[PATCH v2] fuse: prevent possible NULL pointer dereference in fuse_iomap_writeback_{range,submit}()

[Luis Henriques <luis@igalia.com>]

These two functions make use of the WARN_ON_ONCE() macro to help debugging
a NULL wpc->wb_ctx.  However, this doesn't prevent the possibility of NULL
pointer dereferences in the code.  This patch adds some extra defensive
checks to avoid these NULL pointer accesses.

Fixes: ef7e7cbb323f ("fuse: use iomap for writeback")
Signed-off-by: Luis Henriques <luis@igalia.com>
---
Hi!

This v2 results from Joanne's inputs -- I now believe that it is better to
keep the WARN_ON_ONCE() macros, but it's still good to try to minimise
the undesirable effects of a NULL wpc->wb_ctx.

I've also added the 'Fixes:' tag to the commit message.

 fs/fuse/file.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 5525a4520b0f..990c287bc3e3 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -2135,14 +2135,18 @@ static ssize_t fuse_iomap_writeback_range(struct iomap_writepage_ctx *wpc,
 					  unsigned len, u64 end_pos)
 {
 	struct fuse_fill_wb_data *data = wpc->wb_ctx;
-	struct fuse_writepage_args *wpa = data->wpa;
-	struct fuse_args_pages *ap = &wpa->ia.ap;
+	struct fuse_writepage_args *wpa;
+	struct fuse_args_pages *ap;
 	struct inode *inode = wpc->inode;
 	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = get_fuse_conn(inode);
 	loff_t offset = offset_in_folio(folio, pos);
 
-	WARN_ON_ONCE(!data);
+	if (WARN_ON_ONCE(!data))
+		return -EIO;
+
+	wpa = data->wpa;
+	ap = &wpa->ia.ap;
 
 	if (!data->ff) {
 		data->ff = fuse_write_file_get(fi);
@@ -2182,7 +2186,8 @@ static int fuse_iomap_writeback_submit(struct iomap_writepage_ctx *wpc,
 {
 	struct fuse_fill_wb_data *data = wpc->wb_ctx;
 
-	WARN_ON_ONCE(!data);
+	if (WARN_ON_ONCE(!data))
+		return error ? error : -EIO;
 
 	if (data->wpa) {
 		WARN_ON(!data->wpa->ia.ap.num_folios);