special_mapping ldt_special_mapping = {
+ .name = "[ldt]",
+ .fault = ldt_fault,
+ .mremap = ldt_mremap,
+ .close = ldt_close,
+};
+
+static struct vm_area_struct *ldt_alloc_vma(struct mm_struct *mm,
+ struct ldt_mapping *lmap)
+{
+ unsigned long vm_flags, size;
+ struct vm_area_struct *vma;
+ unsigned long addr;
+
+ size = 2 * LDT_ENTRIES_MAP_SIZE;
+ addr = get_unmapped_area(NULL, TASK_SIZE - PAGE_SIZE, size, 0, 0);
+ if (IS_ERR_VALUE(addr))
+ return ERR_PTR(addr);
+
+ vm_flags = VM_READ | VM_LOCKED | VM_WIPEONFORK | VM_NOUSER | VM_SHARED;
+ vma = _install_special_mapping(mm, addr, size, vm_flags,
+ &ldt_special_mapping);
+ if (IS_ERR(vma))
+ return vma;
+
+ lmap->ldts[0].entries = (struct desc_struct *) addr;
+ addr += LDT_ENTRIES_MAP_SIZE;
+ lmap->ldts[1].entries = (struct desc_struct *) addr;
+ return vma;
+}
+
+static int ldt_mmap(struct mm_struct *mm, struct ldt_mapping *lmap)
+{
+ struct vm_area_struct *vma;
+ int ret = 0;
+
+ if (down_write_killable(&mm->mmap_sem))
+ return -EINTR;
+ vma = ldt_alloc_vma(mm, lmap);
+ if (IS_ERR(vma)) {
+ ret = PTR_ERR(vma);
+ } else {
+ /*
+ * The moment mmap_sem() is released munmap() can observe
+ * the mapping and make it go away through ldt_close(). But
+ * for now there is mapping.
+ */
+ lmap->ldt_mapped = 1;
+ }
+ up_write(&mm->mmap_sem);
+ return ret;
+}
+
/* The caller must call finalize_ldt_struct on the result. LDT starts zeroed. */
static struct ldt_struct *alloc_ldt_struct(unsigned int num_entries)
{
@@ -350,7 +450,8 @@ static int read_ldt(void __user *ptr, un

down_read(&mm->context.ldt_usr_sem);

- ldt = mm->context.ldt;
+ /* Might race against vm_unmap, which installs a NULL LDT */
+ ldt = READ_ONCE(mm->context.ldt);
if (!ldt)
goto out_unlock;


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