[PATCH] mm: mm, mmap: do not blow on PROT_NONE MAP_FIXED holes in the stack

[Michal Hocko]

"mm: enlarge stack guard gap" has introduced a regression in some rust
and Java environments which are trying to implement their own stack
guard page.  They are punching a new MAP_FIXED mapping inside the
existing stack Vma.

This will confuse expand_{downwards,upwards} into thinking that the stack
expansion would in fact get us too close to an existing non-stack vma
which is a correct behavior wrt. safety. It is a real regression on
the other hand. Let's work around the problem by considering PROT_NONE
mapping as a part of the stack. This is a gros hack but overflowing to
such a mapping would trap anyway an we only can hope that usespace
knows what it is doing and handle it propely.

Fixes: d4d2d35e6ef9 ("mm: larger stack guard gap, between vmas")
Debugged-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 mm/mmap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f60a8bc2869c..2e996cbf4ff3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2244,7 +2244,8 @@ int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 		gap_addr = TASK_SIZE;
 
 	next = vma->vm_next;
-	if (next && next->vm_start < gap_addr) {
+	if (next && next->vm_start < gap_addr &&
+			(next->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
 		if (!(next->vm_flags & VM_GROWSUP))
 			return -ENOMEM;
 		/* Check that both stack segments have the same anon_vma? */
@@ -2325,7 +2326,8 @@ int expand_downwards(struct vm_area_struct *vma,
 	/* Enforce stack_guard_gap */
 	prev = vma->vm_prev;
 	/* Check that both stack segments have the same anon_vma? */
-	if (prev && !(prev->vm_flags & VM_GROWSDOWN)) {
+	if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
+			(prev->vm_flags & (VM_WRITE|VM_READ|VM_EXEC))) {
 		if (address - prev->vm_end < stack_guard_gap)
 			return -ENOMEM;
 	}
-- 
2.11.0

-- 
Michal Hocko
SUSE Labs