From 2cdcf6481cb3f931b537615c1b9cfbb835e46e67 Mon Sep 17 00:00:00 2001
From: Kent Overstreet <kent.overstreet@gmail.com>
Date: Fri, 18 Dec 2020 04:07:11 -0500
Subject: [PATCH] mm/filemap: fix infinite loop in generic_file_buffered_read()

commit 3644e2d2dda78e21edd8f5415b6d7ab03f5f54f3 upstream.

If iter->count is 0 and iocb->ki_pos is page aligned, this causes
nr_pages to be 0.

Then in generic_file_buffered_read_get_pages() find_get_pages_contig()
returns 0 - because we asked for 0 pages, so we call
generic_file_buffered_read_no_cached_page() which attempts to add a page
to the page cache, which fails with -EEXIST, and then we loop. Oops...

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Reported-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 mm/filemap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 3a983bc1a71c..3b0d8c6dd587 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2203,6 +2203,9 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 	if (unlikely(*ppos >= inode->i_sb->s_maxbytes))
 		return 0;
+	if (unlikely(!iov_iter_count(iter)))
+		return 0;
+
 	iov_iter_truncate(iter, inode->i_sb->s_maxbytes);
 
 	index = *ppos >> PAGE_SHIFT;