IB/uverbs: Fix locking around struct ib_uverbs_file ucontext

We have a parallel unlocked reader and writer with ib_uverbs_get_context()
vs everything else, and nothing guarantees this works properly.

Audit and fix all of the places that access ucontext to use one of the
following locking schemes:
- Call ib_uverbs_get_ucontext() under SRCU and check for failure
- Access the ucontext through an struct ib_uobject context member
  while holding a READ or WRITE lock on the uobject.
  This value cannot be NULL and has no race.
- Hold the ucontext_lock and check for ufile->ucontext !NULL

This also re-implements ib_uverbs_get_ucontext() in a way that is safe
against concurrent ib_uverbs_get_context() and disassociation.

As a side effect, every access to ucontext in the commands is via
ib_uverbs_get_context() with an error check, or via the uobject, so there
is no longer any need for the core code to check ucontext on every command
call. These checks are also removed.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

1 files changed, 24 insertions, 14 deletions

diff --git a/drivers/infiniband/core/uverbs_main.c b/drivers/infiniband/core/uverbs_main.c
index 78d79020ea5c..34df04ed142b 100644
--- a/drivers/infiniband/core/uverbs_main.c
+++ b/drivers/infiniband/core/uverbs_main.c
@@ -136,9 +136,27 @@ static int (*uverbs_ex_cmd_table[])(struct ib_uverbs_file *file,
 static void ib_uverbs_add_one(struct ib_device *device);
 static void ib_uverbs_remove_one(struct ib_device *device, void *client_data);
 
+/*
+ * Must be called with the ufile->device->disassociate_srcu held, and the lock
+ * must be held until use of the ucontext is finished.
+ */
 struct ib_ucontext *ib_uverbs_get_ucontext(struct ib_uverbs_file *ufile)
 {
-	return ufile->ucontext;
+	/*
+	 * We do not hold the hw_destroy_rwsem lock for this flow, instead
+	 * srcu is used. It does not matter if someone races this with
+	 * get_context, we get NULL or valid ucontext.
+	 */
+	struct ib_ucontext *ucontext = smp_load_acquire(&ufile->ucontext);
+
+	if (!srcu_dereference(ufile->device->ib_dev,
+			      &ufile->device->disassociate_srcu))
+		return ERR_PTR(-EIO);
+
+	if (!ucontext)
+		return ERR_PTR(-EINVAL);
+
+	return ucontext;
 }
 EXPORT_SYMBOL(ib_uverbs_get_ucontext);
 
@@ -729,10 +747,6 @@ static ssize_t ib_uverbs_write(struct file *filp, const char __user *buf,
 	if (ret)
 		return ret;
 
-	if (!file->ucontext &&
-	    (command != IB_USER_VERBS_CMD_GET_CONTEXT || extended))
-		return -EINVAL;
-
 	if (extended) {
 		if (count < (sizeof(hdr) + sizeof(ex_hdr)))
 			return -EINVAL;
@@ -791,22 +805,18 @@ out:
 static int ib_uverbs_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	struct ib_uverbs_file *file = filp->private_data;
-	struct ib_device *ib_dev;
+	struct ib_ucontext *ucontext;
 	int ret = 0;
 	int srcu_key;
 
 	srcu_key = srcu_read_lock(&file->device->disassociate_srcu);
-	ib_dev = srcu_dereference(file->device->ib_dev,
-				  &file->device->disassociate_srcu);
-	if (!ib_dev) {
-		ret = -EIO;
+	ucontext = ib_uverbs_get_ucontext(file);
+	if (IS_ERR(ucontext)) {
+		ret = PTR_ERR(ucontext);
 		goto out;
 	}
 
-	if (!file->ucontext)
-		ret = -ENODEV;
-	else
-		ret = ib_dev->mmap(file->ucontext, vma);
+	ret = ucontext->device->mmap(ucontext, vma);
 out:
 	srcu_read_unlock(&file->device->disassociate_srcu, srcu_key);
 	return ret;