mm: make stack guard page logic use vm_prev pointer

Like the mlock() change previously, this makes the stack guard check
code use vma->vm_prev to see what the mapping below the current stack
is, rather than have to look it up with find_vma().

Also, accept an abutting stack segment, since that happens naturally if
you split the stack with mlock or mprotect.

Tested-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/mm/memory.c b/mm/memory.c
index b6e5fd23cc5..2ed2267439d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2770,11 +2770,18 @@ static inline int check_stack_guard_page(struct vm_area_struct *vma, unsigned lo
 {
         address &= PAGE_MASK;
         if ((vma->vm_flags & VM_GROWSDOWN) && address == vma->vm_start) {
-                address -= PAGE_SIZE;
-                if (find_vma(vma->vm_mm, address) != vma)
-                        return -ENOMEM;
+                struct vm_area_struct *prev = vma->vm_prev;
+
+                /*
+                 * Is there a mapping abutting this one below?
+                 *
+                 * That's only ok if it's the same stack mapping
+                 * that has gotten split..
+                 */
+                if (prev && prev->vm_end == address)
+                        return prev->vm_flags & VM_GROWSDOWN ? 0 : -ENOMEM;
 
-                expand_stack(vma, address);
+                expand_stack(vma, address - PAGE_SIZE);
         }
         return 0;
 }