FAILED: patch "[PATCH] binder: Handle start==NULL in binder_update_page_range()" failed to apply to 4.9-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From 2a9edd056ed4fbf9d2e797c3fc06335af35bccc4 Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@xxxxxxxxxx>
Date: Fri, 18 Oct 2019 22:56:31 +0200
Subject: [PATCH] binder: Handle start==NULL in binder_update_page_range()

The old loop wouldn't stop when reaching `start` if `start==NULL`, instead
continuing backwards to index -1 and crashing.

Luckily you need to be highly privileged to map things at NULL, so it's not
a big problem.

Fix it by adjusting the loop so that the loop variable is always in bounds.

This patch is deliberately minimal to simplify backporting, but IMO this
function could use a refactor. The jump labels in the second loop body are
horrible (the error gotos should be jumping to free_range instead), and
both loops would look nicer if they just iterated upwards through indices.
And the up_read()+mmput() shouldn't be duplicated like that.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20191018205631.248274-3-jannh@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 1f73d12409e3..2d8b9b91dee0 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -276,8 +276,7 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 	return 0;
 
 free_range:
-	for (page_addr = end - PAGE_SIZE; page_addr >= start;
-	     page_addr -= PAGE_SIZE) {
+	for (page_addr = end - PAGE_SIZE; 1; page_addr -= PAGE_SIZE) {
 		bool ret;
 		size_t index;
 
@@ -290,6 +289,8 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		WARN_ON(!ret);
 
 		trace_binder_free_lru_end(alloc, index);
+		if (page_addr == start)
+			break;
 		continue;
 
 err_vm_insert_page_failed:
@@ -297,7 +298,8 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
 		page->page_ptr = NULL;
 err_alloc_page_failed:
 err_page_ptr_cleared:
-		;
+		if (page_addr == start)
+			break;
 	}
 err_no_vma:
 	if (mm) {