Re: [PATCH v3] fuse: In fuse_flush only wait if someone wants the return code

[Miklos Szeredi]

On Mon, 28 Nov 2022 at 16:01, Tycho Andersen <tycho@tycho.pizza> wrote:
>
> Hi Milkos,
>
> On Mon, Nov 14, 2022 at 09:02:09AM -0700, Tycho Andersen wrote:
> > v3: use schedule_work() to avoid other sleeps in inode_write_now() and
> >     fuse_sync_writes(). Fix a UAF of the stack-based inarg.
>
> Thoughts on this version?

Skipping attr invalidation on success is wrong.  And there's still too
much duplication, IMO.

How about the attached (untested) patch?

Thanks,
Miklos
---
 fs/fuse/file.c |   89 ++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 63 insertions(+), 26 deletions(-)

--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -18,6 +18,7 @@
 #include <linux/falloc.h>
 #include <linux/uio.h>
 #include <linux/fs.h>
+#include <linux/file.h>
 
 static int fuse_send_open(struct fuse_mount *fm, u64 nodeid,
 			  unsigned int open_flags, int opcode,
@@ -477,48 +478,36 @@ static void fuse_sync_writes(struct inod
 	fuse_release_nowrite(inode);
 }
 
-static int fuse_flush(struct file *file, fl_owner_t id)
-{
-	struct inode *inode = file_inode(file);
-	struct fuse_mount *fm = get_fuse_mount(inode);
-	struct fuse_file *ff = file->private_data;
+struct fuse_flush_args {
+	struct fuse_args args;
 	struct fuse_flush_in inarg;
-	FUSE_ARGS(args);
-	int err;
-
-	if (fuse_is_bad(inode))
-		return -EIO;
+	struct work_struct work;
+	struct file *file;
+};
 
-	if (ff->open_flags & FOPEN_NOFLUSH && !fm->fc->writeback_cache)
-		return 0;
+static int fuse_do_flush(struct fuse_flush_args *fa)
+{
+	int err;
+	struct inode *inode = file_inode(fa->file);
+	struct fuse_mount *fm = get_fuse_mount(inode);
 
 	err = write_inode_now(inode, 1);
 	if (err)
-		return err;
+		goto out;
 
 	inode_lock(inode);
 	fuse_sync_writes(inode);
 	inode_unlock(inode);
 
-	err = filemap_check_errors(file->f_mapping);
+	err = filemap_check_errors(fa->file->f_mapping);
 	if (err)
-		return err;
+		goto out;
 
 	err = 0;
 	if (fm->fc->no_flush)
 		goto inval_attr_out;
 
-	memset(&inarg, 0, sizeof(inarg));
-	inarg.fh = ff->fh;
-	inarg.lock_owner = fuse_lock_owner_id(fm->fc, id);
-	args.opcode = FUSE_FLUSH;
-	args.nodeid = get_node_id(inode);
-	args.in_numargs = 1;
-	args.in_args[0].size = sizeof(inarg);
-	args.in_args[0].value = &inarg;
-	args.force = true;
-
-	err = fuse_simple_request(fm, &args);
+	err = fuse_simple_request(fm, &fa->args);
 	if (err == -ENOSYS) {
 		fm->fc->no_flush = 1;
 		err = 0;
@@ -531,9 +520,57 @@ static int fuse_flush(struct file *file,
 	 */
 	if (!err && fm->fc->writeback_cache)
 		fuse_invalidate_attr_mask(inode, STATX_BLOCKS);
+
+out:
+	fput(fa->file);
+	kfree(fa);
 	return err;
 }
 
+static void fuse_flush_async(struct work_struct *work)
+{
+	struct fuse_flush_args *fa = container_of(work, typeof(*fa), work);
+
+	fuse_do_flush(fa);
+}
+
+static int fuse_flush(struct file *file, fl_owner_t id)
+{
+	struct fuse_flush_args *fa;
+	struct inode *inode = file_inode(file);
+	struct fuse_mount *fm = get_fuse_mount(inode);
+	struct fuse_file *ff = file->private_data;
+
+	if (fuse_is_bad(inode))
+		return -EIO;
+
+	if (ff->open_flags & FOPEN_NOFLUSH && !fm->fc->writeback_cache)
+		return 0;
+
+	fa = kzalloc(sizeof(*fa), GFP_KERNEL);
+	if (!fa)
+		return -ENOMEM;
+
+	fa->inarg.fh = ff->fh;
+	fa->inarg.lock_owner = fuse_lock_owner_id(fm->fc, id);
+	fa->args.opcode = FUSE_FLUSH;
+	fa->args.nodeid = get_node_id(inode);
+	fa->args.in_numargs = 1;
+	fa->args.in_args[0].size = sizeof(fa->inarg);
+	fa->args.in_args[0].value = &fa->inarg;
+	fa->args.force = true;
+	fa->file = get_file(file);
+
+	/* Don't wait if the task is exiting */
+	if (current->flags & PF_EXITING) {
+		INIT_WORK(&fa->work, fuse_flush_async);
+		schedule_work(&fa->work);
+		return 0;
+	}
+
+	return fuse_do_flush(fa);
+}
+
 int fuse_fsync_common(struct file *file, loff_t start, loff_t end,
 		      int datasync, int opcode)
 {