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Subject[RFC] Limit mappings to ten per page per process
On Thu, Feb 08, 2018 at 12:21:00PM -0800, Matthew Wilcox wrote:
> Now that I think about it, though, perhaps the simplest solution is not
> to worry about checking whether _mapcount has saturated, and instead when
> adding a new mmap, check whether this task already has it mapped 10 times.
> If so, refuse the mapping.

That turns out to be quite easy. Comments on this approach?

diff --git a/mm/mmap.c b/mm/mmap.c
index 9efdc021ad22..fd64ff662117 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1615,6 +1615,34 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
return (vm_flags & (VM_NORESERVE | VM_SHARED | VM_WRITE)) == VM_WRITE;
}

+/**
+ * mmap_max_overlaps - Check the process has not exceeded its quota of mappings.
+ * @mm: The memory map for the process creating the mapping.
+ * @file: The file the mapping is coming from.
+ * @pgoff: The start of the mapping in the file.
+ * @count: The number of pages to map.
+ *
+ * Return: %true if this region of the file has too many overlapping mappings
+ * by this process.
+ */
+bool mmap_max_overlaps(struct mm_struct *mm, struct file *file,
+ pgoff_t pgoff, pgoff_t count)
+{
+ unsigned int overlaps = 0;
+ struct vm_area_struct *vma;
+
+ if (!file)
+ return false;
+
+ vma_interval_tree_foreach(vma, &file->f_mapping->i_mmap,
+ pgoff, pgoff + count) {
+ if (vma->vm_mm == mm)
+ overlaps++;
+ }
+
+ return overlaps > 9;
+}
+
unsigned long mmap_region(struct file *file, unsigned long addr,
unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
struct list_head *uf)
@@ -1640,6 +1668,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
return -ENOMEM;
}

+ if (mmap_max_overlaps(mm, file, pgoff, len >> PAGE_SHIFT))
+ return -ENOMEM;
+
/* Clear old maps */
while (find_vma_links(mm, addr, addr + len, &prev, &rb_link,
&rb_parent)) {
diff --git a/mm/mremap.c b/mm/mremap.c
index 049470aa1e3e..27cf5cf9fc0f 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -430,6 +430,10 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
(new_len - old_len) >> PAGE_SHIFT))
return ERR_PTR(-ENOMEM);

+ if (mmap_max_overlaps(mm, vma->vm_file, pgoff,
+ (new_len - old_len) >> PAGE_SHIFT))
+ return ERR_PTR(-ENOMEM);
+
if (vma->vm_flags & VM_ACCOUNT) {
unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
if (security_vm_enough_memory_mm(mm, charged))
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