[RFC PATCH 5/8] mm: Take placement mappings gap into account

[Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>]

When memory is being placed, mmap() will take care to respect the guard
gaps of certain types of memory (VM_SHADOWSTACK, VM_GROWSUP and
VM_GROWSDOWN). In order to ensure guard gaps between mappings, mmap()
needs to consider two things:
 1. That the new mapping isn’t placed in an any existing mappings guard
    gaps.
 2. That the new mapping isn’t placed such that any existing mappings
    are not in *its* guard gaps.

The long standing behavior of mmap() is to ensure 1, but not take any care
around 2. So for example, if there is a PAGE_SIZE free area, and a
mmap() with a PAGE_SIZE size, and a type that has a guard gap is being
placed, mmap() may place the shadow stack in the PAGE_SIZE free area. Then
the mapping that is supposed to have a guard gap will not have a gap to
the adjacent VMA.

For MAP_GROWSDOWN/VM_GROWSDOWN and MAP_GROWSUP/VM_GROWSUP this has not
been a problem in practice because applications place these kinds of
mappings very early, when there is not many mappings to find a space
between. But for shadow stacks, they may be placed throughout the lifetime
of the application.

So define a VM_UNMAPPED_START_GAP_SET flag to specify that a start_gap
field has been set, as most vm_unmapped_area_info structs are not zeroed,
so the added field will often contain garbage. Use
VM_UNMAPPED_START_GAP_SET in unmapped_area/_topdown() to find a space that
includes the guard gap for the new mapping. Take care to not interfere
with the alignment.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/mmap.c          | 21 ++++++++++++++-------
 2 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 9addf16dbf18..160bb6db7a16 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3393,12 +3393,14 @@ extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
 
 struct vm_unmapped_area_info {
 #define VM_UNMAPPED_AREA_TOPDOWN 1
+#define VM_UNMAPPED_START_GAP_SET 2
 	unsigned long flags;
 	unsigned long length;
 	unsigned long low_limit;
 	unsigned long high_limit;
 	unsigned long align_mask;
 	unsigned long align_offset;
+	unsigned long start_gap;
 };
 
 extern unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info);
diff --git a/mm/mmap.c b/mm/mmap.c
index 936d728ba1ca..1b6c333656f9 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1567,14 +1567,17 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
  */
 static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 {
-	unsigned long length, gap;
+	unsigned long length, gap, start_gap = 0;
 	unsigned long low_limit, high_limit;
 	struct vm_area_struct *tmp;
 
 	MA_STATE(mas, &current->mm->mm_mt, 0, 0);
 
+	if (info->flags & VM_UNMAPPED_START_GAP_SET)
+		start_gap = info->start_gap;
+
 	/* Adjust search length to account for worst case alignment overhead */
-	length = info->length + info->align_mask;
+	length = info->length + info->align_mask + start_gap;
 	if (length < info->length)
 		return -ENOMEM;
 
@@ -1586,7 +1589,7 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
 	if (mas_empty_area(&mas, low_limit, high_limit - 1, length))
 		return -ENOMEM;
 
-	gap = mas.index;
+	gap = mas.index + start_gap;
 	gap += (info->align_offset - gap) & info->align_mask;
 	tmp = mas_next(&mas, ULONG_MAX);
 	if (tmp && (tmp->vm_flags & VM_STARTGAP_FLAGS)) { /* Avoid prev check if possible */
@@ -1619,13 +1622,17 @@ static unsigned long unmapped_area(struct vm_unmapped_area_info *info)
  */
 static unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info)
 {
-	unsigned long length, gap, gap_end;
+	unsigned long length, gap, gap_end, start_gap = 0;
 	unsigned long low_limit, high_limit;
 	struct vm_area_struct *tmp;
 
 	MA_STATE(mas, &current->mm->mm_mt, 0, 0);
+
+	if (info->flags & VM_UNMAPPED_START_GAP_SET)
+		start_gap = info->start_gap;
+
 	/* Adjust search length to account for worst case alignment overhead */
-	length = info->length + info->align_mask;
+	length = info->length + info->align_mask + start_gap;
 	if (length < info->length)
 		return -ENOMEM;
 
@@ -1832,7 +1839,7 @@ unsigned long mm_get_unmapped_area_vmflags(struct mm_struct *mm, struct file *fi
 
 unsigned long
 __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
-		unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
+		    unsigned long pgoff, unsigned long flags, vm_flags_t vm_flags)
 {
 	unsigned long (*get_area)(struct file *, unsigned long,
 				  unsigned long, unsigned long, unsigned long)
@@ -1883,7 +1890,7 @@ __get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
 
 unsigned long
 get_unmapped_area(struct file *file, unsigned long addr, unsigned long len,
-		unsigned long pgoff, unsigned long flags)
+		  unsigned long pgoff, unsigned long flags)
 {
 	return __get_unmapped_area(file, addr, len, pgoff, flags, 0);
 }
-- 
2.34.1