[PATCH 3/3] mm: check for VMA being detached before destroying it

[Suren Baghdasaryan <surenb@google.com>]

By the time VMA is freed it has to be detached with the exception of
exit_mmap which is destroying the whole VMA tree. Enforce this
requirement before freeing the VMA. exit_mmap in the only user calling
__vm_area_free directly, therefore it won't trigger the new check.
Change VMA initialization to mark new VMAs as detached and change that
flag once the VMA is added into a tree.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h | 4 ++--
 kernel/fork.c      | 2 ++
 mm/internal.h      | 1 +
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 74e3033c9fc2..9a10fcdb134e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -247,7 +247,7 @@ void setup_initial_init_mm(void *start_code, void *end_code,
 struct vm_area_struct *vm_area_alloc(struct mm_struct *);
 struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
 void vm_area_free(struct vm_area_struct *);
-/* Use only if VMA has no other users */
+/* Use only if VMA has no other users and might still be attached to a tree */
 void __vm_area_free(struct vm_area_struct *vma);
 
 #ifndef CONFIG_MMU
@@ -751,7 +751,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
 	vma->vm_mm = mm;
 	vma->vm_ops = &dummy_vm_ops;
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
-	vma_mark_detached(vma, false);
+	vma->detached = true;
 	vma_numab_state_init(vma);
 }
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 41c964104b58..000fc429345c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -540,6 +540,7 @@ static void vm_area_free_rcu_cb(struct rcu_head *head)
 
 	/* The vma should not be locked while being destroyed. */
 	VM_BUG_ON_VMA(rwsem_is_locked(&vma->vm_lock->lock), vma);
+	WARN_ON_ONCE(!vma->detached);
 	__vm_area_free(vma);
 }
 #endif
@@ -549,6 +550,7 @@ void vm_area_free(struct vm_area_struct *vma)
 #ifdef CONFIG_PER_VMA_LOCK
 	call_rcu(&vma->vm_rcu, vm_area_free_rcu_cb);
 #else
+	WARN_ON_ONCE(!vma->detached);
 	__vm_area_free(vma);
 #endif
 }
diff --git a/mm/internal.h b/mm/internal.h
index 68410c6d97ac..728189e6c703 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1068,6 +1068,7 @@ static inline void vma_iter_store(struct vma_iterator *vmi,
 	vmi->mas.index = vma->vm_start;
 	vmi->mas.last = vma->vm_end - 1;
 	mas_store_prealloc(&vmi->mas, vma);
+	vma_mark_detached(vma, false);
 }
 
 static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
-- 
2.41.0.162.gfafddb0af9-goog