linux-kernel - [PATCH 5.4 173/215] fuse: ignore data argument of mount(..., MS

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Message-Id: <20200720152828.408019065@linuxfoundation.org>
Date:   Mon, 20 Jul 2020 17:37:35 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Stefan Priebe <s.priebe@...fihost.ag>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [PATCH 5.4 173/215] fuse: ignore data argument of mount(..., MS_REMOUNT)

From: Miklos Szeredi <mszeredi@...hat.com>

commit e8b20a474cf2c42698d1942f939ff2128819f151 upstream.

The command

  mount -o remount -o unknownoption /mnt/fuse

succeeds on kernel versions prior to v5.4 and fails on kernel version at or
after.  This is because fuse_parse_param() rejects any unrecognised options
in case of FS_CONTEXT_FOR_RECONFIGURE, just as for FS_CONTEXT_FOR_MOUNT.

This causes a regression in case the fuse filesystem is in fstab, since
remount sends all options found there to the kernel; even ones that are
meant for the initial mount and are consumed by the userspace fuse server.

Fix this by ignoring mount options, just as fuse_remount_fs() did prior to
the conversion to the new API.

Reported-by: Stefan Priebe <s.priebe@...fihost.ag>
Fixes: c30da2e981a7 ("fuse: convert to use the new mount API")
Cc: <stable@...r.kernel.org> # v5.4
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/fuse/inode.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/fs/fuse/inode.c
+++ b/fs/fuse/inode.c
@@ -473,6 +473,13 @@ static int fuse_parse_param(struct fs_co
 	struct fuse_fs_context *ctx = fc->fs_private;
 	int opt;
 
+	/*
+	 * Ignore options coming from mount(MS_REMOUNT) for backward
+	 * compatibility.
+	 */
+	if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE)
+		return 0;
+
 	opt = fs_parse(fc, &fuse_fs_parameters, param, &result);
 	if (opt < 0)
 		return opt;