fix mremap() vs. ioctx_kill() race

teach ->mremap() method to return an error and have it fail for
aio mappings in process of being killed

Note that in case of ->mremap() failure we need to undo move_page_tables()
we'd already done; we could call ->mremap() first, but then the failure of
move_page_tables() would require undoing whatever _successful_ ->mremap()
has done, which would be a lot more headache in general.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 11 insertions, 6 deletions

diff --git a/fs/aio.c b/fs/aio.c
index f8e52a1..58c33dc 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -278,11 +278,11 @@ static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
-static void aio_ring_remap(struct file *file, struct vm_area_struct *vma)
+static int aio_ring_remap(struct file *file, struct vm_area_struct *vma)
 {
 	struct mm_struct *mm = vma->vm_mm;
 	struct kioctx_table *table;
-	int i;
+	int i, res = -EINVAL;
 
 	spin_lock(&mm->ioctx_lock);
 	rcu_read_lock();
@@ -292,13 +292,17 @@ static void aio_ring_remap(struct file *file, struct vm_area_struct *vma)
 
 		ctx = table->table[i];
 		if (ctx && ctx->aio_ring_file == file) {
-			ctx->user_id = ctx->mmap_base = vma->vm_start;
+			if (!atomic_read(&ctx->dead)) {
+				ctx->user_id = ctx->mmap_base = vma->vm_start;
+				res = 0;
+			}
 			break;
 		}
 	}
 
 	rcu_read_unlock();
 	spin_unlock(&mm->ioctx_lock);
+	return res;
 }
 
 static const struct file_operations aio_ring_fops = {
@@ -748,11 +752,12 @@ static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
 {
 	struct kioctx_table *table;
 
-	if (atomic_xchg(&ctx->dead, 1))
+	spin_lock(&mm->ioctx_lock);
+	if (atomic_xchg(&ctx->dead, 1)) {
+		spin_unlock(&mm->ioctx_lock);
 		return -EINVAL;
+	}
 
-
-	spin_lock(&mm->ioctx_lock);
 	table = rcu_dereference_raw(mm->ioctx_table);
 	WARN_ON(ctx != table->table[ctx->id]);
 	table->table[ctx->id] = NULL;