[34-longterm 064/196] UBIFS: fix oops when R/O file-system is fsync'ed

[Paul Gortmaker]

From: Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx>

                   -------------------
    This is a commit scheduled for the next v2.6.34 longterm release.
    If you see a problem with using this for longterm, please comment.
                   -------------------

commit 78530bf7f2559b317c04991b52217c1608d5a58d upstream.

This patch fixes severe UBIFS bug: UBIFS oopses when we 'fsync()' an
file on R/O-mounter file-system. We (the UBIFS authors) incorrectly
thought that VFS would not propagate 'fsync()' down to the file-system
if it is read-only, but this is not the case.

It is easy to exploit this bug using the following simple perl script:

use strict;
use File::Sync qw(fsync sync);

die "File path is not specified" if not defined $ARGV[0];
my $path = $ARGV[0];

open FILE, "<", "$path" or die "Cannot open $path: $!";
fsync(\*FILE) or die "cannot fsync $path: $!";
close FILE or die "Cannot close $path: $!";

Thanks to Reuben Dowle <Reuben.Dowle@xxxxxxxxxx> for reporting about this
issue.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@xxxxxxxxx>
Reported-by: Reuben Dowle <Reuben.Dowle@xxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
 fs/ubifs/file.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/ubifs/file.c b/fs/ubifs/file.c
index 5692cf7..1a411fd 100644
--- a/fs/ubifs/file.c
+++ b/fs/ubifs/file.c
@@ -1312,6 +1312,9 @@ int ubifs_fsync(struct file *file, struct dentry *dentry, int datasync)
 
 	dbg_gen("syncing inode %lu", inode->i_ino);
 
+	if (inode->i_sb->s_flags & MS_RDONLY)
+		return 0;
+
 	/*
 	 * VFS has already synchronized dirty pages for this inode. Synchronize
 	 * the inode unless this is a 'datasync()' call.
-- 
1.7.9.3

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