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Subject: [PATCH 1/2] fuse: Fix race in fuse_writepage_in_flight()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     miklos@szeredi.hu, ktkhai@virtuozzo.com,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Mon, 26 Nov 2018 12:46:10 +0300
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Checking for FR_PENDING in fuse_writepage_in_flight() is racy.
It does not guarantee the first request in misc.write.next list
is not in userspace, since there we take fc->lock, while
fuse_dev_do_read() takes fiq->waitq.lock:

fuse_dev_read()                           fuse_writepage_in_flight()
                                            test_bit(FR_PENDING)
  clear_bit(FR_PENDING)
handle old_req->pages[0] in userspace
                                            copy_highpage(old_req->pages[0], page)
                                            ^^^^^
                                            userspace never sees this pages

The only reliable way to determ, whether we are able to replace
old_req's page, is to completely skip the first request in the list.
This patch makes the function to do that.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 fs/fuse/file.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index b52f9baaa3e7..c6650c68b31a 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1740,6 +1740,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
 {
 	struct fuse_conn *fc = get_fuse_conn(new_req->inode);
 	struct fuse_inode *fi = get_fuse_inode(new_req->inode);
+	struct fuse_req *first_req;
 	struct fuse_req *tmp;
 	struct fuse_req *old_req;
 	bool found = false;
@@ -1764,6 +1765,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
 	}
 
 	new_req->num_pages = 1;
+	first_req = old_req;
 	for (tmp = old_req; tmp != NULL; tmp = tmp->misc.write.next) {
 		BUG_ON(tmp->inode != new_req->inode);
 		curr_index = tmp->misc.write.in.offset >> PAGE_SHIFT;
@@ -1773,7 +1775,7 @@ static bool fuse_writepage_in_flight(struct fuse_req *new_req,
 		}
 	}
 
-	if (old_req->num_pages == 1 && test_bit(FR_PENDING, &old_req->flags)) {
+	if (old_req->num_pages == 1 && old_req != first_req) {
 		struct backing_dev_info *bdi = inode_to_bdi(page->mapping->host);
 
 		copy_highpage(old_req->pages[0], page);