Re: slab-out-of-bounds in iov_iter_revert()

[Al Viro]

On Thu, Sep 17, 2020 at 03:04:40AM +0100, Al Viro wrote:
> On Wed, Sep 16, 2020 at 05:09:49PM -0400, Qian Cai wrote:
> > On Sat, 2020-09-12 at 00:55 +0100, Al Viro wrote:
> > > On Fri, Sep 11, 2020 at 05:59:04PM -0400, Qian Cai wrote:
> > > > Super easy to reproduce on today's mainline by just fuzzing for a few
> > > > minutes
> > > > on virtiofs (if it ever matters). Any thoughts?
> > > 
> > > Usually happens when ->direct_IO() fucks up and reports the wrong amount
> > > of data written/read.  We had several bugs like that in the past - see
> > > e.g. 85128b2be673 (fix nfs O_DIRECT advancing iov_iter too much).
> > > 
> > > Had there been any recent O_DIRECT-related patches on the filesystems
> > > involved?
> > 
> > This is only reproducible using FUSE/virtiofs so far, so I will stare at
> > fuse_direct_IO() until someone can beat me to it.
> 
> What happens there is that it tries to play with iov_iter_truncate() in
> ->direct_IO() without a corresponding iov_iter_reexpand().  Could you
> check if the following helps?

Gyah...  Sorry, that should be

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 6611ef3269a8..92de6b9b06b0 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -3095,7 +3095,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 	loff_t pos = 0;
 	struct inode *inode;
 	loff_t i_size;
-	size_t count = iov_iter_count(iter);
+	size_t count = iov_iter_count(iter), shortened;
 	loff_t offset = iocb->ki_pos;
 	struct fuse_io_priv *io;
 
@@ -3111,7 +3111,8 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 		if (offset >= i_size)
 			return 0;
 		iov_iter_truncate(iter, fuse_round_up(ff->fc, i_size - offset));
-		count = iov_iter_count(iter);
+		shortened = count - iov_iter_count(iter);
+		count -= shortened;
 	}
 
 	io = kmalloc(sizeof(struct fuse_io_priv), GFP_KERNEL);
@@ -3177,6 +3178,7 @@ fuse_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 		else if (ret < 0 && offset + count > i_size)
 			fuse_do_truncate(file);
 	}
+	iov_iter_reexpand(iter, iov_iter_count(iter) + shortened);
 
 	return ret;
 }