From: Miklos Szeredi <mszeredi@redhat.com>
Date: Thu, 20 Oct 2022 17:18:58 +0200
Subject: fuse: fix readdir cache race
Git-commit: 9fa248c65bdbf5af0a2f74dd38575acfc8dfd2bf
Patch-mainline: v6.1-rc4
References: bsc#1205331

There's a race in fuse's readdir cache that can result in an uninitilized
page being read.  The page lock is supposed to prevent this from happening
but in the following case it doesn't:

Two fuse_add_dirent_to_cache() start out and get the same parameters
(size=0,offset=0).  One of them wins the race to create and lock the page,
after which it fills in data, sets rdc.size and unlocks the page.

In the meantime the page gets evicted from the cache before the other
instance gets to run.  That one also creates the page, but finds the
size to be mismatched, bails out and leaves the uninitialized page in the
cache.

Fix by marking a filled page uptodate and ignoring non-uptodate pages.

Reported-by: Frank Sorenson <fsorenso@redhat.com>
Fixes: 5d7bc7e8680c ("fuse: allow using readdir cache")
Cc: <stable@vger.kernel.org> # v4.20
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Acked-by: Luis Henriques <lhenriques@suse.com>

---
 fs/fuse/readdir.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

--- a/fs/fuse/readdir.c
+++ b/fs/fuse/readdir.c
@@ -77,8 +77,10 @@ static void fuse_add_dirent_to_cache(str
 		goto unlock;
 
 	addr = kmap_atomic(page);
-	if (!offset)
+	if (!offset) {
 		clear_page(addr);
+		SetPageUptodate(page);
+	}
 	memcpy(addr + offset, dirent, reclen);
 	kunmap_atomic(addr);
 	fi->rdc.size = (index << PAGE_SHIFT) + offset + reclen;
@@ -516,6 +518,12 @@ retry_locked:
 
 	page = find_get_page_flags(file->f_mapping, index,
 				   FGP_ACCESSED | FGP_LOCK);
+	/* Page gone missing, then re-added to cache, but not initialized? */
+	if (page && !PageUptodate(page)) {
+		unlock_page(page);
+		put_page(page);
+		page = NULL;
+	}
 	spin_lock(&fi->rdc.lock);
 	if (!page) {
 		/*