In the "error_p" label, the code calls __free_page() in a loop from
pages[0] to pages[nr_pages -1].

When pages[i] are allocated in a loop with calls to alloc_page() and one
of these allocations fails, the code jumps to the "error_p" label without
saving the real number of successful allocations and without using this
as the limit of the free cycle.

For the above-mentioned reasons, Syzbot reports a bug:
"[syzbot] KASAN: null-ptr-deref Read in __free_pages".[1]

Fix this bug by saving the real number of allocated pages and, in those
cases where the inth iteration of alloc_page() fails and the code jumps
to the "error_p" label, use that number as the upper limit for the index
of the 'for' loop that calls __free_pages(pages[i]).

[1] https://lore.kernel.org/lkml/00000000000084e0cf05daafb25f@google.com/T/#m143407dade7ed9126253175728d6f38505d2393c

Reported-and-tested-by: syzbot+d55757faa9b80590767b@syzkaller.appspotmail.com
Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 kernel/watch_queue.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 00703444a219..5b0531020cf8 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -220,7 +220,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 	struct page **pages;
 	unsigned long *bitmap;
 	unsigned long user_bufs;
-	int ret, i, nr_pages;
+	int ret, i, nr_pages, allocated_pages;
 
 	if (!wqueue)
 		return -ENODEV;
@@ -254,6 +254,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 
 	for (i = 0; i < nr_pages; i++) {
 		pages[i] = alloc_page(GFP_KERNEL);
+		allocated_pages = i;
 		if (!pages[i])
 			goto error_p;
 		pages[i]->index = i * WATCH_QUEUE_NOTES_PER_PAGE;
@@ -271,7 +272,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 	return 0;
 
 error_p:
-	for (i = 0; i < nr_pages; i++)
+	for (i = 0; i < allocated_pages; i++)
 		__free_page(pages[i]);
 	kfree(pages);
 error:
-- 
2.34.1