fuse: fix pipe buffer lifetime for direct_io

jira VULN-7917
cve CVE-2022-1011
commit-author Miklos Szeredi <[email protected]>
commit 0c4bcfd
upstream-diff Used 4.19 LT commit 99db282
              because page info is in fuse_req in this kernel as opposed
              to fuse_args in 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 protected]>
Fixes: c302162 ("fuse: support splice() reading from fuse device")
	Cc: <[email protected]>
	Signed-off-by: Miklos Szeredi <[email protected]>
(cherry picked from commit 0c4bcfd)
	Signed-off-by: Brett Mastbergen <[email protected]>

Author: bmastbergen

fs/fuse/dev.c

@@ -932,7 +932,17 @@ 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) {

fs/fuse/file.c

@@ -1386,6 +1386,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 		nbytes += frag_size;
 	}
 
+	req->user_pages = true;
 	if (write)
 		req->in.argpages = 1;
 	else

fs/fuse/fuse_i.h

@@ -267,6 +267,8 @@ struct fuse_req {
 	/** refcount */
 	atomic_t count;
 
+	bool user_pages;
+
 	/** Unique ID for the interrupt request */
 	u64 intr_unique;