[PATCH 4.18 291/350] xen: remove size limit of privcmd-buf mapping interface

[Greg Kroah-Hartman]

4.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Juergen Gross <jgross@xxxxxxxx>

commit 3941552aec1e04d63999988a057ae09a1c56ebeb upstream.

Currently the size of hypercall buffers allocated via
/dev/xen/hypercall is limited to a default of 64 memory pages. For live
migration of guests this might be too small as the page dirty bitmask
needs to be sized according to the size of the guest. This means
migrating a 8GB sized guest is already exhausting the default buffer
size for the dirty bitmap.

There is no sensible way to set a sane limit, so just remove it
completely. The device node's usage is limited to root anyway, so there
is no additional DOS scenario added by allowing unlimited buffers.

While at it make the error path for the -ENOMEM case a little bit
cleaner by setting n_pages to the number of successfully allocated
pages instead of the target size.

Fixes: c51b3c639e01f2 ("xen: add new hypercall buffer mapping device")
Cc: <stable@xxxxxxxxxxxxxxx> #4.18
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/xen/privcmd-buf.c |   22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

--- a/drivers/xen/privcmd-buf.c
+++ b/drivers/xen/privcmd-buf.c
@@ -21,15 +21,9 @@
 
 MODULE_LICENSE("GPL");
 
-static unsigned int limit = 64;
-module_param(limit, uint, 0644);
-MODULE_PARM_DESC(limit, "Maximum number of pages that may be allocated by "
-			"the privcmd-buf device per open file");
-
 struct privcmd_buf_private {
 	struct mutex lock;
 	struct list_head list;
-	unsigned int allocated;
 };
 
 struct privcmd_buf_vma_private {
@@ -60,13 +54,10 @@ static void privcmd_buf_vmapriv_free(str
 {
 	unsigned int i;
 
-	vma_priv->file_priv->allocated -= vma_priv->n_pages;
-
 	list_del(&vma_priv->list);
 
 	for (i = 0; i < vma_priv->n_pages; i++)
-		if (vma_priv->pages[i])
-			__free_page(vma_priv->pages[i]);
+		__free_page(vma_priv->pages[i]);
 
 	kfree(vma_priv);
 }
@@ -146,8 +137,7 @@ static int privcmd_buf_mmap(struct file
 	unsigned int i;
 	int ret = 0;
 
-	if (!(vma->vm_flags & VM_SHARED) || count > limit ||
-	    file_priv->allocated + count > limit)
+	if (!(vma->vm_flags & VM_SHARED))
 		return -EINVAL;
 
 	vma_priv = kzalloc(sizeof(*vma_priv) + count * sizeof(void *),
@@ -155,19 +145,15 @@ static int privcmd_buf_mmap(struct file
 	if (!vma_priv)
 		return -ENOMEM;
 
-	vma_priv->n_pages = count;
-	count = 0;
-	for (i = 0; i < vma_priv->n_pages; i++) {
+	for (i = 0; i < count; i++) {
 		vma_priv->pages[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
 		if (!vma_priv->pages[i])
 			break;
-		count++;
+		vma_priv->n_pages++;
 	}
 
 	mutex_lock(&file_priv->lock);
 
-	file_priv->allocated += count;
-
 	vma_priv->file_priv = file_priv;
 	vma_priv->users = 1;