Subject: [04/12] fuse: fix pipe buffer lifetime for direct_io

From: Miklos Szeredi <[email-masked]>
Date: Mon, 7 Mar 2022 16:30:44 +0100

commit 0c4bcfdecb1ac0967619ee7ff44871d93c08c909 upstream.

In FOPEN_DIRECT_IO mode, fuse_file_write_iter() calls
fuse_direct_write_iter(), which normally calls fuse_direct_io(), which then
imports the write buffer with fuse_get_user_pages(), which uses
iov_iter_get_pages() to grab references to userspace pages instead of
actually copying memory.

On the filesystem device side, these pages can then either be read to
userspace (via fuse_dev_read()), or splice()d over into a pipe using
fuse_dev_splice_read() as pipe buffers with &nosteal_pipe_buf_ops.

This is wrong because after fuse_dev_do_read() unlocks the FUSE request,
the userspace filesystem can mark the request as completed, causing write()
to return. At that point, the userspace filesystem should no longer have
access to the pipe buffer.

Fix by copying pages coming from the user address space to new pipe
buffers.

Reported-by: Jann Horn <[email-masked]>
Fixes: c3021629a0d8 ("fuse: support splice() reading from fuse device")
Cc: <[email-masked]>
Signed-off-by: Miklos Szeredi <[email-masked]>
Signed-off-by: Zach O'Keefe <[email-masked]>
Signed-off-by: Greg Kroah-Hartman <[email-masked]>
Signed-off-by: Ben Hutchings <[email-masked]>
---
 fs/fuse/dev.c    | 12 +++++++++++-
 fs/fuse/file.c   |  1 +
 fs/fuse/fuse_i.h |  2 ++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 585c52dbb2e3..13cf7c260a83 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -992,7 +992,17 @@ static int fuse_copy_page(struct fuse_copy_state *cs, struct page **pagep,
 
 	while (count) {
 		if (cs->write && cs->pipebufs && page) {
-			return fuse_ref_page(cs, page, offset, count);
+			/*
+			 * Can't control lifetime of pipe buffers, so always
+			 * copy user pages.
+			 */
+			if (cs->req->user_pages) {
+				err = fuse_copy_fill(cs);
+				if (err)
+					return err;
+			} else {
+				return fuse_ref_page(cs, page, offset, count);
+			}
 		} else if (!cs->len) {
 			if (cs->move_pages && page &&
 			    offset == 0 && count == PAGE_SIZE) {
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 8aef8e56eb1b..aa4f0d632bed 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1319,6 +1319,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 			(PAGE_SIZE - ret) & (PAGE_SIZE - 1);
 	}
 
+	req->user_pages = true;
 	if (write)
 		req->in.argpages = 1;
 	else
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index 7e4b0e298bc7..6104534ddb7e 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -310,6 +310,8 @@ struct fuse_req {
 	/** refcount */
 	atomic_t count;
 
+	bool user_pages;
+
 	/** Unique ID for the interrupt request */
 	u64 intr_unique;